It is almost impossible to correctly classify all proteins. The following classifications are given only to introduce a broader idea to the students.

A. Classification based on functions

1. Catalytic proteins, e.g. enzymes

2. Structural proteins, e.g. collagen, elastin

3. Contractile proteins, e.g. myosin, actin.

4. Transport proteins, e.g. hemoglobin, myoglobin, albumin, transferrin

5. Regulatory proteins or hormones, e.g. ACTH, insulin, growth hormone

6. Genetic proteins, e.g. histones

7. Protective proteins, e.g. immunoglobulins, interferons, clotting factors.

 B. Classification based on Composition and Solubility

B-1. Simple Proteins

According to definition, they contain only amino acids.

 i. Albumins: They are soluble in water and coagulated by heat. Human serum albumin has a molecular weight of 69,000. Other examples are lactalbumin of milk and egg albumin.

ii. Globulins: These are insoluble in pure water, but soluble in dilute salt solutions. They are also coagulated by heat. Examples are egg globulin, serum globulins, legumin of peas.

iii. Protamines: These are soluble in water, dilute acids and alkalies. They are not coagulated by heating. They contain large number of arginine and lysine residues, and so are strongly basic. Hence, they can combine with other acidic proteins. Protamine zinc insulinate is a common commercial preparation of insulin.

 iv. Prolamines: They are soluble in 70-80% alcohol, but insoluble in pure water. They are rich in proline but lack in lysine. Examples are zein from corn, gliadin of wheat, hordein of barley.

 v. Lectins: Lectins are precipitated by 30–60% ammonium sulphate. They are proteins having high affinity to sugar groups. Lectin from Dolichos biflorus will agglutinate human blood group A1 RBCs. Phytohemagglutinin (PHA), a lectin from Phaseolus vulgaris (red kidney bean) agglutinates all RBCs and WBCs. Concanavalin-A (ConA) from legumes will specifically attach to mannose and glucose.

 vi. Scleroproteins: They are insoluble in water, salt solutions and organic solvents and soluble only in hot strong acids. They form supporting tissues. Examples are collagen of bone, cartilage and tendon; keratin of hair, horn, nail and hoof.

B-2. Conjugated Proteins

They are combinations of protein with a non-protein part, called prosthetic group (Table 1). Conjugated proteins may be classified as follows:

 i. Glycoproteins: These are proteins combined with carbohydrates. Hydroxyl groups of serine or threonine and amide groups of asparagine and glutamine form linkages with carbohydrate residues. Blood group antigens and many serum proteins are glycoproteins. When the carbohydrate content is more than 10% of the molecule, the viscosity is correspondingly increased; they are sometimes known as mucoproteins or proteoglycans.

 ii. Lipoproteins: These are proteins loosely combined with lipid components. They occur in blood and on cell membranes.

iii. Nucleoproteins: These are proteins attached to nucleic acids, e.g. Histones. The DNA carries negative charges, which combines with positively charged proteins.

 iv. Chromoproteins: These are proteins with colored prosthetic groups. Hemoglobin (Heme, red); Flavoproteins (Riboflavin, yellow), Visual purple (Vitamin A, purple) are some examples of chromoproteins.

 v. Phosphoproteins: These contain phosphorus. Casein of milk and vitellin of egg yolk are examples. The phosphoric acid is esterified to the hydroxyl groups of serine and threonine residues of proteins.

 vi. Metalloproteins: They contain metal ions. Examples are Hemoglobin (Iron), Cytochrome (Iron), Tyrosinase (Copper) and Carbonic anhydrase (Zinc).

Table1. Examples of conjugated proteins

B-3. Derived Proteins They are degradation products of native proteins. Progressive hydrolysis of protein results in smaller and smaller chains: Protein → peptones → peptides → amino acids.

 C. Classification Based on the Shape

 C-1. Globular Proteins

They are spherical or oval in shape. They are easily soluble. Examples are albumins, globulins and protamines.

C-2. Fibrous Proteins The molecules are elongated or needle shaped. Their solubility is minimum. They resist digestion. Collagen, elastin and keratins are examples.

 D. Classification Based on Nutritional Value

D-1. Nutritionally Rich Proteins

They are also called as complete proteins or first class proteins. They contain all the essential amino acids in the required proportion. On supplying these proteins in the diet, children will grow satisfactorily. A good example is casein of milk.

 D-2. Incomplete Proteins

They lack one essential amino acid. They cannot promote body growth in children; but may be able to sustain the body weight in adults. Proteins from pulses are deficient in methionine, while proteins of cereals lack in lysine. If both of them are combined in the diet, adequate growth may be obtained.

 D-3. Poor Proteins

 They lack in many essential amino acids and a diet based on these proteins will not even sustain the original body weight. Zein from corn lacks tryptophan and lysine.

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مواضيع ذات صلة

Brown Adipose Tissue Promotes Thermogenesis

Hormones Regulate Fat Mobilization

Adipose Tissue is The Main Store of Triacylglycerol in The Body

Precipitation Reactions of Proteins

Study of Protein Structure

Imbalance in the Rate of Triacylglycerol Formation & Export Causes Fatty Liver

The Liver Plays A Central Role in Lipid Transport & Metabolism

HDL Takes Part in Both Lipoprotein Triacylglycerol & Cholesterol Metabolism

Tertiary Structure of Proteins

Secondary Structure of Proteins

Primary Structure of Proteins

Chylomicrons & Very-low Density Lipoproteins are Rapidly Catabolized