The Biological Importance of Proteins

Proteins constitute a large part of the structure of cells and are present in all tissues. Many proteins also have special physiological functions.

(1) Membrane Proteins. Proteins and lipids form the major structural components of cell membrane. The membrane associated proteins consist of integral or intrinsic proteins and peripheral or extrinsic proteins.

(2) Enzymes are special proteins produced within an organism which are capable of catalyzing specific catalytic reactions. They are biocatalysts which influence the rate of a chemical reaction, usually without undergoing any change themselves. All enzymes are proteins.

(3) Hormones. Several hormones are peptides and proteins. They play an important role in the regulation of metabolic reactions.

(4) Blood Proteins. The blood proteins include the plasma proteins and hemoglobin.
 
Plasma Proteins. Electrophoresis at pH 8.6 in barbital buffer separates 6 major plasma components, albumin, a1-globulin, a2-globulin, β-globlin, γ-globulin and fibrinogen. Starch-gel electrophoresis breaks up the banks into 20 zones containing 50 plasma proteins.

Albumin (MW 76000) is synthesized in the liver and is the most abundant plasma protein. It maintains the colloid osmotic pressure of plasma and takes part in the transport of a variety of materials. 

A1-Chymotrypsin (MW 45,000) inhibits the photolytic action of protein-digesting enzymes like trypsin, chymotrypsin and plasmin. 

Haptoglobin is an a2-globulin. When R.B.C. are damaged, hemoglobin realized into the plasma is bound by haptoglobin and thus prevented from being excreted through the urine.
 
A2-macroglubin (MW 820,000) is an inhibitor of proteolytic enzymes.

Transferrin (MW 90,000) is a β-globulin involved in the transfer of Fe in plasma.

Gamma (γ) globulins (immunoglobulin, antibodies) are produced in the spleen and lymphatic cells in response to foreign substances (antigens). There are five classes of immunoglobulin, IgG, IgA, IgM, IgD, and IgE.

Complement. The complement system consists of heat labile substances in the serum. They cause lysis of R.B.C and destruction of certain bacteria in the presence of an appropriate antibody.
Hemoglobin is a conjugated protein consisting of globin + haeme (an Fe poryhyrin prosthetic group. It consists of 4 polypeptide chains, 2 alpha chains and 2 beta chains.

(5) Antibiotics. Some antibiotics like gramicidin S, tyrocidin penicillin G are peptides.

(6) Nucleoproteins are conjugated proteins (Protein + Nucleic acid) of cell nuclei. The proteins include protamines, histones and non-histone chromosomal (NHC) proteins.

(7) Multiple Protein Assemblies contain several compounds which together from a functional unit.
Collagen a fibrous protein found in many connective tissues consists of three helically coiled liner chains, each of about 1,000 amino acids.

Flagella. Microtubules of flagella are made up of rubelins. They are mixed dimmers of two closely related subunits along with small amounts of high molecular weight proteins.