Classification Of Proteins

Based on their function proteins are of different types.

Classification Of Proteins Based On Their Function

Antibodies are specialized proteins that circulate in the bloodstream which help in the defence against foreign invaders like antigens, microbes, etc. They immobilize the invaders so they can be destroyed by the white blood cells.
Contractile proteins help in the contraction and movement of muscles. These proteins are myosin and actin. Actin helps with muscle contraction and cellular movements during cell division. Myosin supplies energy to actin to help it perform its functions.
Enzymes are also proteins that catalyze biological reactions. The absence of enzymes leads to disorders as in the case of lactose intolerance caused by the deficiency of lactase enzymes. Digestive enzymes like amylase, lipase, protease, etc are also common examples.
Hormonal proteins play the role of messengers that coordinate the various functions inside the body. Eg. Insulin controls blood sugar levels, somatropin is a growth hormone that promotes protein synthesis in muscles, and oxytocin is the stimulating hormone that causes contraction during childbirth.
Secretory proteins such as fibroin produce the spider web or cocoon of silkworms.
Structural proteins support the body parts that they integrate into. Examples are collagen seen in the skin, keratin in hair, and elastin in tendons and ligaments.
Storage proteins serve as reserves for various amino acids that are required for other functions. Albumin in egg white, casein in milk, ferritin that stores iron in haemoglobin, etc are perfect examples.
Transport proteins carry other molecules from one place to another. Haemoglobin is the best example that carries oxygen to all parts of the body. Cytochrome in the electron transport chain is another example that carries electrons.
Protective proteins function as a defense mechanism in one way or the other. They are of different types. Plant toxins and snake venoms are common examples that protect the organisms from predators. Blood-clotting proteins in the vascular system protect it from injuries. Antibodies and antibiotics protect from infections and diseases.

Protein Classification Based On Chemical Composition And Solubility

Based on their chemical composition there are two types of proteins- simple and complex.

1. Simple Proteins

Simple proteins are otherwise called homoproteins. They are a result of the bonding of only amino acids. There will be no additions from other elements, compounds, etc., in the peptide bond structure. Simple proteins are soluble in water. Examples of simple proteins are collagen, keratin, and albumin.

2. Conjugated Proteins

Conjugated proteins are hetero-proteins that have amino acids and additional non-protein parts. They are insoluble in water. The non-protein part may be formed by carbohydrates, phosphates, or plastids. These are called glycoproteins, phosphoproteins, chromoproteins, etc. The subdivisions of conjugated proteins are,

Glycoproteins contain one or more units of carbohydrates in the peptide backbone. Usually, there are 15020 units of carbohydrates such as mannose, arabinose, galactose, fucose, glucose, N Acetylglucosamine- a monosaccharide derivative of glucose, N-acetylneuraminic acid in mammalian cells, etc. Examples of glycoproteins include glycophorin in the erythrocyte membrane, fibronectin in the extracellular matrix, blood plasma proteins, etc.
Phosphoproteins are a modified form of protein that has a phosphate group attached either to the serine or threonine residues. Their function is to provide structural support and reserve. Examples are dentin for support in teeth and caseins or egg yolk phosvitin for storage.
Chromoproteins contain a coloured prosthetic group attached to them. -They are proteins that contain coloured prosthetic groups. Haemoglobin and myoglobin are the most common examples that have one and four heme groups attached to them. Another example is chlorophylls bound to a porphyrin ring with a central magnesium atom, rhodopsin in the retina functions as a light receptor.
Nucleoproteins are the histones that form a prosthetic group in nucleic acids. They are an integral part of chromatin.
Lipoprotein has a lipid molecule attached to it such as cholesterol and phospholipids.
Metalloprotein has metals bound to the proteins. Ceruloplasmin in the liver that carries copper to various parts of the body. Insulin contains zinc, kinase has magnesium, etc. There also are iron-containing proteins in the form of heme or a free state as in ferritin, hemosiderin, and transferrin.

Solubility of Proteins in Water

There are hydrophobic and hydrophilic proteins that are insoluble and soluble in water. This affinity to water molecules is the base of the secondary structure formation of proteins. The static form of proteins is more soluble in water than those with helices.

When it comes to the tertiary structure, they are formed in such a way that the hydrophobic amino acids are turned towards the core while the hydrophilic residues are moved towards the periphery.

At the same time, the solubility of proteins shows an increase in the presence of salt in the solution. This solubility increases up to a level and then decreases as the salt concentration is increased further. This increase in solubility is called salting in and the later decrease is called salting out.

Protein Classification Based On Shape

Based on their shape, proteins are of two types- fibrous and globular. Each of these is again subdivided into various groups.

A. Fibrous proteins

Fibrous proteins are mainly mechanical and structural in function. They support the cell and maintain the shape of the organisms. They are insoluble in water and are made of hydrophobic amino acids which facilitates their packaging as a complex structure.

The polypeptides of these proteins form sheets or long filaments. Their secondary structure will be a repeating form of either of these forms. These proteins in invertebrates help their body be flexible and provide strength and protection as well. Based on their functions, there are different types of fibrous proteins.

Collagen

There are the most abundant forms of proteins in living beings that comprise a major portion of protein in organisms. It is a structural protein that has a parallel and helix form. They form a cable-like form to provide strength to the structure. It is mainly seen in the skin and internal organs of organisms such as in the connective tissue, external scaffoldings of organisms, etc.

They are part of the matrix of bones and tendons. Cartilage is a form of this protein and is also seen in the cornea. Present in various tissues, it functions in different ways to give strength and stability to the organism. In the cornea, it is crystalline and transparent while in the skin it forms multidirectional, intertwined fibers to support the tensile strength.

α-Keratins

This type of protein forms the nonliving parts such as nails, claws, horns, wool, beaks, hair, hooves, and mostly the larger portion of the outer skin layer. The flexibility and stiffness of these parts are due to the presence of disulfide bonds in the protein. The number of such bonds makes the protein flexible or stiff. More number of disulfide bonds make the protein more rigid while less number of such bonds make it soft and more flexible. Wool has less number of disulfide bonds while nails and claws are rich in disulfide bonds.

Elastins

Elastin is responsible for the skin elasticity and that of blood vessels. They have a randomly coiled structure to facilitate this function..

Fibroin

These are proteins secreted by silkworms and spiders that help them form a weblike structure. They have a semi-crystalline structure. In silkworms, the fibroin protein has an outer layer of another protein called sericin which helps them make the cocoon.

B. Globular Proteins

Globular proteins are spherical in shape. They have a more compact and complex structure than the fibrous proteins. They form tertiary, quaternary, and other structures. These proteins are usually soluble in water and they are part of various biological membranes. Their functions include regulation of biological processes, transportation, protection, etc. Examples are,

Enzymes, insulin
Cell receptors such as neurotransmitters, hormones
Immunoglobulins
Membrane transporters
Transport proteins such as hemoglobin, myoglobins, etc

Denaturation of Proteins

As strong as the bonds inside the proteins are, they are vulnerable to certain conditions that can denature these molecules. The denaturation happens in their 3-dimensional conformation which could degrade them to their tertiary, secondary, or even primary form of just a polypeptide.

Such changes may be reversible or irreversible. These changes can cause the proteins to lose their biological activity as well. The common protein denaturing agents are of two types