ALL Chemistry Carbohydrates
ALL Chemistry Carbohydrates
ALL Chemistry Carbohydrates
I- DEFINITION :
Carbohydrate are aldehyde or ketone derivatives of polyhydric
alcohols or any substances derived from them .
A- Naming of monosaccarides :
1- According to the presence of aldehyde or ketone group :
a- Aldoses : monosaccarides containing aldehyde group (
-CHO ) , The suffix –ose means sugar .
b- Ketoses : monosaccarides containing ketone group ( -C=O ) ,
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2- According to the number of carbon atoms :
a- Trioses : monosaccharides containing 3 carbons .
b- Tetroses : 4 carbons .
c- Pentoses : 5 carbons .
d- Hexoses : 6 carbons .
e- Heptoses : 7 carbons .
3- According to both presence of aldehyde or ketone groups
and number of carbon atoms :
a- Aldotrioses and ketotrioses .
b- Aldotetroses and ketotetroses .
c- Aldopentoses and ketopentoses .
d- Aldohexoses and ketohexoses .
4- System for numbering the carbons :
The carbons are numbered starting from aldehyde group as
carbon number 1. In case of ketoses the carbon of ketone
group group is the carbon number 2.
B- Classification of monsaccharides :
1- Trioses : monosaccharides containing 3 carbon atoms .
a- Aldotrioses : Glyceraldehyde “glycerose” .
b- Ketotrioses : Dihydroxyacetone .
2- Tetroses : monosaccarides containing 4 carbon atoms :
a- Aldotetroses : Erythrose and threose .
b- Ketotetrose : Erythrulose .
Note : The suffix-ulose means Keto sugar .
3- Pentose : monosaccharides containing 5 carbon atoms .
a- Types :
1) Aldopentoses : Ribose , arabinose , xylose and lyxose .
2) Ketopentoses : Ribulose and xylulose .
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b- Importance ( function ) of pentoses :
1) Ribose and deoxyribose enter in the structure of nucleic acids
RNA and DNA .
2) Ribose enters in the structure of ATP , GTP and other high
energy phosphate compounds .
3) Ribose enters in the structure of coenzymes NAD , NADP and
flavoproteins .
4) Ribose phosphate and ribulose phosphate are intermediates in
pentose phosphate shunt ( a minor pathway for glucose oxidation ).
5) Arabinose and xylose are constituents of glycoprotein in plants
and in animals .
6) Lyxose is a constituent of lyxoflavin isolated from human heart
muscle .
7) Xylulose is an intermediate in uronic acid pathway ( a minor
pathway for glucose oxidation ) .
4- Hexoses :
a- Types :
1) Aldohexoses : glucose , glactose and mannose .
2) Ketohexose : fructose .
b- Importance :
1) Glucose is the most importance sugar in carbohydrates :
a) Dietary carbohydrates are absorbed in the form of glucose .
b) In the liver and other tissues , glucose is converted to all
carbohydrates in the body e . g . glycogen , galactose , ribose and
fructose .
c) Glucose is the major source of energy in mammals .
2) Fructose “ Fruit sugar “ :
a) It can be converted into glucose in liver .
b) It is the main source of energy in mammals .
3) Galactose :
a) It can be converted into glucose in liver .
b) Synthesized in mammary gland to make the lactose of milk
( milk sugar ) .
4) Mannose : A constituent of many glucoproteins .
5- Heptoses :
As sedoheptulose : is formed in the oxidation of glucose through
the pentose phosphate pathway .
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Ring ( cyclic ) Structure of sugars
The simple open formula of sugars fails to explain some reactions e . g .
glucose which has aldehyde group does not give all the reactions of
aldehyde. This indicates that the –CHO group must be masked or
combined in some way . In solution , the sugar which has an aldehyde
group undergoes the following :
1- Hydration of aldehyde group to form group ( alcohol ) .
2- Intermolecular reactions occur by subsequent condensation between
one of the –OH of aldenol group and the –OH group of C4 or C5 to form
ring structure ( hemiacetal structure ), here the carbonyl group becomes
asymmetric carbon atoms .
3- If the remaining –OH is on the right side , so it is α - sugar .
If the remaining –OH is on the left side , so it is β - sugar .
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4- Pyranose and furanose :
A) The 1-5 ring form is called pyranose as it resembles the organic
compound pyran e . g . α and β glucopyranose .
B) The 1-4 ring form is called furanose as it is resmbles the
organic compound furan e . g . α and β glucofuranose .
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Glucose in solution is present mainly (99 %) as glucopyranose and
(1% ) as glucofuranose, of 99 % of glucopyranose ( 36 % ) are present
as α - D form and ( 63 % ) as β – D form .
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4- Racemic mixture : It is the mixture containing equal number of
molecule of 2 optically active sugars ; one is dextrorotatory and the
other is levorotatory . Thus , it shows no optical activity ( Provided
that the angle of rotation is equal in both sides ) .
5- Resolution : It is the separation of optically inactive racemic
mixture into its optically active substance .
B- Optical isomerism :
It is the ability of substance to present in more than one form
(isomer) . A substance containing one asymmetric carbon atom can exist
in a number of isomers = 2n where n is the number of asymmetric carbon
atoms . e . g . glucose has 4 asymmetric carbon atoms so the number of
its isomers equal 24 = 2 x 2 x 2 x 2 = 16 isomers .
1- Configuration ( Enantiomers )
A)The simplest carbohydrates are monosaccharides trioses ; for
example glyceraldehyde which has one asymmetric carbon and thus 2
optically active forms : L and its mirror image D forms .
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2- Anomeric carbon and anomers :
A) Anomeric carbon : is the asymmetric carbon atom obtained
from active carbonyl sugar group : carbon number 1 in aldoses and
carbon number 2 in ketoses .
B) Anomers : These are isomers obtained from the change of
position of hydroxyl attached to the anomeric carbon e . g . α and β
glucose are 2 anomers . Also α and β fructose are 2 anomers .
C) Mutarotation : It is a gradual change of specific rotation of
any optically active substance having free aldehyde ( -CHO ) or
ketone (C=O) group .
1) α -Glucose freshly dissolved in water has specific rotation of
+112 .
2) β -Glucose when freshly dissolved in water , has specific ratation
of +19 .
3) when both anomers are left for some times , α and β sugars slowly
change into an equilibrium mixture which has specific rotation of
+52.5
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3- Aldose – Ketose isomerism : Fructose has the same molecular formula
as glucose but differs in structure formula . One contains keto group
(C=O) and the other contains aldehyde group ( -CHO ) . Both are
isomers.
4- Epimeric carbon and epimers :
A) Epimeric carbon is the asymmetric carbon atom other than
carbon of aldehyde or Ketone gbroup e . g . carbons number 2 , 3
and 4 of glucose .
B) Epimers : are isomers resulting from the change of position
of groups around the epimeric carbons . Glucose , glactose and
mannose are epimers .
1) Glucose has 3 epimeric carbons , 2 , 3 and 4 .
2) Galactose : epimer of carbon 4 .
3) Mannose : epimer of carbon 2 .
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VI- PROPERTIES OF MONOSACCHARIDES :
A- Physical properties :
1- All monosaccharides are soluble in water .
2- All monosaccharides show the property of optical activity .
3- All monosaccharides can exist in α and β forms .
4- All monosaccharides can undergo mutarotation .
B- Chemical Properties :
1- Oxidation : oxidation of sugars give acids
2- Reduction : Reduction of carbonyl group gives the
corresponding alcohol e . g . glucose gives sorbitol , ribose gives
ribitol , galactose gives galacticol.
3- Reducing sugars : Sugars containing free aldehyde or
ketone group can reduce other reagents e . g . they can reduce
cupric ions of Fehling and Benedict’s reagents into cuprous ions :
Cupric ( blue ) + sugar Cuprous ( red ) + oxidized sugar
A) These tests are one of the earlist tests for the presence of
sugar in urine of diabetics .
B) These tests are nonspecific , because they can be reduced
also by other hexoses or other reducing compounds as vit . c .
4- Reactions with phosphoric and sulphuric acids :
A) Reactions of phosphoric acid with monosaccharides gives
phosphate esters e . g . glucose gives glucose-6-phosphate and
glucose-1-phosphate . Phsphorylated sugars are important
intermediates in carbohydrate metabolism .
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give a violet ring . This is the idea of Molish’s test ; a general test
for all carbohyrates .
5- Fermentation : Fermentation is the action of bacterial or
Yeast enzymes on carbohydrate .
A) Fermentation of sugas give ethyl alcohol and CO2 .
B) All D-monosaccharides are fermentable .
C6H12O6 2 CH3-CH2-OH + 2 CO2
6- Osazone formation : Osazone are characteristic crystals resulting
from the reaction of sugars with phenylhydrazine . All sugars having
free carbonyl group can form osazone crystal .
SUGAR DERIVATIVES :
A- Sugar acids : are produced by oxidation of carbonyl carbon , last
hydroxyl carbon or both .
1- Aldonic acids : Oxidation of carbonyl carbon to carboxylic
group gives aldonic acid e . g . glucose is oxidized to gluconic
acid .
2- Uronic acid : Oxidation of last hydroxyl carbon gives uronic
acid e. g . glucose is oxidized to glucuronic acid .
3- Aldaric acids : These are dicarboxylic acids produced by
oxidation of both carbonyl carbon and last hydroxyl carbon e . g .
glucose is oxidized to glucaric acid ( saccharic acid ) .
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C- Deoxysugars : Are sugars in which one of the hydroxyl groups has
been replaced by a hydrogen atom i.e one oxygen is missed .
1- Deoxyribose : occuring in nucleic acid DNA .
2- L-Fucose ( 6-deoxygalactose ) : occurring in glycoproteins .
D- Amino sugars : In these sugars , the hydroxyl group is replaced by an
amino or an acetylamino group .
1- Amino sugars are constituents of glycoproteins , gangliosides and
glucosaminoglycans .
2- Examples :
A) Glucosamine : occurring in heparin and hyaluronic acid .
B) Galactosamine : occurring in chondroitin sulphate .
C) Mannosamine : occurring in neuraminic and sialic acids .
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VIII- GLYCOSIDIC BOND AND GLYCOSIDES :
Glycosidic bond : It is hhte bond between a carbohydrate and another
compound to form a complex carbohydrate .
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2- Sugar nucleotide as ATP , GTP and other nucleotides : a
glycone here is purines and pyrimidines .
3- Glycolipids : as cerebrosides .
4- Glycoproteins .
5- Cardiac glycosides : Aglycone here is steroid
II- DISACCHARIDES :
These are formed by condensation of 2 molecules of
monosaccharides bond together by glycosidic bond . Its general formula
is Cn(H2O)n-1
A- The most important disaccharides are :
1- Maltose = α- glucose + α-glucose (α 1-4 glucosidic
bond ) .
2- Isomaltose = α-glucose + α-glucose (α 1-6 glucosidic
bond ) .
3- Lactose = β-glucose + β -galactose (β 1-4 glucosidic
bond ) .
4- Sucrose = α-glucose + β -fructose (α 1-B2 glucosidic
bond ) .
5- Cellobinose = β-glucose + β -glucose (β 1-4 glucosidic bond
).
6- Trehalose = α-glucose + α-glucose (α 1-1 glucosidic
bond ) .
B- Naming of glycosidic bonds : glycosidic bonds sugars are named
according to :
1- The numbers of the connected carbons .
2- The position of the anomeric carbon of the sugar , If it is in α
position , the linkage is an α-bond . If it is in the β position . the
linkage is a β -bond .
3- Example : lactose consists of β -glucopyranose and β
-galactopyranose . The bond is between carbon 1 of β
-galactopyranose and carbon 4 of glucopyranose . The bond is
therefore β 1-4 galactosidic linkage .
C- Maltose : Also called malt sugar :
1- Structure : It is formed of 2 molecules of α-D
glucopyranose linked together by α 1-4 glucosidic bond .
2- Sources :
Malt .
Maltose is produced during digestion of starch by amylase enzyme
3- Properties : Maltose contains free carbonyl ( aldehyde )
group so having the following properties :
A) It is a reducing agent ( can reduce Benedict’s reagent) .
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B) It can be present in α and β forms .
C) It can show mutarotation .
D) It can form characterstic osazone crystals .
D- Isomaltose :
1- Structure : It is similar to maltose , being formed of 2
molecules of α-D glucopyranose , but linked together by α 1-6
glucosidic bond .
2- Sources : Isomaltose is produced during digestion of starch
and glycogen by amylase enzyme .
3- Properties : The same as maltose .
E- Lactose :
1- Structure : It is formed of 2 molecules of β -D-
galactopyranose and β -D-glucopyranose linked together by β 1-4
glucosidic ( galactosidic ) bond.
2- Sources : it is the sugar present in milk . In human milk , its
concentration is 7.4 g/dl . It may appear in urine in late pregnancy
and during lactation .
3- Properties : Lactose contains free carbonyl group , so
having the following properties :
A) It is reducing sugar ( can reduce Benedict’s reagent ) .
B) It can present in α and β forms .
C) It can be show mutarotation .
D) It can form characteristic osazone crystals .
E) Lactose is digested by intestinal enzyme called : Lactase into
galactose and glucose. Deficiency of this enzyme stops the
digestion of lactose. this leads to its fermentation by intestinal
bacteria , diarrhea and abdominal distension .
F- Sucrose :
1- Structure : it is formd of 2 molecules of α-D-glucopyranose and
β -D-fructofuranose linked by α1 B 2 glycosidic bond .
2- Sources : cane and beet sugar . It is also present in pine apple
and carrot .
3- Properties : sucrose contains no free carbonyl group ( because
both the anomeric carbons ; carbon 1 of α-glucose and carbon 2 of
β -fructose are involved in glycosidic bond ) so fructose has the
following properties :
A) It is not a reducing sugar ( can not reduce Benedict’s reagent
).
B) It can not be present in α and β forms .
C) It can not show mutarotation .
D) It can not form osazone crystals .
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E) Sucrose is dextrorotatory . On hydrolysis by inverase
( sucrase ) enzyme , it gives a mixture of equal number of glucose
and fructose molecules . This mixture is called invert sugar and it is
levorotatory .
G- Cellobiose :
1- Structure : It is formed of 2 units of β -D- glucopyranose linked by
β 1----------4 glucosidic bond .
2- Sources : It is obtained by partial hydrolysis of cellulose .
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POLYSACCARIDES :
These are carbohydrates , formed of more than 10 sugar units .
They are classified into :
A- Homopolysaccharides which contain repeated same sugar units .
B- Heteropolysaccharides which contain repeated different sugar units .
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Properties :
1- Starch gives blue colour with iodine . Amylopectin gives red
colour with iodine .
2- Partial hydrolysis ( digestion ) by amylase enzyme gives
various forms of dextrins .
2- Dextrins : These are hydrolytic products of starch . They are formed
of α-glucose units but simpler than starch . They include amylodextrin ,
erythrodextrin and achrodextrin .
3- Glycogen : ( also called animal starch ) :
Structure : It is highly branched chain homopolysaccharide .
Each branch is composed of 12-14 glucose units , linked together by 1-4
glycosidic bonds and by 1-6 glycosidic bond at branch point (like
amylopectin) .
Sources : Glycogen is the storage form of carbohydrates in human and
animals . It is synthesized and stored in liver , muscles and tissues .
Properties : It gives red colour with iodine .
4- Cellulose :
a- Structure : It is long straight non branching chains of glucose units (β
-D-glucopyranose ) linked together by β 1---4 glycosidic bond . The
chains are strenthened by cross linked hydrogen bonds .
b- Sources : Cellulose is the chief constituent of the framework of plant ;
leaved vegetables , fruits , wood , cotton , …etc .
c- Properties :
1- Cellulose gives no colour with iodine .
2- Cellulose is insoluble in water .
3- Cellulose in diet cannot be digested by many mammals
including humans because of the absence of hydrolase enzyme that
attachs β -linkage .
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4- Its presence in diet is important because it cannot be digested
, so it will increase the bulk of stool . This stimulates the intestinal
movement and prevents constipation .
5- Cellulose can be utilized and serve as a source of energy in
herbivores because their gut contain bacterial enzymes that can
attach β -Linkage .
5- Inulin
a- Structure : it is frutosan i.e . formed of repeated units of fructose
linked together by β 1-2 bonds .
A) Sources : Root of artichokes and other plants .
B) Properties : soluble in warm water .
C) Medical importance : Inulin clearance is one of a diagnostic
tests for investigation of glomerular filtration rate .
6- Chitin :
A) Structure : It is a polymer of N-acetylglucosamine linked
together by glycosidic bonds .
B) Sources : It is important polysaccharides invertebrates .
B- Heteropolysaccharides : They include glucosaminoglycans ,
proteoglycans and glycoproteins .
GLYCOSAMINOGLYCANS, (MUCOPOLYSACCHARIDES) :
A – Introduction :
1- They are formed of repeating disaccharides units [ acidic sugar amino
sugar ]n .
The acidic sugar is either D-glucuronic acid or its carbon 5 epimer L-
induronic acid .
The amino sugar is either D-glucoamine or D=galactosamine in which
the amino group is usually acetylated . The amino sugar may also be
sulphated at carbon 4 or 6 .
2- Most of GAGs are present extracellulary except heparine .
3- Most of them form the structural components of connective tissue such
as bone , elastin and collagen .
4- They act as lubricants and cushion for other tissues because they have
the property of holding large quantities of water .
5- When a solution of glycosaminoglycans is compressed , the water is
“squeezed out” and the glycosaminoglycans are forced to occupy a
smaller volume , when the compression is released , the
glycosaminoglycans return back to their original , hydrated volume
because of the repulsion of their nagative charges . This property is the
cause of resilence of synovial fluid and the vitreous humor of the eye .
B- Glycosaminoglycans include :
1- Hyaluronic acid :
Structure : Repeated disaccharides unit consists of
1) glucuronic acid .
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2) N-acetylglucosamine .
N . B . It is the only GAGs which contains no sulfate group .
Site :
1) Synovial fluid .
2) Vitrous body of the eye .
3) Embryonic tissue .
4) Cartilage .
5) Loose connective tissue .
C- Function :
1) It permits cell migration during wound repair and
morphogenesis , i . e . differentiation of cells in the form of organs
and tissues in the early embryo .
2) It makes extracellular matrix loose because of its ability to
attract water .
3) It makes cartilage compressible because of its high
concentration in this tissue .
4) It acts as a lubricant in joints .
d- Role in disease :
Hyaluronic acid facilitates cell migration .It is produced in
increased amounts by tumor cells . This facilitates migration of these
cells through the extracellular matrix and spread of the tumor .
2- Chondritin 4 – and 6 – sulfate :
Structure : It is usually present in association with protein to form
proteoglycan aggregates . The repeated disaccharides unit consists of :
1) Glucouronic acid .
2) N-acetylgalactosamine with sulfate on either C-4 or C-6 .
Site : It is the most abundant GAGs in the body . It is found in :
1) Cartilage , tendons , ligements and bones .
2) Aorta , skin , cornes , umbilical cord and in certain neurons .
c- Functions :
1) In cartilage : it binds collagen and hold fibers in strong
network .
2) Help to maintain the shape of skeletal system .
3) Have role in compressibility of cartilage in weight bearing .
3- Keratan sulfate :
Structure : The repeated disaccharides unit consists of :
1) Galactose ( no uronic acid ) , with sulfate on C-6 .
2) N-Acetylglucosamine with sulfate on C-6 .
Site :
1) Cornea .
2) It is found as proteoglycan in cartilage .
C- Functions :
1) It plays an important role in corneal transparency .
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4- Dermatan sulfate :
1- Structure : The repeated disaccharide unit consists of :
1) L-Induroic acid .
2) N-Acetylgalactosamine with sulfate on C-6 .
Sits :
1) Cornea .
2) Selera .
3) Skin , blood vessels and heart valves .
Functions :
1) In cornea , it plays together with keratan sulfate an important
role in corneal transparency .
2) Its presence in sclera may play a role in maintaining the
overall shape of the eye .
5- Heparine :
Structure : The repeated disaccharides unit consists of :
1) Induronic acid with sulfate on C-2 .
2) Glucosamine with sulfate on C-3 and C-6 .
Site : Heparine present in mast cells ( intracellular compound ) Mast
cells are located along the wall of blood vessels of liver , lungs , skin ,
heart , kidney and spleen .
Functions :
1) It act as anticoagulant .
2) Heparine sulfate ( which is the same as heparine in structure
except some glucosamines are acetylated and there are fewer
sulfate groups ) ; has the following functions :
1) It is a component of cell membrane and act as
receptors .
2) It participates in cell adhesion and cell-cell
interaction .
3) It is present in basement membrane of the kidney and
plays an important role determining the charge selectiveness
of golmerular filtration .
- PROTOGLYCANS AND GLUCOPROTEINS :
Both are proteins containing carbohydrates , They differ from each
other in that they present in different sites , contain different sugars and
have different shape and size .
Proteoglycans : These are chains of glycosaminoglycans attached to
protein molecule e . g . hyaluronic acid , chondroitin sulfate , keratan
sulfate , dermatan sulfate , heparine and heparan sulfate . They serve as a
ground substance and associated with structure elements of tissues as
bone elastin and cartilage. The carbohydrate part is present in very long
unbranched chains ( more than 50 monosaccharides molecules ) attached
to protein core .
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Glycoproteins ( mucoproteins ) :
1- Structure : They consist of :
Protein core :
Carbohydrate chains which are branched short chain ( from 2-15
monosaccharides units ) such usually called oligosaccharides chain .
They include :
1) Hexoses : Galactose and mannose .
2) Acetylhexosamines : N-acetyl glucosamine and N-acetyl-
galactosamine .
3) Pentoses : Arabinose and xylose .
4) Methylpentose
5) Sialic acid .
6) They contain no uronic acids or sulfate groups .
2- Functions :
Glycoproteins are components of extracellular matrix .
They are components of mucins of gastrointestinal and urogental
tracts , where they act as protective biologic lubricants .
Glycoproteins are components of cell membrane as :
1) Blood group antigens ( A , B , AB ) .
2) Cell surface recognition receptors : for hormones , other cells
and viruses .
3) Glycophorin : Which is glycoprotein present in human red cell
membrane . It spans the lipid membrane . It has free polypeptide
portions outside both the external and internal ( cytoplasmic )
surfaces.
4) Plasma proteins : globular proteins – except albumin – present in
plasma are glycoproteins .
5) Most secreted enzymes and proteins ( as hormones ) are
glycoproteins.
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¬
ENZYME
Definitions
A- Catalysts: These are organic or inorganic substances that acce¬lerate
the rate of chemical reactions.
1- The organic catalysts are enzymes: They are:
a- Highly specific i.e. catalyze one or two reactions only.
b- Protein in nature, so they are denaturated by heat.
2- The inorganic catalysts are metals as zinc, magnesium chloride ions.
They are:
a- Non specific i.e. catalyze many reactions.
b- Not affected by heat.
B- Enzymes: These are specific protein catalysts, that accelerate the rate
of chemical reactions. Enzyme structure is not changed by entering the
reactions, and it does not affect the equilib¬rium constant (i.e. end
products) of the reactions.
C- Rate of chemical reaction: It is the change in the amount (moles,
grams) of starting materials (substrates) or products per unit time.
D- Substrate: Is the substance upon which the enzyme acts.
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B- Allow reactions to occur at a rate appropriate to the needs of the
cells
Substrates -------------------------------- Products
ENZYMES:
(en = in , zyme = yeast).
Enzymes are biologically active proteins that are synthesized within the
A- Properties of enzymes:
1. They are invariably protein
2. They act within a moderate PH and temperature range
3. They are highly specific, catalyzing only one type of
chemical reaction.
B. Enzyme specificity:
The specificity of the enzyme is determined by
1. The functional group of the enzyme and its co-factor
2. .The functional group of the substrate
3. The physical proximity of these various functional groups
D- Enzyme activity:
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1- Requirements for enzyme activity:
*- Enzymes are either simple or conjugated proteins.
*- If the enzyme is a simple protein, only the native conformation of the
protein is required for activity.
*- If the enzyme is a conjugated protein, it is called: holo¬enzyme and its
activiy will depend upon:
1) Conformation of the protein which is called a apoenzyme
2) The availability of a non-protein part, which is called cofactor.
.
Some enzymes require metals as Mg2+ and Cu2+as co-factors.
Some enzymes require coenzymes as NAD+ and FAD.
Apoenzyme Cofactor
E- Coenzymes
The co-enzymes are heat stable organic compound responsible for the
catalytic action of the enzymes. Co-enzymes are frequently containing
Vit B complex. They are classified according to their function into
1) Hydrogen carriers:
NAD and NADP .
FAD and FMN .
Lipoic acid.
Coenzyme Q.
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Co-enzymes for transfer of 2 hydrogen
B. Co-enzyme Q or Ubiquinone
It is a component of the respiratory chain in the mitochondria and acts as
electron carrier
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C. Flavin nucleotide co-enzyme
Riboflavin (vitamin B2) is the constituent of several enzymes which are
involved in intermediary metabolism
1. Riboflavin monophosphate also knowen as flavin mono-nucleotide
(FMN): It is the constituent of cytochrome C reductase and the amino
acid dehydrogenase
D. Lipoic acid
It is involved in the complete system of oxidative decarboxylation of
pyruvate or ketoglutarate
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Thiamine (Vit B1) is a component of the co-enzyme TPP that acts as
decarboxylase in the oxidative decarboxylation of α-keto acids. It is
component of the enzyme pyruvate dehydrogenase
B. Co-enzyme A
Pantothenic acid is a component of Co-enzyme A. It acts as Co-acylase in
the acyl transfer reaction.
C-Pyridoxal Phosphate
Pyridoxine (vit B6) is involved in the enzyme system of transaminases,
cystathionine synthetase and heme biosynthesis
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D. Biotin or Vitamin H
It acts as co-carboxylase in the transfer of pyruvate to oxalacetate and
acetyl CoA into malonyl CoA
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ENZYMES NOMENCLATURE (NAMING):
Enzyme nomenclature has histori¬cally been a source of confusion.
There are several ways of naming enzymes:
A- Trivial names e.g. trypsin, pepsin.
B- Some enzymes were named by attaching the suffix -ase to the name of
the substrates e.g. maltose & maltase.
C- Some enzymes were named according to the type of the reaction
e.g. aminotransferase.
D- To standardize enzyme nomenclature, the International Union of
Biochemistry (lUB) made a systemic name to each enzyme. This name
can indicates:
1- The substrate acted upon.
2- The coenzyme involved in the reaction.
3- The type of reaction catalyzed.
e.g. Lactate - NAD+ - oxidoreductase enzyme.
(Its old name was lactate dehydrogenase)
Alcohol Dehydrogenase
E.C (1) : Class of enzyme: oxidoreductase.
E.C. 1.1) : Group upon which the enzyme acts is : CH-OH
E.C. 1.1.(1) : The coenzyme is NAD+.
E.C. 1.1.1.(1) : Alcohol e.g. ethanol is the substrate.
CLASSIFICATION OF ENZYMES:
- There are 6 classes of enzymes which are:
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A- Oxidoreductases : This group of enzymes catalyzes an oxidation¬
reduction reaction between two substrates:
S (oxidized) + Y (reduced) --------------- S (reduced) + Y (oxidized).
1- Oxidoreductases are further classified according to the sub¬strate
oxidized and to the mechanism of oxidation.
The mechanism of oxidation is either by addition of oxygen (oxidases)
or by removal of hydrogen (dehydrogenases).
* Dehydrogenase are group of the enzymes catalyzes the removal of 2 H
or electron from the substrate to the coenzymes e.g Lactate
dehydrogenase
* Oxidase: they are group of the enzyme catalyze transfer of hydrogen or
electron to oxygen with the formation of hydrogen peroxide e.g Glucose
oxidase
* Oxygenase: enzymes catalyzes the incorporation of either molecular
oxygen into the substrate e.g lipoxygenase or one oxygen e.g
monoxygenase
Phosphohexose isomerase_
Glucose-6-phosphate Fructose-6-phosphate
31
6- Ligases (or synthetases): This group of enzymes catalyzes join¬ing of
two substrates using the energy released in the hydroly¬sis of a high
energy phosphate compound as ATP or GTP.
. Example: Glutamine synthetase.
Glutamate + NH3---------------------------- Glutamine + H2O
MECHANISM OF ENZYME ACTION:
A- Energy of activation:
1- All the reactions that proceed from initial substrates (ini¬tial state) to
products (final state) consume energy. This is called free energy of the
reaction.
2- However the substrates do not become products directly, but must be
energized (absorb energy) to reach an activated or transition state. This
energy is called activation energy.
3- At transition state, there is a high probability that a che¬mical bond
will be made or broken to form the product.
4- The definition of activation energy: is the amount of energy required to
raise all the molecules in one mole of a substa¬nce to the transition state.
5- The effect of enzymes is to decrease the energy of activation of
substrates
Active site:
The substrate is bounded to a specific region on the enzyme called active
site which is characterized by
Smal portion of the total volume of the enzyme
It is composed of R group comes from side chain of the amino acids and
the specificity depends on arrangement of these groups
32
1) During enzyme action, there is a temporary combination between the
enzyme and its substrate forming enzyme¬ substrate complex. This
occurs at active site of enzyme.
2)This is followed by dissociation of this complex into enzyme again
and products.
E + S------- ES ------- E + P
ENZYME KINETICS:
It is the study of the rate or velocity of reac¬tions catalyzed by enzymes.
A- Initial velocity: the rate (velocity) at which a reaction pro¬ceeds is
measured as the decrease in the concentration of reac¬tants (substrate) or
the increase in concentration of the products with time.
1- If an enzyme is incubated with its substrate and the appea¬rance of
the product with time is recorded on a graph, the resulting line will have
the hyperbolic shape as in the fo¬llowing figure 4
33
2- The rate (velocity) of the reaction, which corresponds to the slope of
this curve, is initially constant but gradually decreases.
3- The decline in the rate of the reaction may be due to deple¬tion of the
substrate, inhibition of the enzyme by its pro¬duct, or denaturation of the
enzyme. Each of these events affects the conditions of the reaction, and in
turn affects the velocity of the reaction.
4- Thus only in the initial portion of the reaction are the con¬ditions
accurately known. For this reason, only the initial velocity (Vi) is used in
calculating the kinetic parameters of the reaction. The units of velocity
are concentration per unit time, e.g. micromoles per minute.
34
increase in the amount of substrate causes no increase in the velocity of
the reaction. This is true if all other conditions especially enzyme
concentration remain constant. At low substrate concentration, not all
enzymes are saturated. So the rate of reaction will increase.
At higher substrate concentration, all enzymes get satura¬ted with
substrates and any more increase of substrate concentration will result in
no increase in the rate of the reaction (plateau curve). The following
diagram can explain:
A- Michaelis-Menten Equation:
1) This equation describes the dependence of reaction ve¬locity on
substrate concentration.
2) Michaelis and Menten proposed that in any enzymatic reaction, the
enzyme (E) combines with substrate (S) to form an enzyme-substrate
(ES) complex.
K1
E + S -------------------- ES
ES then breaks down either to enzyme and substrate again or to enzyme
and product (P)
K2
ES-------------------- E + P
35
K-1
ES------------------------ E + S
4) The rates(velocities) at which the partial reactions of this model occur
are described by the rate constants K1 , K-1 and K2.
5) According to this model, the increase in the initial velocity (Vo)
observed with an increase in (S) is due to an increase in the amount of ES
formed. At Vmax , all of the enzyme is involved in an ES complex.
Vmax [S]
Vi = -------------------------------
[S] + Km
K1 + K2
Km = --------------------------
K-1
If Km = [S] then
Vmax
Vi = ---------------------
2
36
Lineweaver-Burk plot:-
Here the reciprocal of V i.e 1/V is plotted versus the reciprocal of [S] 1/S.
The curve is straight line and it becomes more practical to estimate Km
because a few give saturation curve.
3- Effect of temperature:
a- The optimal temperature for enzymatic activity in human body is that
temperature similar of that of the cells 37 ºC
b- At zero temperature, the enzyme is inactive. The re¬action velocity
increases with increase of temperature until a maximum velocity is
reached. This increase in reaction velocity is due to the increase in the
number of molecules having sufficient energy to pass over the energy
barrier and form the products of the reaction. Further elevation in
temperature resulted in decrease in reaction velocity.
37
4. Effect of pH
The optimal pH for an enzyme activity is that pH at which the enzymes
acts maximally.
Increase or decrease in pH, the ionic state of both enzyme and substrate
will be changed and the rate of the reaction decline
Each enzyme has its own optimal pH , pancreatic lipase 7.5 while
pepsin 2
Enzyme-inhibitor
Any substance that can diminish the velocity of an enzymatic reaction is
called inhibitor. The inhibition of the enzymes can be classified either
reversible or irreversible
1.Reversible inhibitors:- these inhibitors bind to the enzyme through non-
covalent bond
*- Dilution of the enzyme-inhibitor complex results in diss¬ociation of
the reversibly bound inhibitor and recovery of enzyme activity.
*- Reversible inhibition may be competitive or non competitive
2- Irreversible inhibitors: This type of inhibition occurs when an
inhibited enzyme does not regain activity upon dilution of the enzyme
inhibitor complex
38
*- Effect of competitive inhibitor on Vmax
The effect of a competitive inhibitor is reversed by increa¬sing substrate
concentration. Therefore, at a sufficiently high substrate concentration,
the reaction velocity reaches Vmax observed in absence of the inhibitor
Effect on Km: A competitive inhibitor increase Km of the substrate i.e
more substrate is needed
Effect on Line-weaver-Burk plot: Inhibited and uninhibited reactions
show different X axis intercepts indicate increase in Km
39
Uncompetitive inhibitor
This inhibitors binds only on [ES] complex. This results in apparent
changes in both Vmax and Km which is reflected in double reciprocal
plot
40
Isoenzymes
Isozymes are physically distinct forms of the enzyme with the same
catalytic activity: they catalyze the same reaction but at different
rates( have different Km value). They migrate differently in an electric
field.
- The best known example is:
. Lactate dehydrogenase enzyme (LD) which is a tetrameric i.e.
contains 4 polype¬ptide chains. These 4 chains are a mixture of different
proportions of 2 chains H and M. chains Hand M (H from heart & M
from muscle).
These two subunit combined in 5 different ways so that there are 5
isoenzymes of LDH
41
Regulation of the enzyme activity
Control of metabolic regulation of a pathway occurs through modi-
fication of the enzymatic activity of the key enzyme in the pathway( rate
limiting enzyme). The activity of rate limiting enzyme can be regulated in
a number of ways
Induction
It is the increase in enzyme production through hormonal activation of
the mechanism controlling their gene expression
Example: Insulin hormone is hypoglycemic hormone induced the
synthesis of group of the enzyme responsible for glucose oxidation
( glucokinase or phosphofructokinase) or glycogen synthesis (glycogen
synthetase)
42
2- Phosphorylation reactions are catalyzed by a family of enzymes, called
protein kinase. It utilizes ATP as a phosphate donor. Phosphate groups
are cleaved from phosphorylated en¬zymes by the action of
phosphoprotein¬ phosphatase.
3- Depending on the specific enzyme, the phosphorylated form may be
more or less active than the unphosphorylated enzyme. For example,
phosphorylation of glycogen phosphorylase increases activity, whereas
the addition of phosphate to glycogen syn¬thase results in a less active
enzyme.
B. Non covalent modification This is done by non-covalent binding of
certain metabolities to allosteric site. De novo synthesis of fatty acids
acetyl CoA carboxylase is regulated by feed back nhibition by non-
covalent binding of fatty acids
It means that the end product of a series of reactions directly inhibits the
first enzyme of that series.
Enzyme Compartmentation
Compartmentalization of the metabolic pathway allows the separation of
the process that proceeds in opposite direction and may otherwise
interfere with one another e.g the anabolic process involved in the
biosynthesis of fatty acids from acetyl CoA are located in cytosol while
catabolic process concerned with the oxidation of fatty acids are located
with the mitochondria.
Clinical Enzymology
Enzyme level determination in plasma can be used ax an index or
diagnostic agent in the diagnosis of certain diseases. There are two types
of plasma enzyme
Functional Enzymes
They are the enzymes that are normally present in plasma in higher
concentration as they perform certain physiological function. E.g Blood
clotting factors and lipoprotein lipase
Non-Functional enzymes
They perform no known physiological function in plasma as their names
implies. Their presence in plasma at levels higher than normal level
reflect tissues destruction
e.g ALT and AST in liver disease
CPK in heart disease
LDH in heart disease
43
NUCLEOTIDES
The nucleotides are important intracellular molecules having the
following functions
1- They enter in the structure of the nucleic acids RNA and DNA.
2- Purines enter in the structure of
a- ATP (a source of energy) is the main form chemical energy
available.
b- The physiological mediators Cyclic AMP: the second messenger
involves in the action of many hormones
c- Component of some coenzymes ; FAD , NAD and NADP.
d-S~adenosylmethionine (methyl donner).
3- Pyrimidines enter in the structure of
44
a- UDP-glucose (glycogen and glucouronic acid
synthesis).
b- UDP-galactose (lactose synthesis).
c- CDP - acylglycerol (phospholipid synthesis).
45
SOURCES OF DIFFERENT ATOMS OF PURINE
AND PYRIMIDINE BASES
Although human ingest dietary nucleic acids and nucleotides, survival
does not requir their absorption and utilization. Human can synthesize
ample amounts of purine and pyrimidine nucleotides de novo ( from
intermediates).
Biosynthesis of Purine
De- novo synthesis pathway
46
47
Inhibitors of purine biosynthesis
1- Azaserine
It is a glutamine antagonist , stop purine synthesis at the reaction 5.
2- Diazonorleucine
Blocks reaction 2
3- 6- Mercaptopurine
Blocks the formation of AMP and GMP
IMP--------/----------- AMP--------/--------GMP
Site of purine biosynthesis:
Liver is the major site for purine biosynthesis.
48
Purine biosynthesis utilizes 6 ATP molecules and requires glycine
glutamine aspartate and methenyltetrahydrofolate. To avoid the loss of
unnecessary energy and nutrients. this pathway should be regulated to
provide, the exact needs of purines. The regulation depends upon the
activity of PP-ribose-P synthetase enzyme. It is allosterically inhibited by
GMP, GDP, AMP and ADP i.e. excess purines-inhibit more formation
and vice versa.
Catabolism of purines
In humans , the end products of purine catabolism is uric acid.
Site Mainly liver.
Mechanism
49
Sources of uric acid
1- Catabolism of purines in the liver (99%).
2-Uric acid produced in the intestine by the action of bacteria on puri¬nes
present in diet (1%).
Regulation of uric acid formation
*This depends on the activity of xanthine oxidase enzyme , which is
present in the liver kidney and intestine.
*Absence of this enzyme led to decrease uric acid formation.
Plasma level of uric acid
2.5- 7 mg/l00 ml
The form of uric acid in the plasma and other body fluids depends upon
the pH of that fluid
At the pH around 7.
The uric acid is present in the form of a salt sodium urate. e.g. Blood ,
CSF.
At the pH below 5.75:
The predominant form is uric acid e.g. urine.
Effect of hyperuricemia
50
When the plasma level of sodium urate exceeds certain level (above 7
mg/
100 ml)-------- formation of crystals of sodium urates--------deposition
of these crystals in soft tissues (these urate deposits are called : tophi);
1- In Joints
Urate crystals will be phagocytosed by leukocytes---------- Acute
inflammatory reaction called acute gouty arthritis. The joints that firstly
affected are the small joints especially those of big toes.
2 In kidneys
Deposition of uric acid tophi may lead to uric acid stone.
3-In the cartilage
As that of the ear----------- distruction of cartilage.
HYPOURICEMIA
It is the decrease of blood uric acid level below 2 mg/l00 ml.
Causes
1-Deficiency of activity of xanthine oxidase enzyme. Here xanthine and
hypoxanthine can not be converted to uric acid. This leads to
a-Hypouricemia.
b-Excess xanthine and hypoxanthine excretion (xanthinurea).
2-Drugs
e.g.acetylsalicylic acid (aspirine) inhibits urate excretion.
PYRIMIDINE BIOSYNTHESIS
51
Regulation of pyrimidine biosynthesis
Carbamoyl phosphate synthetase II is
a- Stimutated by PP-ribose-phosphate.
b- Inhibited by UTP and other purines.
NUCLEIC ACID
Nucleic acid are polymers of nucleotides. The nucleotides are linked
together by phosphodiester bonds between 3' –hydroxyl on the sugar of
one nucleotides and the 5'-phosphate on the sugar of the another
nucleotides. In long chains formed. One end contains the phosphate
group (5' end) and the other contains the free hydroxyl at 3' end.
52
*DNA is formed of two antiparallel double chains (strands) of
nucleotides in the form of double helix. The sugar and phosphate
molecules form the backbone outside while the bases are arranged inside.
So the bases are not part of the backbone but lie between the 2 strands.
The helix is 20µ in diameter. In crystalline DNA, the bases are separated
by 3.4 A and there is a complete turn of the helix every 10 base pair.
53
The genetic information of DNA is stored as a sequence of purine and
pyrimidine nucleotides in the DNA molecule.
Pairing rule
It was found that in DNA molecules the concentration of deoxy-
adenosine
nucleotides (adenine-ribose~P) = Deoxy-Thymidine nucleotides
(thymine-ribose
-P) , and the concentration of deoxy-guanosine nucleotides (guanine-
rihose-p)
=Deoxy-cytidine nucleotides (cytosine-ribose-P).
The pairing of bases of both strands of DNA are complementary and not
identical
always adenine is paired only with thymine and Guanine is paired
only with cytosine A------- T G------C
Shape of DNA:
Chromosomal DNA may be linear or circular DNA is complexed with
histones in eukaryotic chromosome which profoundly influence the
structure of the chromosome. Histones are small proteins rich in positive
charge amino acids (Basic amino acids as arginine and lysine). Histone
are divided into five classes H1 H2A H2B H3 and H4. Also non histone
proteins and possibly RNA affect the packing of DNA. When single
DNA is complexed with histones it forms chromatin threat.
Sense and anti-sense strands
The 2 strands of DNA molecules are anti-parallel. i.e. one strand runs in
the 5' to 3' direction and the other strand runs in the 3' to 5' direc¬tion.
This resembles 2 parallel streets each going one way but in opposite
directions. Accordingly , one strand is considered as the sense strand , the
opposite strand might be considered comple -mentary or anti-sense
strand.
54
Bonds between the bases
In double stranded DNA1 adenine is paired with thymine by 2 hydrogen
bonds and guanine is paired with cytosine by 3 hydrogen bonds. Thus G-
C bond is stronger than A -T bond. The higher the G-C content of a DNA
molecule , the more difficult is to separate the 2 strands
Functions of DNA
1) Replication of DNA (=Reproduction)
The genetic information are stored in the nucleotide sequence of DNA of
parent cell passes to daughter cell. So the new DNA is typically identical
to that of parent cell.
2) Transcription :
DNA is the source of information for the synthesis of all protein
molecules formed in the same cell. This can be done by a process called
transcription.
REPLICATION OF DNA
It is the process for transferring genetic information on DNA from
generation to generation. When a cell divides to give 2 daughter cells,
replication of DNA allows the daughter cells to have the same DNA
formation of the mother cells. Replication is semiconservative process i.e
on the two strands of DNA molecules, there is always one strand of old
DNA and the other is newly synthesized.
The 2 strands of parent DNA are separated from each other. Then each
one acts as a template upon which a new strand is formed.
Semiconservative replication
During cell division the double strands of DNA molecule are separated.
By using free nucleotides which are present in the nucleus (ATP GTP
,TTP and CTP), each strand of DNA induces the formation of a
complementary stra¬nd which is identical in structure to the one which
has separated
This reactions need a large amount of energy i.e. each nucleotide that
incorporated in the DNA molecule utilize 2 high energy phosphate.
Steps of replication
1. The parent strands unwind at a unique site (called oriC site) with the
help of an unwinding helicase enzyme, using ATP hydrolysis as a source
of energy
55
2. Another protein, a single strand binding protein binds to the unwind
DNA to prevent reannealing
3.The exosed bases on the separated strands serve as a templete for the
new strand
4. The replication is continous in one strand “leading strand” and
discontinous for the other “ lagging stand”
5.The leading strand is synthesized continously by DNA polymerase III
that can bind nucleotide only from 5' ---3' direction.
6. Because there is no DNA polymerase III that can bind nucleotide from
3' ---5' direction, The enzyme replicate the other strand by turning it on
its back (3' to 5' direction Turned on 5' to 3' direction). It is called
Okasaki fragments
7.DNA polymerase I fills the gap between the fragments by nucleotides
then DNA ligase cause ligation of the fragments to form continous strand
56
Ribonucleic acid = RNA
There are 3 types of RNA
- Messenger RNA mRNA
- Transfer RNA tRNA
-Ribosomal RNA rRNA
All are formed in the nucleus under the control of DNA and the enzyme
RNA polymerase.
RNA is differ from DNA in the following features
RNA DNA
Sugar Ribose Deoxyribose
Base Uracil Thymine
Strands Single Double
Stability Breaks down at high pH Resistant
Due to presence of 2 OH
Messanger RNA
57
Transfer RNA = tRNA:
* It is a single strand of nucleotides formed of
Bases Adenine guanine cytosine and uracil.
- Sugar : Ribose.
- Phosphate
* This single strand is arranged to form 3 loops and 2 free ends as
shown
* It is the amino acids carrying RNA. It has the shape of clove leaf
58
nucleus and pass to the cyto¬plasm. There are at least 20 species of tRNA
in every cells at lease one to each of the 20 amino acids
- So tRNA are the carriers of amino acid. tRNA has 2 different regions
the anti¬codon region and the terminal region for amino acid which has
three nucleotides at 3 terminus CCA. Amino acids attaches to 3-OH of
the terminal adenine
-The middle arm contain the anticodon region which is formed of 3 bases
complementary to a cer¬tain codon on mRNA.
So for each codon in mRNA specific tRNA will be attached which means
a specific amino acids.
RIBOSOMAL RNA = (rRNA )
*The cytoplasmic ribosomes are the site of protein synthesis i.e. They
contain enzymes needed for this process.
*Ribosomes contain only proteins and RNA (rRNA).
*rRNA are formed in the nucleus as a large precursor molecule. In the
nucleolus , this precursor will undergo alteration to be converted to
ribosome subunits.
*Each ribosome consists of 2 subunits : one about twice the size of the
other.
*The whole ribosome and each subunit have its own sedimentation rate.
This rate is measured by what is called Svedberg units
Whole ribosome Large subunit Small
subunit
59
Eukaryotes 80 S 60 S 40
S
Prokaryota 70 S 50 S
30 S
The large subunit (60 S) is the binding site for t RNA , while the small
subunit (40 S) is the binding site for mRNA.
PROTEIN BIOSYNTHESIS
Principle of protein biosynthesis
* Body proteins are of various types plasma proteins contractile proteins
of muscle all enzymes , some hormones
* Protein biosynthesis is subdivided among the various tissues.
60
nucleotides. If the genetic code is represented by two nucleotides only,
we can have
4x4 =16 code words
but the genetic code is represented by 3 nucleotides and can provide x3
4x4x4 = 64
3 codons which are called non-sense codons, do not code for any amino
acids. This leaves 61 codons for 20 amino acids i.e the number of codons
is about three times the number of amino acids. This exess in the genetic
code is explained by the fact that many amino acids are represented by
more than one genetic code.
61
acyl-¬tRNA synthetase, glycine acyl-tRNA synthetase. They help the
trans¬fer of different amino acids to tRNA as follows
-The amino acid becomes attached to the terminal nucleotide of the tRNA
(the base of which is adenine). The aminoacyl-tRNA is travelled to the
ribosomes where protein biosyn¬thesis occurs.
Transcription
Transcription is the synthesis of mRNA of a certain protein under the
control of RNA polymerase
Transcription is local. It means that it occurs in a certain area of DNA
that carries the genetic infor¬mation (=codons) about this protein(s). This
area of DNA is a genome or structural gene.
Transcription is conservative, DNA is used only as a templet and
unchanged after transcription
62
This is the signal after which the 2 strands of DNA molecule in the
genome are separated from each other.
Then under the control of RNA polymerase enzyme ,one strand acts as a
tempelate upon which mRNA is formed. This strand is termed: sense
strand. The other DNA strand is termed : antisense strand. Moreover , one
strand of DNA molecule will act as a sense strand for one gene and
antisense for another gene.
The formation and elongation of RNA molecules begins by its 5' end and
elongates towards its 3' end. This occurs anti-parrallel to the sense strand
that acts as a tempelate for it.
63
The high energy phosphate compound, ATP, GTP, CTP and UTP act as
a donner
for different nucleotides needed for the formation and elongation of
RNA. This consumes a large amounts of energy.
e.g. ATP--------------------------------AMP
RNA molecule ends at a certain site in the sense strand. This site is
termed : stop site. The newly formed RNA leaves the nucleus to the
cytoplasm (ribosomes) where protein synthesis occurs.
Summary of transcription
1. RNA polymerase holoenzyme is formed of 5 subunits 2α, 2β and
2σ. 2σ is responsible for recognition and binding of RNA polymerase to
the initiation site while the core 2α and 2β that are very rich in GC
nucleotides are responsible for elongation of RNA.
1- Binding of RNA polymerase enzyme to the promoter sequence
site.
2- The 2 strands of genome in the DNA are separated into sense and
anti-sense strands (enzyme unwinds a short stretch of DNA).
3- At the sense strand of the genome , the enzyme select the correct
ribonucleotides and catalyze the formation of RNA molecule which
begins at its 5' end.
4- Then elongation of the RNA molecule from the 5' to the 3' end
continuous antiparrallel to its tempelate.
5- The high energy phosphate compounds ATP , GTP , CTP and
U'TP act as a donners for different nucleotides needed for the formation
and elongation of RNA. This consumes a large amounts of energy.
e.g. ATP---------------- AMP
6- RNA molecule ends at a certain site in the sense strand. This site is
termed stop site.
7- The newly formed RNA leaves the nucleus to the cytoplasm
(ribosomes) where protein synthesis occurs.
64
i.e. changes of RNA after their synthesis
mRNA together with tRNA and rRNA undergo some modification after
their transcription and before they are released from the nucleus to the
cyto¬plasm. These changes may be
1- Addition of some new nucleotides (AMP) to the 3' end of the
transcripted mRNA. This addition helps the transfer of mRNA from the
nucleus to the cytoplasm.
2- Cleavage of the transcripted RNA
e.g. r RNA is synthesized as one simple long strand , which is cleaved
to give 2 fragments (40S) and (60S) so, the rRNA molecules are ready to
be incorporated into the ribosome.
a- INITIATION
* mRNA binds to 40S ribosomal subunit. This binding needs a protein
fac¬tor which is termed : initiation factor 3 (IF-3)..
65
*The aminoacyl-tRNA interacts with GTP and another protein factor that
is termed : Initiation factor 2. (IF-2) ----- complex
66
*The complete ribosome contains 2 sites for tRNA molecules. These are
P site (Peptidyl site) and A site (aminoacyl site).
*The first amino acid in the polypeptide chain (first aminoacyl
tRNA)gets attached to the P site. The subsequent aminoacyl tRNA are
attached to A site.
b- ELONGATION
The binding of the proper aminoacyl tRNA in A site depends on proper
codon recognition. Aminoacyl tRNA enters A site as follows
Aminoacyl tRNA + Elongation factor 1 (EF-1) + GTP--------- Complex
Aminoacyl tRNA
site A
The free NH2 group of amino acid 2 (aa2) attracts the COOH group of
the amino acid 1 (aa1). This leads to the transfer of peptide chain to site
A under the influence of peptidyl transferase enzyme with release of the
peptidyl tRNA.
Site P is now free. In the presence of the translocase enzyme , GTP and
elongation factor 2 , the ribosome moves in the direction of a 5'------3'
direction on mRNA to translocate the newly formed peptidyl tRNA in
site P,
67
* Site A is now free. It occupies by a third aminoacyl~tRNA
according to codon-anticodon and the process is repeated. The
polypeptide chain is thus increased each time by one amino acid.
C- TERMINATION
After many cycles of elongation , the terminating codon of mRNA is
reached at site A. There is no tRNA with anticodon to recognize the
terminating codon of mRNA. Releasing factor hydrolyzes the bond
between the peptide and tRNA at site P , and release the protein
molecule.
POST-TRANSLATIONAL REGULATION
Some proteins after being translated undergo additional changes to be
active. These changes may be
1- Addition of carbohydrate
To form glycoprotein e.g. Immunoglobulin.
68
gene(s) of certain protein(s). Each component is formed of a number of
nucleotide
in DNA molecule. These components are
1- Promoter.
2- Operator.
3- Structural gene(s).
4- Regulator gene.
Structural
genes
Regulator gene Promotor Operator
Inducer is a substance that can bind with the repressor -------- Opera¬tor
is free ------------- Promotor together with RNA-polymerase enzyme will
initiate transcription of structural genes to synthesize proteins.
Examples of inducers and repressors
1- Clucocorticoids act as inducers for the synthesis of gluconeogenic
enzv¬mes.
2- Insulin acts as an inducer for glycolytic enzymes and as repressor
for gluconeogenic enzymes.
69
70