The name protein (proteios Greek  = pre-eminent or first) was first suggested, in 1838, by a Swedish chemist Berzelius to a Dutch chemist Mulder, who referred it to the complex organic nitrogenous substances found in the cells of the living beings. They are naturally occurring nitrogenous polymers of different alpha-amino acids linked by peptide (—CONH) linkage. On hydrolysis, they give a mixture of alpha-amino acids. Thus proteins are biopolymers of alpha-amino acids. They contain carbon, hydrogen, nitrogen, sulphur, and oxygen as constituent elements. They may contain cobalt, manganese, zinc, iron, copper, etc.

They are present in animals as well as plants. In the animal kingdom, it occurs in forms such as silk, wool, hair, nail, skin, haemoglobin of blood, and blood plasma. In the plant kingdom, it occurs in high concentration in seeds. Their sources are pulses, milk, eggs, fish, meat, etc. They are important because they regulate metabolic processes. They are essential constituents of all living matter.

Classification of Proteins:

Classification on the Basis of Composition:

Simple proteins:

They on acid hydrolysis give only alpha-amino acids. e.g.

• Albumins: egg albumin, Serum albumin
• Globulins: Tissue, vegetable,  and Serum globulin
• Glutenins: Glutenin in wheat.
• Protamines: Occur in the nucleic acid.

Conjugated proteins:

They on hydrolysis (acids or alkalies or enzymes) give alpha-amino acids and non-protein group. e.g.

• Glycoproteins: Contain carbohydrate as a prosthetic group. e.g. egg white, mucin of saliva
• Nucleoproteins: Contain nucleic acid as a prosthetic group. e.g. components of viruses, chromosomes, and ribosome structures.
• Chromoproteins: Contain chlorophyll as a prosthetic group. e.g. Haemoglobin
• Phosphoproteins: Contain Phosphoric acid as a prosthetic group. e.g. Casein of milk and vielline of egg yolk.
• Lipoproteins: Contain fats as a prosthetic group. e.g. membrane structure, lipids transported in the blood.
• Flavoprotein: Contain flavine Adenine Dinucleotide (FAD) as a prosthetic group. This protein is important in the electron transport chain in respiration.
• Metal-protein: Contain metal as a prosthetic group. e.g. the protein in plants nitrate reductase, which converts nitrates into nitrites.

Derived proteins:

Natural proteins undergo structural change because of heat, chemical reagents, enzymes, acids, or alkalies and form degraded products of proteins (derived).

The flow is Proteins → Proteoses → Peptones → Polypeptides → Simple peptides → amino acids

Classification Based on Structure:

Globular proteins:

• They have a globular or elliptical shape. They are also called as spheroproteins.
• They are soluble in water, acids, and bases. They form a colloidal solution with water.
• They are made up of not only primary, secondary but also tertiary and occasionally quaternary structures.
• They form enzymes, antibodies, and some hormones (insulin), etc. They are needed for the formation of chemical messengers like hormones in the body. They are essential for the formation of transporters of other particles through the membrane.
• e.g. egg albumin (egg white), Casein in milk

Fibrous proteins:

• They have a fiber-like structure. They are also called as scleroproteins.
• They are elongated strand-like structures and are usually present in the form of rods or wires.
• They are insoluble in water, weak acids, and weak bases but soluble in strong acids and alkalis.
• They have primary and secondary structures. They are made up of a single unit or structure which is repeated multiple times.
• They perform a structural function in the cell. They are needed for the formation of tough structures like connective tissue, tendons, and fibers of the muscle.
• e.g. collagen, elastin, keratin of hair, Nails, horn, feathers. fibroin of silk.

Classification Based on Functions:

Enzymic Proteins.

• They are the most varied and most highly specialized proteins and shows catalytic activity. Almost all enzymes are globular proteins.
• Enzymes catalyze a variety of reactions.
• Examples: Urease, amylase, catalase, cytochrome C, alcohol dehydrogenase

Structural Proteins.

They are usually inert to biochemical reactions. They maintain the native form and position of the organs. The cell wall and primary fibrous constituents of the cell have structural proteins.

• Collagen: It is found in connective tissue such as tendons, cartilage, a matrix of bones, and the cornea of the eye. Leather is almost pure collagen.
• Elastin: It is found in ligaments. It is capable of stretching in two dimensions.
• Keratin: It constitutes almost the entire dry weight of hair, wool, feathers, nails, claws, quills, scales, horns, hooves, tortoiseshell, and much of the outer layer of skin.
• Fibroin: It is the major component of silk fibres and spider webs.
• Resilin: The wing hinges of some insects are made of resilin, which has nearly perfect elastic properties.

Transport or Carrier Proteins:

• Certain proteins, especially in animals, are involved in the transport of many essential biological factors to various parts of the organisms.
• Hemoglobin of erythrocytes carries oxygen to tissues. The blood plasma contains lipoproteins, which carry lipids from the liver to other organs. Ceruloplasmin transports copper in the blood.

Nutrient and Storage Proteins:

Ovalbumin is the major protein of egg white. The milk protein, casein stores amino acids. The seeds of many plants store nutrient proteins, required for the growth of the germinating seedlings. Ferritin, found in some bacteria and in plant and animal tissues, stores iron.

Contractile or Motile Proteins:

• They give an ability to contract, move about, and change shape to cells. Actin and myosin function in the contractile system of skeletal muscle and also in many nonmuscle cells.
• Microtubules are built up of Tubulin.

Defense Proteins:

• They defend organism against invasion by other species or protect them from injury.
• Immunity: The antibodies (or immunoglobulins), the specialized proteins made by the lymphocytes of vertebrates, can precipitate or neutralize invading bacteria, viruses or foreign proteins from another species.
• Blood Clotting: Fibrinogen and thrombin, although enzymic, are blood-clotting proteins that prevent loss of blood when the vascular system is injured.

Regulatory Proteins:

• They regulate the cellular or physiological activity. They are called hormones.
• For example, insulin regulates sugar metabolism and growth hormone which is required for bone growth.
• The cellular response to many hormonal signals is often mediated by a class of GTP-binding proteins called G-proteins.
• Some of them bind to DNA and regulate the biosynthesis of enzymes and RNA molecules involved in cell division.

Toxic proteins.

Snake venom, bacterial toxins, and toxic plant proteins are toxic. They have defensive functions.

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