Characteristics of Plasma Protein | Biochemistry

The following points highlight the top three characteristics of plasma protein. The characteristics are: 1. Albumin 2. Globulins 3. Fibrinogen.

Plasma Protein: Characteristic # 1. Albumin:

a. About half of the total plasma proteins is albumin.

b. It consists of 610 amino acids arranged in a single peptide chain.

c. It has a molecular weight of about 69,000 and is synthesized in the liver.

d. It is precipitated by full-saturation with ammonium sulphate.

e. The normal concentration of serum albu­min is 6.3 – 7.9 gm./100 ml by precipita­tion method.

f. It exerts 80 per cent of the colloid osmotic pressure of plasma.

g. It plays an important role in the exchange of water between tissue fluid and blood.

h. Its concentration decreases in severe pro­tein deficiency, liver diseases and nephri­tis leading to the development of edema.

Plasma Protein: Characteristic # 2. Globulins:

a. Globulins are separated by half-saturation with ammonium sulphate.

b. The molecular weight ranges from 90,000 to 1,300,0000.

c. Globulins are separated into α₁, α₂, β- and γ-globulin fractions, α- and β- globu­lins are synthesized in liver but γ- globu­lins are formed in the cells of reticulo-endothelial system.

d. The nature of different globulin fractions are discussed below:

α₁-Globulins:

This fraction is of several complex proteins containing carbohy­drates and lipids. These are orosomucoid, α₁– glycoprotein and a,-lipoproteins. Its normal concentration in serum is 0.2-0.4 gm./100 ml.

α₂-Globulins:

This fraction also includes complex proteins such as α₂-glycoprotein, plasminogen, prothrombin, haptoglobulins, ceruloplasmin and α₂-macroglobulin. The normal concentration of it in se­rum is 0.5-0.9 gm./100 ml.

β-Globulins:

This fraction consists of li­pid containing three different β-lipoproteins. It also contains siderophilin. Its nor­mal concentration in serum is 0.6-1.2 gm/ 100 ml.

γ-Globulins:

These are immunoglobulin’s having antibody activity. They are classi­fied as IgG, IgA ad IgM on the basis of their electrophoretic mobility.

Mucoproteins and Glycoproteins:

In addition to globulins they contain carbohydrates such as galactose, mannose, fructose and hexosamine. They are found in α₁– and α₂-globulin fractions. Those which contain more than 4% hexosamine are des­ignated as mucoproteins and those which contain less than 4% hexosamine are glycoproteins. Some glycoproteins have specific binding function for thyroxine and Cortisol.

Lipoproteins:

Two lipoproteins (α and β) are found in the α- and β-globulin fractions. Human serum lipoproteins are separated into different frac­tions. Fraction A contains the β lipoproteins with densities less than 1.063; fraction B contains the α₂-lipoproteins with densities of 1.063-1.107; and fraction C contains the a-lipoproteins with densi­ties of 1.107-1.220.

The lipoproteins are large mol­ecules having molecular weights of 1,300,000. They are the major carriers of the lipids of the plasma.

Metalloproteins:

The two important metalloproteins are transferrin (containing iron) and ceru­loplasmin (containing copper). Transferrin trans­ports iron and ceruloplasmin transports copper into different tissues.

Transferrin is found in the β-globulin fraction while ceruloplasmin in α₂-globulin frac­tion. Metalloprotein concentration is reduced in pernicious anemia, chronic infections or liver dis­eases and its concentration is increased in iron de­ficiency or pregnancy.

Plasma Protein: Characteristic # 3. Fibrinogen:

a. Its molecular weight is between 3,50,000 and 4,50,000.

b. It is the precursor of fibrin (the substance of the blood clot).

c. Like globulin, it is precipitated by half- saturation with ammonium sulphate.

d. It normally constitutes 4-6% of the total proteins of the plasma.

e. It is formed in the liver and its concentra­tion in blood falls rapidly in the exces­sive destruction of liver tissue.

f. It is a large asymmetrical molecule which is highly elongated having an axial ratio of about 20: 1.

Functions:

i. Nutritive:

Albumin is largely involved in the nutritive functions of the plasma pro­teins owing to its high concentration. It is effective as a source of protein in hypoproteinemic patients.

ii. Water Distribution:

The colloid osmotic pressure of plasma proteins plays an im­portant role in the distribution of water between the blood and the tissues. Plasma albumin is responsible for this function due to its low molecular weight and quan­titative dominance over other proteins. In kidney diseases where protein loss from the body is more, large amount of water moves to tissues producing edema.

iii. Buffering Actions:

The serum proteins can combine with acids or bases to maintain the pH of blood. They act as acids and combine with cations (mainly sodium) at the normal pH of the blood.

iv. Transport:

The plasma proteins transport lipids and the fat-soluble vitamins (e.g., A, D & E). Bilirubin is associated with albu­min and also with fractions of the α-globulins. Thus, bilirubin is transported along with them. β₁-metal combining globulins (siderophilin) is responsible for the trans­port of iron in the plasma.

Thyroxine is transported in association with an α- globulin (thyroxine-binding protein, TBP) and Cortisol by a mucoprotein (transcorrin). Many drugs and dyes are transported in the plasma in combination with albumin. Half of the calcium of plasma is bound to protein for transport. Hemoglobin liberated intravascularly is carried to the reticuloendothelial system by complexing with the hepatoglobins.

v. Viscosity:

Because of the presence of pro­tein in plasma, it is a viscous fluid. The viscosity of blood provides resistance to flow of blood in the blood vessels to main­tain blood pressure at normal level.

vi. Coagulation:

Plasma contains pro­thrombin, fibrinogen and other factors in­volved in coagulation of blood.

vii. Immunity:

γ-globulins are present in plasma and these γ-globulins protect body against bacterial infections.

viii. Enzymes:

Plasma contains several en­zymes of diagnostic importance’s in some diseases.

Leukocytes:

a. They contain proteins, nucleoproteins, fats, lecithin, cholesterol, purines, enzymes and inorganic salts.

b. They are highly supplied with proteases.

Blood Platelets:

a. They are un-nucleated and so they contain no DNA, but RNA is present.

b. They contain glycogen, ADP, ATP, and the enzymes of glycolysis.

c. Large amounts of catecholamine’s, 5-hydroxytryptamine (serotonin), and hista­mine are present in them.

d. Phospholipids are also present in the form of lipoproteins which activates pro­thrombin in blood coagulation.

e. They also contain contractile protein (thrombosthenin) which is involved in the process of clot retraction.