Classification of Proteins

Proteins have been classified in many different ways.

(1) Classification Based On Solubility Characteristic

Proteins may be classified as simple proteins and conjugated proteins.

(1) Simple Proteins consist of only amino acids or their derivatives. When hydrolyzed by acids, alkalis or enzymes, simple proteins yield only amino acids or their derivatives. They include the following groups:

a. Albumins. These are water soluble proteins found in all body cells and also in the blood stream.

b. Globulins are insoluble in water but are soluble in dilute salt solutions of strong acids and bases.

c. Glutelins are soluble in dilute acids and alkalis. The proteins glutenin from water is an example. They occur only in plant material.

d. Prolamines are soluble in 70-80% alcohol. They include gliadin from wheat and zein from corn. They are found only in plant material.

e. Aluminous or scleroproteins are insoluble in all neutral solvents and in dilute alkalis and acids. They are found in connective tissues and in hair and nails.

f. Histones are water soluble proteins in which basic amino acids predominate. They are rich in arginine or lysine. In eukaryotes the DNA of the chromosomes is associated with histones to form nucleoproteins.

g. Protamines are water soluble basic polypeptides with a low molecular weight. They are very rich in the amino acid argunine.

(2) Conjugated Proteins consist of simple proteins in combination with some non-protein component. The non-protein groups are called prosthetic groups. Conjugated proteins include the following groups.

a. Nucleoproteins (protein + nucleic acid). Nucleoproteins are proteins in combination with nucleic acids. Nucleoprotamines are combinations of nucleic acids with the basic simple protein protamine.

b. Glycoproteins (Protein + Carbohydrate). Glycoproteins are proteins linked to carbohydrate. In most glycoproteins the linkage is between asparagines (Asn) and N-acetyl-D-glycosamine (GlcNAc). Salivary glands and mucous glands of the digestive tract, however, secrete mucoprotiens in which the linkage is between N-acetylglycosamine and serine/threonine of the protein.

c. Phosphoproteins (protein + phosphate). Phosphoproteins are proteins in combination with a phosphate-containing radical other than a nucleic acid or phospholipids.

d. Chormoproteins are proteins in combination with a prosthetic group that is a pigment.

e. Lipoproteins are proteins conjugated with lipids. There are four types of lipoproteins, high density lipoproteins (HDL) or a-lipoproteins, low density lipoproteins (LDL) or β- lipoproteins, very low density lipoproteins (VLDL) or pre-β- lipoproteins and chylomicrons.

f. Metalloproteinase is proteins conjugated to metal ion(s) which are not part of the prosthetic group. They include caeruloplasmin, an enzyme with oxidase activity that may transport copper in plasma, and siderophilin that is bound to iron.