Classification of Proteins

Proteins are classified on the basis of their composition, shape, and function. According to physicochemical properties, proteins are classified as simple proteins and conjugated proteins.

Simple Proteins:

Simple proteins are naturally occurring proteins. Upon hydrolysis simple proteins produce only amino acids. Following are some types of simple proteins.

• a) Globulins:

These proteins are found both in plants and animals. They are insoluble in water but soluble in dilute salt solutions. These proteins undergo coagulation upon heating.

Examples: Myosin, Lactoglobulin, Ovoglobulin, Serum globulin and Legumin

• b) Albumins:

        These proteins are found both in plants and animals. They are soluble in water and precipitate out in saturated ammonium sulfate solution. These proteins undergo coagulation upon heating.

Examples: Lactalbumin, Ovalbumin, Serum albumin and Legumelin

• c) Prolamins:

        These proteins are insoluble in absolute ethanol and water but soluble in 75 % ethanol solution. These proteins are rich in proline amino acid and deficient in lysine amino acid.

Examples: Zein of maize and Gliadin of wheat

• d) Globins:

        These proteins form hemoglobin when combined with heme. Different species have different globin proteins but have the same heme part. These proteins are rich in histidine amino acids.

• e) Histones:

        These proteins are rich in arginine amino acid and deficient in tryptophan amino acid. These proteins combine with DNA and form chromatin materials.

Conjugated or Compound Proteins:

Conjugated proteins are present in nature in combination with prosthetic groups. Prosthetic groups are non-protein compounds like sugars, lipids, nucleic acids etc. These proteins are classified on the basis of prosthetic groups. Some types are discussed below.

• a) Glycoproteins:

In glycoproteins the proteins are in combination with sugars (carbohydrates). The sugar part is also called glycan. In these proteins the protein part usually ranges from 15 to 99 %. These proteins are found in egg white, serum, tendons, human urine, bones, and cartilage. They are also present in animal secretions like saliva. Few hormones like TSH and FSH are also glycoproteins.

• b) Phosphoproteins:

In phosphoproteins the proteins are in combination with phosphoric acid.

Examples: Casein (milk) and Vitellin (egg Yolk)

• c) Nucleoproteins:

In nucleoproteins the proteins are in combination with nucleic acids (RNA and DNA). They are present in tissues containing nucleic acid material like sperms.

Examples: Chromosomes and Ribosomes

• d) Lipoproteins:

In lipoproteins the proteins are in combination with lipids. The lipids can be cholesterol, fatty acids, lecithin, and triglycerides. These proteins are present in egg yolk, blood plasma, cell membrane, and nervous tissues.

• e) Metalloproteins:

In metalloproteins the proteins are in combination with metals.

Examples: Carbonic anhydrase (Zinc), Ferritin (Iron), and Ceruloplasmin (Copper)

• f) Chromoproteins:

In chromoproteins the proteins are in combination with colored prosthetic groups.

Examples: Hemoglobin (Heme has red color), Flavoproteins (riboflavin has yellow color)

Proteins are also classified according to their functions. Following are some functional classes of proteins.

• a) Structural Proteins:

The structures of tissues are governed by structural proteins.

Examples: Keratin (forms hair, nails, skin) Elastin (provide stretchiness to tissues), and Collagen (found in tendons, skin, bones, and cartilages)

• b) Catalytic Proteins:

The catalytic proteins are the enzymes of simple or conjugated proteins.

Examples: Lyases (breaks bonds by means other than oxidation and hydrolysis), Ligases (joins to molecules), Isomerases (catalyzes isomerization reactions)

• c) Transport Proteins:

These proteins serve as carrying (transporting) different molecules.

Examples: Transferrin (transports iron), Hemoglobin (O₂ gas carrier)

• d) Regulatory Proteins:

These proteins regulate the expression of genes. These proteins act as hormones.

Examples: Insulin (controls the amount of glucose in blood), Growth hormones

• e) Storage Proteins:

The proteins store proteins for body nutritional needs.

Examples: Ovalbumin (egg white),  Casein (milk)

• f) Contractile Proteins:

These proteins assist in contraction of muscles.

Examples: Actin and Myosin

• g) Immune Proteins:

These proteins are associated with the immune system.

Examples: Cytokines (develops communication between immune cells), Immunoglobulins (act as antibodies)

Proteins are also classified on the basis of their shape and molecular length.

• a) Globular Proteins:

            These proteins have an axial ratio of 10. They have ovoid or spheroid shape. The axial ratio is given as follows.

These proteins have irregular amino acid sequences. These are functional proteins, enzymes and transport proteins belonging to this class. These proteins are mostly water soluble and are sensitive to pH change and heat. Examples include catalase, insulin, and hemoglobin.

• b) Fibrous Proteins:

            These proteins have an axial ratio greater than 10. Their shape is long like a thread. These proteins have a regular amino acid sequence with repetitive amino acids. These are structural proteins and provide strength and support to tissues. These proteins are mostly water insoluble and are less sensitive to pH change and heat. Examples include collagen and keratin.