Classification of proteins on the basis of functional and chemical nature

A Functional classification of proteins:-

Based on the functions they perform, proteins are classified into the following

groups (with examples)

1. Structural proteins : Keratin of hair and nails, collagen of bone.

2. Enzymes or catalytic proteins : Hexokinase, pepsin.

3. Transport proteins : Hemoglobin, serum albumin.

4. Hormonal proteins : Insulin, growth hormone.

5. Contractile proteins : Actin, myosin.

6. Storage proteins : Ovalbumin, glutelin.

7. Genetic proteins : Nucleoproteins.

8. Defense proteins : Snake venoms, Immunoglobulins.

9. Receptor proteins for hormones, viruses.

B Protein classification based on chemical nature and

solubility:-

This is a more comprehensive and popular classification of proteins. It is based

on the amino acid composition, structure, shape and solubility properties.

Proteins are broadly classified into 3 major groups

1. Simple proteins : They are composed of only amino acid residues.

2. Conjugated proteins : Besides the amino acids, these proteins contain a non-

protein moiety known as prosthetic group or conjugating group.

3. Derived proteins : These are the denatured or degraded products of simple

and conjugated proteins.

Modern Classification of protein:-

1. Simple proteins

(a) Globular proteins : These are spherical or oval in shape, soluble in

water or other solvents and digestible.

(i) Albumins : Soluble in water and dilute salt solutions and

coagulated by heat. e.g. serum albumin, ovalbumin (egg),

lactalbumin (milk).

(ii) Globulins : Soluble in neutral and dilute salt solutions e.g. serum

globulins, vitelline (egg yolk).

(iii) Glutelins : Soluble in dilute acids and alkalies and mostly found

in plants e.g. glutelin (wheat), oryzenin (rice).

(iv) Prolamines : Soluble in 70% alcohol e.g. gliadin (wheat), zein

(maize).

(v) Histones : Strongly basic proteins, soluble in water and dilute

acids but insoluble in dilute ammonium hydroxide e.g. thymus

histones.

(vi) Globins : These are generally considered along with histones.

However, globins are not basic proteins and are not

precipitated by NH4OH.

(vii) Protamines : They are strongly basic and resemble histones but

smaller in size and soluble in NH4OH. Protamines are also

found in association with nucleic acids e.g. sperm proteins.

(viii) Lectins are carbohydrate-binding proteins, and are involved in

the interaction between cells and proteins. They help to

maintain tissue and organ structures. In the laboratory, lectins

are useful for the purification of carbohydrates by affinity

chromatography e.g. concanavalin A, agglutinin.

(b) Fibrous proteins : These are fiber like in shape, insoluble in water and

resistant to digestion. Albuminoids or scleroproteins are predominant

group of fibrous proteins.

(i) Collagens are connective tissue proteins lacking tryptophan.

Collagens, on boiling with water or dilute acids, yield gelatin

which is soluble and digestible.

(ii) Elastins : These proteins are found in elastic tissues such as

tendons and arteries.

(iii) Keratins : These are present in exoskeletal structures e.g. hair,

nails, horns. Human hair keratin contains as much as 14%

cysteine.

2. Conjugated proteins

(a) Nucleoproteins : Nucleic acid (DNA or RNA) is the prosthetic group

e.g. nucleohistones, nucleoprotamines.

(b) Glycoproteins : The prosthetic group is carbohydrate, which is less than

4% of protein. The term mucoprotein is used if the carbohydrate

content is more than 4%. e.g. mucin (saliva), ovomucoid (egg white).

(c) Lipoproteins : Protein found in combination with lipids as the prosthetic

group e.g. serum lipoproteins.

(d) Phosphoproteins : Phosphoric acid is the prosthetic group e.g. casein

(milk), vitelline (egg yolk).

(e) Chromoproteins : The prosthetic group is coloured in nature e.g.

hemoglobins, cytochromes.

(f) Metalloproteins : These proteins contain metal ions such as Fe, Co, Zn,

Cu, Mg etc., e.g. ceruloplasmin (Cu), carbonic anhydrase (Zn).

3. Derived proteins : The derived proteins are of two types. The primary

derived are the denatured or coagulated or first hydrolysed products of

proteins. The secondary derived are the degraded (due to breakdown of

peptide bonds) products of proteins.

(a) Primary derived proteins

(i) Coagulated proteins : These are the denatured proteins produced

by agents such as heat, acids, alkalies etc. e.g. cooked proteins,

coagulated albumin (egg white).

(ii) Proteans : These are the earliest products of protein hydrolysis by

enzymes, dilute acids, alkalies etc. which are insoluble in

water. e.g. fibrin formed from fibrinogen.

(iii) Metaproteins : These are the second stage products of protein

hydrolysis obtained by treatment with slightly stronger acids

and alkalies e.g. acid and alkali metaproteins.

(b) Secondary derived proteins : These are the progressive hydrolytic

products of protein hydrolysis. These include proteoses, peptones,

polypeptides and peptides.