Classification of proteins on the basis of functional and chemical nature

Classification of Proteins

Proteins are complex macromolecules that perform a wide variety of biological functions. They can be classified based on function, chemical nature, and modern structural properties.

A. Functional Classification of Proteins

Proteins can be classified based on the role they play in organisms:

1.     Structural proteins – Provide mechanical support

o    Examples: Keratin (hair and nails), Collagen (bone)

2.     Enzymes (Catalytic proteins) – Speed up biochemical reactions

o    Examples: Hexokinase, Pepsin

3.     Transport proteins – Carry molecules across membranes or in blood

o    Examples: Hemoglobin, Serum albumin

4.     Hormonal proteins – Regulate physiological processes

o    Examples: Insulin, Growth hormone

5.     Contractile proteins – Facilitate movement

o    Examples: Actin, Myosin

6.     Storage proteins – Store nutrients and amino acids

o    Examples: Ovalbumin (egg), Glutelin (wheat)

7.     Genetic proteins – Associated with nucleic acids

o    Example: Nucleoproteins

8.     Defense proteins – Protect organisms from pathogens or toxins

o    Examples: Immunoglobulins, Snake venom proteins

9.     Receptor proteins – Bind hormones, viruses, or ligands

o    Example: Hormone receptors

B. Classification Based on Chemical Nature and Solubility

This system classifies proteins based on amino acid composition, structure, shape, and solubility.

1. Simple Proteins

Composed entirely of amino acids. They are further divided into:

(a) Globular Proteins – Spherical or oval, water-soluble, digestible

·         Albumins: Soluble in water and dilute salts, heat-coagulable (e.g., Serum albumin, Ovalbumin, Lactalbumin)

·         Globulins: Soluble in neutral or dilute salt solutions (e.g., Serum globulins, Vitelline)

·         Glutelins: Soluble in dilute acids/alkalis; mostly plant proteins (e.g., Glutelin, Oryzenin)

·         Prolamines: Soluble in 70% alcohol (e.g., Gliadin, Zein)

·         Histones: Strongly basic, soluble in water/dilute acids, insoluble in ammonium hydroxide (e.g., Thymus histones)

·         Globins: Not basic; soluble proteins associated with oxygen transport

·         Protamines: Strongly basic, smaller than histones, soluble in ammonium hydroxide (e.g., Sperm proteins)

·         Lectins: Carbohydrate-binding proteins; maintain tissue structure and used in affinity chromatography (e.g., Concanavalin A, Agglutinin)

(b) Fibrous Proteins – Insoluble, fiber-like, resistant to digestion

·         Collagens: Connective tissue proteins; gelatin obtained by boiling

·         Elastins: Found in elastic tissues like tendons and arteries

·         Keratins: Present in hair, nails, horns; high cysteine content (~14% in human hair)

2. Conjugated Proteins

Proteins containing a non-protein prosthetic group:

·         Nucleoproteins: DNA or RNA as prosthetic group (e.g., Nucleohistones, Nucleoprotamines)

·         Glycoproteins: Contain carbohydrates (<4%); mucoproteins if >4% (e.g., Mucin, Ovomucoid)

·         Lipoproteins: Proteins with lipid prosthetic groups (e.g., Serum lipoproteins)

·         Phosphoproteins: Contain phosphate groups (e.g., Casein, Vitelline)

·         Chromoproteins: Contain colored prosthetic groups (e.g., Hemoglobin, Cytochromes)

·         Metalloproteins: Contain metal ions (e.g., Ceruloplasmin-Cu, Carbonic anhydrase-Zn)

3. Derived Proteins

Derived proteins are denatured or hydrolyzed products of simple or conjugated proteins.

(a) Primary Derived Proteins – Initial denaturation or hydrolysis products

·         Coagulated Proteins: Denatured by heat, acids, or alkalies (e.g., Cooked proteins, Coagulated albumin)

·         Proteans: Early hydrolysis products, insoluble in water (e.g., Fibrin from fibrinogen)

·         Metaproteins: Second-stage hydrolysis products (e.g., Acid/alkali metaproteins)

(b) Secondary Derived Proteins – Progressive hydrolytic products

·         Include proteoses, peptones, polypeptides, and peptides formed during enzymatic or chemical breakdown of proteins

Summary Table (Optional for Quick Revision)

Classification	Subtype / Examples	Solubility / Property
Globular	Albumin, Globulin	Water / Salt soluble
Fibrous	Collagen, Keratin	Insoluble, fiber-like
Conjugated	Nucleoprotein, Glycoprotein, Metalloprotein	Depends on prosthetic group
Derived	Coagulated, Proteans, Peptones	Denatured or hydrolyzed