Proteins: Definition, Classification, Functions

Proteins

Definition

Proteins may be defined as the high molecular weight polymers composed of α-amino acids united to one another by peptide linkage (-CO-NH-). Proteins are the major consitituents of all living organisms. They contain carbon, hydrogen, nitrogen, oxygen and sulphur.

Classification

They are classified into 3 main groups.

1. Simple protein : These proteins on hydrolysis yield only α-amino acids. (eg). albumin, globulin

2. Conjugated protein : These are proteins composed of simple proteins combined with non-protein part called as prosthetic groups.

They are further subdivided into:

·           Nucleo protein : Proteins present along with nucleic acids. (eg) Histones and Protamines.

·           Phosphoprotein : These are protein containing phosphoric acid (eg) casein of milk.

·           Glycoprotein : These are proteins containing carbohydrate moiety as prosthetic group. (eg.) Gonadotropic hormone, mucous glycoprotein mucin (saliva) and osseomucoid (bone)

·           Chromoprotein : These proteins contain heterocyclic compounds like porphyrins as the prosthetic group. (eg) Hemoglobin and Myoglobin.

·           Lipoproteins : These are proteins conjugated with lipids (eg) chlyomicron, very low density lipoprotein (VLDL), low density lipoprotein (LDL) and high density lipoprotein (HDL).

·           Metalloproteins : These proteins contain metal as prosthetic group (eg) Siderophilin (Fe) and Ceruloplasmin (Cu).

3. Derived proteins : These are proteins derived from the simple and conjugated proteins by the action of acids, alkalies or enzymes. They are the products resulting from partial to complete hydrolysis of proteins. (eg.) proteoses, peptones and peptides.

Functions

·           Proteins are the essence of life processes.

·           They are the fundamental constituents of all protoplasm and are involved in the structure of a living cell and in its functions.

·           Enzymes are proteins which act as biocatalysts.

·           Proteins serve as components of the tissues holding the skeletal elements together.

·           Nucleoproteins serve as carrier of genetic characters and hence govern inheritance of traits.

·           They execute their activities in the transport of various compounds.

·           Some hormones are proteins which regulate numerous physiological functions.

·           They function in the homeostatic control of the volume of the circulating blood.

·           They are involved in blood clotting through thrombin, fibrinogen and other protein factors.

·           They act as the defense against infections by means of antibodies.

Functional diversity of proteins

·           Proteins are the fundamental constituents of all protoplasm and are involved in the structure and functions of living cells. Proteins have many different biological functions.

·           Catalytic proteins

·           Enzymes are proteins which have catalytic power, far beyond that of man-made catalysts. They enhance the rate of biochemical reactions. (eg.) amylase, carbonic anhydrase etc.

·           Nucleoproteins

·           Hi stones are basic proteins found in association with nucleic acids. They serve as carriers of genetic characters and hence govern inheritance of traits.

·           Hormonal proteins

·           Some hormones are proteins which regulate numerous physiological functions. (eg) growth hormone, insulin and glucagon.

·           Storage proteins

·           They have the function of storing aminoacids as nutrients and as building blocks for the growing embryo. (eg) casein of milk, ovalbumin of egg white.

·           Transport proteins

·           They are capable of binding and transporting specific types of molecules via blood. (eg) hemoglobin and albumin.

·           Contractile proteins

·           Proteins such as actin and myosin in skeletal muscle function as essential elements in contractile and motile systems.

·           Defensive proteins

·           Some proteins have protective or defensive functions. The blood proteins - thrombin and fibrinogen participate in blood clotting. Antibodies or immunoglobulins are protective proteins which prevent the onset of diseases in the body.

·           Structural proteins

·           Proteins such as collagen, keratin etc. serve as structural elements.

·           Toxic proteins

·           Ricin of caster bean, diphtheria toxin and botulinum toxin represents another group of proteins which cause dysfunctions and disorders in the body.