Proteins are the most versatile macromolecules in living systems and serve crucial functions in essentially all biological processes. They function as catalysts, they transport and store other molecules such as oxygen, they provide mechanical support and immune protection, they generate movement, they transmit nerve impulses, and they control growth and differentiation.
Amino acids are relatively simple molecules containing both an amine group and an acid group.
Classification based on the source of protein molecules
The proteins have been traditionally divided into two well-defined groups: animal proteins and plant proteins.
Animal proteins are the proteins derived from animal sources such as eggs, milk, meat and fish. They are usually called higher-quality proteins because they contain (and hence supply) adequate amounts of all the essential amino acids.
On the other hand, plant proteins are called lower-quality proteins since they have a low content (limiting amount) of one or more of the essential amino acids.
Classification based on the shape of protein molecule
On the basis of the shape of protein molecule, the proteins have been grouped under two categories: globular and fibrous
1. Globular or Corpuscular Proteins
These have an axial ratio (length : width) of less than 10 (usually not over 3 or 4) and, henceforth, possess a relatively spherical or ovoid shape. These are usually soluble in water or in aqueous media containing acids, bases, salts or alcohol, and diffuse readily. As a class, globular proteins are more complex in conformation than fibrous proteins, have a far greater variety of biological functions and are dynamic rather than static in their activities. Tertiary and quaternary structures are usually associated with this class of proteins. Nearly all enzymes are globular proteins, as are protein hormones, blood transport proteins, antibodies and nutrient storage proteins.
2. Fibrous or Fibrillar Proteins
These have axial ratios greater than 10 and, henceforth, resemble long ribbons or fibres in shape. These are mainly of animal origin and are insoluble in all common solvents such as water, dilute acids, alkalies and salts and also in organic solvents. Most fibrous proteins serve in a structural or protective role.
Classification based on composition and solubility (Most Accepted Classification)
This divides proteins into 3 types:
1. Simple Proteins or Holoproteins
These are of globular type except for scleroproteins which are fibrous in nature. This group includes proteins containing only amino acids, as structural components. On decomposition with acids, these liberate the constituent amino acids.
These are further classified mainly on their solubility basis as follows:
1. Protamines and histones
6. Scleroproteins or Albuminoids
2. Conjugated or Complex Proteins or Heteroproteins
These are also of globular type except for the pigment in chicken feathers which is probably of fibrous nature. These are the proteins linked with a separable nonprotein portion called prosthetic group. The prosthetic group may be either a metal or a compound.
On decomposition with acids, these liberate the constituent amino acids as well as the prosthetic group. Their further classification is based on the nature of the prosthetic group present. The various divisions are:
3. Derived Proteins
These are derivatives of proteins resulting from the action of heat, enzymes or chemical reagents. This group also includes the artificially-produced polypeptides.
I. Primary derived proteins
These are derivatives of proteins in which the size of protein molecule is not altered materially.
1. Proteans: Insoluble in water; appear as first product produced by the action of acids, enzymes or water on proteins. e.g., edestan derived from edestin and myosan derived from myosin.
2. Metaproteins or Infraproteins: Insoluble in water but soluble in dilute acids or alkalies; produced by further action of acid or alkali on proteins at about 30–60°C. e.g., acid and alkali metaproteins.
3. Coagulated Proteins: Insoluble in water; produced by the action of heat or alcohol on proteins. e.g., coagulated eggwhite.
II. Secondary derived proteins
These are derivatives of proteins in which the hydrolysis has certainly occurred. The molecules are, as a rule, smaller than the original proteins.
1. Proteoses: Soluble in water; coagulable by heat; produced when hydrolysis proceeds beyond the level of metaproteins; primary proteoses are salted out by half saturation with (NH4)2.SO4 and precipitated by HNO3 and picric acid; secondary proteoses are salted out only by complete saturation with (NH4)2.SO4 but are not precipitated by HNO3 or picric acid. e.g., albumose from albumin; globulose from globulin.
2. Peptones: Soluble in water; noncoagulable by heat; produced by the action of dilute acids or enzymes when hydrolysis proceeds beyond proteoses; neither salted out by (NH4)2SO4 nor precipitated by HNO3 or picric acid.
3. Polypeptides: These are combinations of two or more amino acid units. In fact, the proteins are essentially long chain polypeptides
Classification based on biological function
- Enzyme proteins. Examples: Urease, Amylase, Catalase, etc.
- Structural proteins. Examples: Collagen, Elastin, Fibroin, etc.
- Transport or carrier proteins. Examples: Hemoglobin, Lipoproteins
- Nutrient and storage proteins. Examples: Casein, Ferritin, etc.
- Contractile or motile proteins. Examples: Actin, Myosin, etc.
- Defense proteins. Examples: Fibrinogen, Thrombin, etc.
- Regulatory proteins. Examples: Insulin, G-Proteins, etc.
- Toxic proteins. Examples: Snake venom, Bacterial toxins, Ricin, etc.