Carbohydrates

Part 1
Introduction
 Organisms rely on the oxidation of complex
organic compounds to obtain energy
 Three general types of such compounds are:
 Carbohydrates (CHO)
 Amino acids
 And lipids
 CHO are the primary source of energy for
brain, erythrocytes and retinal cells
 Stored primarily as liver & muscle glycogen

M. Zaharna Clin. Chem. 2009

Carbohydrates: CHO
 Compounds containing C, H, O
 General formula (CH2O)n
 All CHO contain C=O and –OH functional groups
 There are some derivatives of this formula
(addition of phosphates, amines…)
 Classification of CHO is based on four different
properties:
1. The size of the base carbon chain
2. The location of the CO functional group.
3. The number of sugar units
4. The stereochemistry of the compound.

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 Can be classified based on the number of
carbons in the molecule
 Trioses ( 3 Carbons)
 Tetroses
 Pentoses
 And hexoses
 The smallest CHO is
glyceraldehyde ( 3 Carbon)

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 Hydrates of aldehyde or ketone derivatives
 Aldose form – aldehyde as functional group
 Ketose form – ketone as functional group

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Sterochemistry
 Mirror image forms
 D = right side OH, L = left side OH
 D & L designations are based on the
configuration about the single asymmetric C

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 Classification based on the number of
sugar units in the chain
1. Monosaccharide
2. Disaccharide (2 sugars linked together)
3. Oligosaccharide (2 – 10 linked sugars)
4. Polysaccharide (Long sugar chains)

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Monosaccharides
 Simplest sugars; cannot be broken down into any simpler
sugar
 3 carbons = triose, 4 carbons = tetraose, 5 carbons =
pentose, & 6 carbons = hexose
 Important pentose (5 carbon) sugars include ribose and 2-
deoxyribose

M. Zaharna Clin. Chem. 2009

Monosaccharides
 Example: glyceraldehydes (3 carbon
compound- smallest CHO)
 D- and L- form used to describe possible
isomers of glucose. Ex: ( D-glucose and L-
glucose.)

M. Zaharna Clin. Chem. 2009

Disaccharides
 Formed from two monosaccharide with the
production water.
 Most common form is sucrose (table sugar), which
is glucose and fructose
 Other forms include:
 Lactose (glucose and galatose)
 and maltose (malt product)

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Common Disaccharides

Glucose + Glucose

Glucose + Galactose

Glucose + Fructose

Sucrose ( table sugar )

M. Zaharna Clin. Chem. 2009
Polysaccharides
 Plants (cellulose); not digested by humans.
 Starch: principle CHO (polysaccharide)
storage product of plants
 Glycogen: principle CHO storage product in
animal.
 Glycoside linkage of CHO involves many
CHO-
 Formed by the combination of
monosaccharide.

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Glucose Metabolism
 Glucose is a primary source of energy.
 Various tissues and muscles throughout the
body including extracellular fluid depend on
glucose for energy.
 If glucose levels fall below certain levels the
nervous tissue lose its primary energy source
and is incapable of maintaining normal
function.

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Fate of glucose
 CHO is digested (starch and glycogen).
 Amylase digest the no absorbable forms of
CHO to dextrin and disaccharide which are
hydrolyzed to monosaccharide by maltose.
 Maltose is an enzyme released by intestinal
mucosa.
 Sucrase hydrolyze sucrose to glucose & fructose
 Lactase: hydrolyze lactose to glucose & galatose.

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Lactose intolerance
 Lactose intolerance: due to a deficiency of
lactase enzyme on or in the intestinal lumens,
which is need to metabolize lactose.
 Results in an accumulation of lactose in
stomach as waste lactic acid- causing the
stomach upset and discomfort.

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Glucose metabolism
 Disaccharides are converted into
monosaccharide – absorbed by the stomach
transported to the liver by the hepatic portal
venous blood supply.
 Glucose is the only CHO to be directly use for
energy or stored as glycogen.
 Others have to be broken down then utilized
for energy and storage.

M. Zaharna Clin. Chem. 2009

Glucose metabolism

 After glucose is absorbed it can go into one

of three metabolic pathways based on
 availability of substrate and
 nutritional status of cell.
 Ultimate goal to convert glucose to CO2 and
H2O.
 Requires ATP and ADP, O2 in the final step.
 NADH acts as intermediate – ATP is gained

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Glucose metabolism
 1st step in all pathways is Glucose is
converted to glucose -6 phosphate using
ATP- catalyzed by hexokinase.
 Glucose-6- phosphate enters the pathways:
1. Embden-Meyerhof (glucose → pyruvate)
2. Hexose Monophosphate
3. Glucogenesis (storage of glucose as glycogen)

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Pathways in glucose metabolism
 Glycolysis
 Breakdown of glucose for energy production

 Glycogenesis
 Excess glucose is converted and stored as glycogen

 High concentrations of glycogen in liver and skeletal muscle

 Glycogen is a quickly accessible storage form of glucose

 Glycogenolysis
 Breakdown of glycogen into glucose

 Glycogenolysis occurs when plasma glucose is decreased

 Occurs quickly if additional glucose is needed

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Pathways in glucose metabolism
 Gluconeogenesis
 Conversion of non-carbohydrate carbon substrates to glucose
 Gluconeogenesis takes place mainly in the liver

 Lipogenesis
 Conversion of carbohydrates into fatty acids
 Fat is another energy storage form, but not as quickly
accessible as glycogen

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Hormone regulation
 Hormones effect the entry of glucose into cells and
fate in the cells within the body.
 As needed hormones regulate release of glucose.
 after meals glucose increase, without hormones to shut off
secretion, the mechanism of glucose release would
steadily increase.
 Hormones work together to meet 3 requirements:
1. Steady supply of glucose.
2. Store excess glucose
3. Use stored glucose as needed

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Insulin
 Primary hormone responsible for the
entry of glucose in the cell.
 Synthesized in the beta cells of islets
of langerhans in the pancreas.
 Insulin release cause increase
movement of glucose into the cells and
increase glucose metabolism
 Is the only hormone that decreases
glucose levels and is referred as a
hypoglycemic agent.
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Glucagon
 Peptide hormone that is synthesized by the
alpha cells of the islets cells of the pancreas
and released during stress and fasting states.
 Released in response to decreased body
glucose.
 Main function is to:
 increase hepatic glycogenolysis,
 and increase gluconeogenesis.
 Hyperglycemic agent

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M. Zaharna Clin. Chem. 2009
Epinephrine (adrenaline)
 Hormone produced by the adrenal gland
 Increases plasma glucose by:
 inhibiting insulin secretion,
 increasing glycogenolysis
 and promotes lipolysis.
 Release during times of stress

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Glucocorticoids
 Cortisol is released when stimulated by
ACTH.
 Cortisol increases plasma glucose by:
 increasing gluconeogenesis,
 increase glycogen breakdown in the liver
 and lipolysis.
 Insulin antagonist

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Thyroxine
 The thyroid gland releases
thyroxine.
 Increases glucose levels by:
 increasing glycogenolysis,
 gluconeogenesis
 And intestinal absorption of
glucose.

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Somatostatin
 Produced by the delta cells of the lslets of
langerhans of the pancreas.
 The inhibition of insulin, glycagon
 Therefore only minor overall effect

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Hyperglycemia
 Increased in plasma glucose levels ( > 110
mg/dl)
 During a hyperglycemia state, insulin is
secreted by the beta cells of the pancreatic
islets of langerhans.
 Insulin enhances membrane permeability to cells
in the liver, muscle, and adipose tissue.
 Due to hormone imbalance

M. Zaharna Clin. Chem. 2009

M. Zaharna Clin. Chem. 2009
Diabetes Mellitus
 Metabolic diseases charaterized by
hyperglycemia resulting from defect in insulin
secretion, insulin action or both.
 Two major types: (in 1979)
 Type I, (insulin dependent) and Type 2, (non
insulin dependent)
 1995: further categories by WHO:
 Type 1 diabetes, type 2 diabetes, other specific
types and gestation diabetes mellitus.

M. Zaharna Clin. Chem. 2009

Type 1 diabetes
 Deficiency or loss of insulin production due to beta
cell destruction.
 Commonly occurs in children (juvenile diabetes)
 Genetics play a minimal role, can be due to
exposure to environmental substances or viruses.
 Clinical picture: less than 20 yrs old, polyuria, weight
loss, increased levels
 Treatment: insulin

M. Zaharna Clin. Chem. 2009

Type 2 diabetes mellitus

 Due insulin resistance and relative insulin

deficiency .
 Seen adults greater than 20 yrs old, most
common adult form.
 Genetics play a larger role in addition to diet
and genetics.

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Other specific types
 Secondary condition, genetic defect in beta
cell function or insulin action, pancreatic
disease, disease of endocrine origin, drug or
chemical induced.
 Characteristics of the disease depends on
the primary disorder.

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Gestational diabetes mellitus
 Glucose intolerance that is induced by
pregnancy
 Caused by metabolic and hormonal changes
related to the pregnancy.
 Glucose tolerance usually returns to normal
after delivery.

M. Zaharna Clin. Chem. 2009