Proteins- Definition, Classification and Properties for dmlt ddt dotat and paramedical students

Proteins- Definition, Classification and Properties: 

Contents:

1. Definition of Proteins
2. Biological Importance of Proteins
3. Classification of Proteins.
4. Important Tests of Proteins
5. Estimation of Proteins

1. Definition of Proteins

Proteins may be defined as the high molecular weight mixed polymers of α-amino acids joined together with peptide linkage (-CO-N H-).
Proteins are the chief constituents of all liv­ing matter. 
They contain carbon, hydrogen, nitro­gen and sulphur and some contain phosphorus also.

2.Biologicalmportance of Proteins.

i.Proteins are the essence of life processes.

ii. They are the fundamental constituents of all protoplasm and are involved in the struc­ture of the living cell and in its function.

iii Enzymes are made up of proteins.

iv. Many of the hormones are proteins.

v. They are involved in blood clotting through thrombin, fibrinogen and other protein factors.

vi. They act as the defence against infections by means of protein antibodies.

3.Classificationof Proteins.

I. Simple proteins

(i) Albumins:

Soluble in water, coagulable by heat and precipitated at high salt concentrations.   Examples – Serum albumin, egg albumin, lactalbumin (Milk), leucosin (wheat), legumelin (soyabeans).

ii) Globulins:

Insoluble in water, soluble in dilute salt solutions and precipitated by half  saturated salt solutions.

Examples – Serum globulin, vitellin (egg yolk), tuberin (potato), myosinogen (muscle), legumin (peas).

(iii) Glutelins:

Insoluble in water but soluble in dilute  acids and alkalis. Mostly found in plants.

Examples – Glutenin (wheat), oryzenin (rice).

(iv) Prolamines: Insoluble in water and absolute alcohol  but soluble in 70 to 80 per cent alcohol.

Examples – Gliadin (wheat), zein (maize).

(v) Protamines:

Basic proteins of low molecular weight.  Soluble in water, dilute acids and alkalis,  Not coagulable by heat. Examples– Salmine (salmon sperm).

(vi) Histones:

Soluble in water and insoluble in very I dilute ammonium hydroxide.   Examples– Globin of hemoglobin and thymus histones.

II. Conjugated Proteins

(i) Nucleoproteins:

Composed of simple basic proteins (pro­tamines or histones) with nucleic acids,   found in nuclei. Soluble in water.

Examples – Nucleoprotamines and nucleohistones.

(ii) Lipoproteins:

Combination of proteins with lipids, such as fatty acids, cholesterol and   phospholipids etc.

Examples – Lipoproteins of egg-yolk, milk and cell membranes, lipoproteins of blood.

(iii) Glycoproteins:

Combination of proteins with carbohydrate (mucopolysaccharides).

Examples – Mucin (saliva), ovomucoid (egg white), osseomucoid (bone).

(iv) Phosphoproteins:

Contain phosphorus radical as a prosthetic group.

Examples – Caseinogen (milk), ovovitellin (egg yolk).

(v) Metalloproteins:

Contain metal ions as their prosthetic  groups. The metal ions generally are Fe, Co. Mg, Mn, Zn, Cu etc.

Examples – Siderophilin (Fe), ceruloplasmin (Cu).

III. Derived Protein

A. Primary derivatives

(i) Proteans:

Derived in the early stage of protein hydrolysis by dilute acids, enzymes or alkalis.     Examples– Fibrin from fibrinogen.

(ii) Metaproteins:

Derived in the later stage of protein hydrolysis by slightly stronger acids and alkalis.   Examples– Acid and alkali metaproteins.

(iii) Coagulated:

They are denatured proteins formed by the action of heat. X-rays, ultraviolet rays etc .   Example: Cooked proteins, coagulated albumins.

B. Secondary derivatives

(i) Proteoses:

Formed by the action of pepsin or trypsin. Precipitated by saturated solution of ammonium sulphate, incoagulable by heat.

Examples – Albumose from albumin, globulose from globulin.

(ii) Peptones: .

Further stage of cleavage than the proteoses. Soluble in water, incoagu­lable by heat and not precipitated by saturated ammonium sulphate solutions.

(iii) Peptides:

Compounds containing two or more amino acids. They may be di-, tri-, and porypeptides.

Examples – Glycyl-alanine, leucyl-glutamic acid.