Proteins, প্রোটিন, আমিষ

Proteins: Proteins are polymers of alpha amino acid bonded by peptide linkage.

Classification of proteins according to composition

A) Simple proteins: The proteins which yield only alpha amino acids upon hydrolysis.

1.Albumins: Albumins are water soluble proteins which are coagulated by heat. Examples: egg albumin and serum albumin.

2.Globulins: Globulins are insoluble in water but soluble in dilute salt solutions which are coagulated by heat. Examples: serum globulin and vegetable globulin. 

3.Scleroproteins:  Scleroproteins are insoluble in water. Examples: keratin in hair and finger nails.

4.Glutenins: Glutenins are insoluble in water but soluble in dilute acids and alkalis, which are coagulated by heat. Examples: glutenin from wheat. 

5.Histones:  Histones are soluble in water but insoluble in dilute ammonium hydroxide, which are not coagulated by heat. Examples: globin in hemoglobin.

6.Prolamines:  Prolamines are insoluble in water but soluble in 70% ethanol, which are not coagulated by heat. Examples: zein from corn and gliadin from wheat.

7.Protamines: Protamines  are soluble in water and dial dilute ammonium hydroxide, which are not coagulated by heat. Examples: Salmine from Salmon and sturine from sturgeon.

B) Conjugated proteins: Conjugated proteins are those which yield alpha amino acid and a non protein material upon hydrolysis. The non protein material is called the prosthetic group.

1.Glycoproteins: Glycoproteins contain a carbohydrate derivative as their prosthetic group. Example: mucin a constituent of saliva.

2.Phosphoproteins: Phosphoproteins contain phosphoric acid as prosthetic group. Example: casein from milk. 

3.Chromoproteins: Chromoproteins contain pigmented prosthetic group. Example: hemoglobin and cytochrome.

4.Nucleoproteins: Nucleoproteins contain nucleic acid as prosthetic group. Examples: nuclein and nucleohistones. 

5.Lipoproteins: Lipoproteins contain cholesterol esters and phospholipids as prosthetic group. They are found in brain and nervous tissue and cell membranes.

Classification of proteins according to functions

1. Structural proteins: These are fibrous proteins. Example: collagen from skin, cartilage and bone.

2. Contractile proteins: Examples: myosin, actin from muscles.

3. Hormones: Example: insulin.

4. Enzymes: They serve as catalyst of biochemical reactions in living organisms. Examples: pepsin and trypsin.

5. Antibodies: Antibodies are produced to remove the invading species from the system. Example: gamma globulins from blood.

6. Blood proteins: Examples: albumins, hemoglobin and fibrinogen. They maintain osmotic pressure, oxygen transport and blood coagulation.