Enzymes: Their Activity and Role in Metabolism

The cell protoplasm contains one thousand or more proteins,
each bringing about one of the many chemical changers required
for the maintenance of life..


Introduction
The cell protoplasm contains one thousand or more proteins, each bringing about one of the many chemical changers required for the maintenance of life. These proteins are called enzymes.
Enzymes are soluble and colloidal organic catalysts which are synthesized in the body cell. The word enzyme was first introduced by Khune in 1878 and is derived from the original Greek word 'enzyme', which means "in-yeast". The word enzyme was given on speculation that the yeast cells role in fermentation was in reality due to a protein present within the cell and acting there as a catalyst.
"An enzyme may be defined as a complex biological catalyst that is produced by a living organism in its cells to regulate the various physiological processes of the body". It is believed that for every enzyme in the living cell, there is a separate gene responsible for that enzyme's synthesis. As genes are self-duplicating units which remain unchanged in composition as they are passed on from one generation to another, there will be no change in enzyme and so the metabolism, behaviour and physical characteristics of a given species remain the same for generations. Only when there is a loss or change in one or more genes, there will be a change in the enzymes produced by these genes.

Chemical Nature
Many enzymes have been obtained in a highly purified form, and, indeed, may have been isolated as crystals. Since all of these crystalline enzymes have been proved to be protein, it is generally accepted that all enzymes are protein in nature. They are water soluble and most of them are colourless, but some are brown,

Terminology and Classification of Enzymes
Enzymes may be conveniently classified into several major groups according to the chemical reactions which they catalyse.

Transferases
This is a group of enzymes that catalyse the transfer of some group or radical, 'R', from one molecule A to another molecule B.

Hydrolases
They catalyse the hydrolytic fission of substrate by direct addition of the elements of water across the bond which is split.
The group includes the extracellular digestive enzymes and many intracellular enzymes viz. polysaccharides (amylases), glycosidases, esterases, peptidases (pepsin, chymotrypsin, trypsin), Amidases and hydrolytic deaminases (guanine deaminase).In the transmission of nerve impulses, the enzyme, acetylcholinesterase catalyses the breakdown of acetylcholine to choline and acetic acid.

Lyases
These are a group of enzymes that reversibly catalyse the removal of chemical groups from substrates non hydrolytically. Those which add or remove water are sometimes called as Hydrolases viz. aconitase, fumarase, enolase. Non -oxidative decarboxylases remove CO 2 viz. Amino acid decarboxylase, carboxylase, carbonic anhydrase, aldolase (ketose 1 -phosphate aldolase) is included in this group because it catalyses the reversible addition of two molecules of triose phosphate to form fructose di-phosphate [1][2][3].

Isomerases
These are the enzymes that catalyse the interconversion of a compound to one of its isomers viz. in the anaerobic breakdown of carbohydrates to lactic acid, triose phosphate isomerase converts 3-phosphoglyceraldehyde to dihydroxy acetone phosphate.

Ligases or Synthetases:
These are the enzymes that catalyse the linking together of two molecules, coupled with the breaking of a pyrophosphate bond of ATP viz. glutamine synthetase, Acetyl co-enzyme-A synthetase [4,5].

General Properties of Enzymes
a) An enzyme is a protein that is synthesized in a living cell and catalyses or speeds up a reaction.
b) The enzymes work at extremely low concentrations.

c)
An enzyme being a protein, loses its catalytic properties, if subjected to agents like heat, strong acids and bases, organic solvents or other materials which denature the protein. and destruction, therefore, they must be constantly replenished by further synthesis in the body.
f) The enzymes are highly specific i.e. each enzyme usually acts on a single substrate or a group of closely related substrates.
On the basis of above discussion, it is clear that a particular enzyme is effective in accelerating a reaction in either direction, for it does not determine the direction of a reaction. Thus, most enzymatic reactions are reversible and are of great help in metabolism.