CLASSIFICATION OF PROTEINS

Disclaimer: we based all our information on the book but the information in the book is
somewhat short so we provided additional information especially with the examples to
explain further and able to understand more about the topic.

PROTEINS
- polymer of L-alpha Amino Acid.
- Proteins are necklaces of amino acids - long chain molecules.
- As enzymes, they are the driving force behind all the biochemical reactions which
make biology work.
- As structural elements, they are the main constituents of our bones, muscles, hair,
skin, and blood vessels.
- As antibodies, they recognize invading elements and allow the immune system to get
rid of the unwanted invaders.

CLASSIFIED BASED ON biological importance, chemical differences, shape and activities

I. BIOLOGICAL IMPORTANCE
A. Enzymes
● biggest class of proteins, metabolic proteins, and biological catalyst in
biochemical reactions.
● Examples:
○ Trypsin
■ produced in the pancreas and secreted into the small
intestine, where it helps break down proteins into
smaller peptides
○ Amylase
■ assists in the digestion of carbohydrates
■ breaks down complex carbohydrates such as starch
and glycogen into simpler sugars like maltose and
glucose, which can be easily absorbed by the body.
● Both trypsin and amylase illustrate the characteristic properties of
enzymes. They are highly specific, acting on particular substrates
(proteins for trypsin and carbohydrates for amylase) and
catalysing specific chemical reactions.

B. Structural Proteins
● Structural proteins are a class of proteins that provide support and
stability to cells, tissues, and organisms by forming the structural
framework.
● Examples:
○ Collagen
■ most abundant protein in the human body
■ major component of connective tissues, such as skin,
tendons, ligaments, and bones
■ provide structural support and help to maintain the
shape and integrity of tissues.
 ○ Elastin
■ imparts elasticity to tissues, allowing them to stretch
and recoil
■ particularly abundant in tissues like the lungs, blood
vessels, and skin, where elasticity is required.

C. Contractile Proteins
● Contractile proteins are a class of proteins that are involved in muscle
contraction, enabling movement and various physiological processes
in living organisms.
● The two primary contractile proteins found in muscle tissue are actin
and myosin. They work together in a coordinated manner to facilitate
muscle contraction
○ Actin
■ globular protein that forms thin filaments in muscle
fibres. These filaments are composed of polymerized
actin molecules
○ Myosin
■ motor protein that forms thick filaments in muscle
fibers.
D. Transport proteins
● facilitate the movement of molecules across cellular membranes
or within the bloodstream
● Examples:
○ Hemoglobin - carries oxygen molecules and carbon dioxide in
blood
○ Myoglobin - stores oxygen in muscle
E. Hormones
● They regulate metabolic and cellular activities in cells and tissue.
● Not all hormones are protein in nature
● Examples:
○ Insulin
■ regulates or controls carbohydrate metabolism
■ Lowers the effect of excess free glucose
○ Growth hormones
■ stimulates growth of bones

F. Genetic Proteins
● also known as nucleoproteins or DNA-binding proteins
● interact with DNA molecules
● play crucial roles in various aspects of gene regulation, DNA
replication, repair, recombination, and transcription
● Examples:
○ Histones
■ Histones are proteins around which DNA is wrapped to
form a structure called chromatin.
○ DNA Polymerase
 ■ DNA polymerases are enzymes responsible for
synthesizing new DNA strands during DNA replication
and repair.

G. Defensive proteins/ Protective proteins

● serve as defense mechanisms in living organisms, providing
protection against various pathogens, toxins, and other harmful
substances.
● Examples:
○ Fibrinogen - precursor of fibrin in blood clotting
○ Thrombin - involved in clotting mechanism

H. Storage proteins
● Storage proteins are a class of proteins that serve as reservoirs of
amino acids and other essential nutrients in living organisms.
● Examples:
○ Casein - milk protein
○ Ovalbumin - egg white protein
○ Zein - seed protein
○ Gliadin - seed protein of wheat
○ Ferritin - stores iron
I. Toxins
● These toxins are usually proteins or peptides that interfere with
normal cellular processes and disrupt physiological functions.
● Examples:
○ Snake venom - enzymes that hydrolyses phosphoglycerides
○ Ricin - toxic protein of cottonseed
○ Clostridium botulinum toxin - causes bacterial food poisoning

II. CHEMICAL DIFFERENCES

A. Simple Proteins
● Basically composed of amino acids only.
Examples:
a. Albumin
● constitute the most important group of simple proteins
● water soluble; may be precipitated from solutions of high salt
concentration. They are coagulated by heat.
● E.g: egg albumin, serum albumin. Lactalbumin.
b. Globulin
● insoluble in salt free water; soluble in dilute salt solution but when
concentration of salt is brought to 30-50 percent saturation they are
insoluble.
● E.g globulin of blood serum, egg white, ovoglobulin, fibrinogen of
blood.
c. Histones
● hydrolysis products of nucleoproteins
● small molecular weight proteins which are basic
● soluble in acid solutions
 ● usually form complex with nucleic acid
d. Albuminoids
● insoluble in water and all neutral solvents
● are not coagulated by heat
● E.g keratin of hair, skin, hofs and nails collagen of the bone and
cartilage elastin of the yellow elastic fibres of the connective tissues.

B. Conjugated proteins
● Simple proteins linked to non-protein groups. (prosthetic groups)
● Hydrolysis products are amino acids + non-protein groups
Examples:
● chromoproteins - pigments (hemoglobin)
○ coloured prosthetic groups
○ Hemoglobin- A common example is haemoglobin, which
contains a heme cofactor (Heme, red)
○ Other example; Flavoproteins (Riboflavin, yellow)

● mucoprotein - carbohydrate (mucin)

○ any of various complex conjugated proteins (such as mucins)
that contain polysaccharides
○ occur in body fluids and tissues
○ composed of o-linked carbohydrates as well as highly
glycosylated proteins, which are held together by disulfide
bond

● glycoprotein - carbohydrate (globulin)

○ proteins combined with carbohydrates

Note: DIFFERENCE BETWEEN MUCOPROTEIN AND GLYCOPROTEIN

The prosthetic groups in mucoproteins and glycoproteins are oligosaccharides
(carbohydrates consisting of a small number of simple sugar molecules) usually containing
from four to 12 sugar molecules; the most common sugars are galactose, mannose,
glucosamine, and galactosamine.

mucoprotein glycoprotein

type of conjugated a type of protein in which

protein that contains carbohydrate
polysaccharides molecules are attached
attached to them to the peptidoglycan
chain

● nucleoprotein - nucleic acid (virus)

○ prosthetic group present in nucleoproteins is nucleic acids
○ nucleoproteins are proteins of cell nuclei and are main
component of chromatin
○ Histones -
 ● lecithoprotein - phospholipids (lipoprotein)
○ water-soluble, conjugated protein in which the prosthetic group
is a lipid
○ transport lipids in the blood and lymph from the small intestine
to the liver and from the liver to fat deposits.
● flavoprotein - flavin nucleotide (a.a. oxidase)
○ ubiquitous proteins that use flavins as prosthetic groups
○ involved in oxidation reaction in cells

● metalloprotein - metal (ferritin)

○ contain a heme prosthetic group (Fe-protoporphyrin XI) that is
able to form reversible binding with gaseous ligands as oxygen
(O2), carbon dioxide (CO2), or nitric oxide (NO).
○ responsible for catalyzing some of the most difficult and yet
important functions, including photosynthesis, respiration,
water oxidation, molecular oxygen reduction, and nitrogen
fixation.

● phosphoprotein - phosphate (casein)

○ contain phosphorus
○ Casein of milk and vitellin of egg yolk are examples

C. Derived proteins
● These proteins are the derivatives of either simple or complex proteins
resulting from the action of heat, enzymes and chemicals.
● Products of hydrolysis of simple and conjugated proteins.
● Coagulated proteins that are formed by heating or treatment of
alcohol, aids, bases.
● They are classified as primary derived protein and secondary
derived protein.

1. Primary Derived Proteins

- No change in molecular weight after heating
Examples:
a. Proteans:
■ Obtained as a first product after the action of acid or enzymes or
water on protein.
■ They are insoluble in water.
■ Eg. Edestan, myosin
b. Metaprotein:
■ They are produced by further action of acid or alkali on protein
at 30-60°C.
■ They are water insoluble but soluble in dil acid or alkali.
■ Also known as Infraprotein.
■ Eg. Curd
c. Coagulated protein:
■ They are produced by the action of heat or alcohol on protein.
■ They are insoluble in water.
 ■ Eg. Coagulated egg
2. Secondary Derived Proteins
● Products of more extensive hydrolysis
● The derived protein in which size of original protein is altered
● Examples:
○ Proteoses
■ produced by the action of dilute acid or digestive
enzymes when the hydrolysis proceeds beyond the
level of metaprotein
■ soluble in water
■ not coagulated by heat.
■ Eg. Albumose, Globulose

III. SHAPE
A. Fibrous protein:
● structural proteins of animals and include the proteins of hair, skin,
nails and connective tissues.
● Arranged in parallel along a single axis.
● Insoluble in water and generally resistant to denaturation.
● They are elongated or fiber-like proteins.
● Axial ratio (length: breadth ratio) is more than 10
● They are static in nature with simple structure.
● They have less biological functions
● E.g. collagen, keratin, myosin, fibrin
B. Globular
● arranged in tight, compact spherical shape
● Soluble in water
● they are proteins of meat, milk and eggs
● more sensitive to denaturation than fibrous proteins
● They are spherical or globular in shape.
● Axial ratio is always less than 10
● They are dynamic in nature (can flow or move) with higher degree of
complexity in structure.
● They have variety of biological functions

IV. ACTIVITIES
A. Active Proteins
● Have the ability to bind with other molecules.
B. Passive Proteins
● Do not have the ability to bind with other molecules to perform
their functions