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Chemical Nature of Enzymes

Having laid the foundation of the enzymic conversion of compounds, several enzymes were isolated from living cells. The true nature of enzymes was established by James Sumner in 1926 when he extracted and crystallized an enzyme, urease from  jack beans. Hel also determined its chemical nature and composition. It is now well known that all enzymes, without exception, are proteins having a three-dimensional structure. About 150 enzymes have been prepared in pure crystalline form. Many of them contain a non-protein component, called prosthetic group, which may be covalently or non-covalently linked with the enzyme molecule. Enzymes therefore are defined as biological catalysts, a kind of protein molecules formed by the living cells which catalyze a particular reaction or a group of closely related reactions.

Properties of enzymes

All enzymes are synthesized within the cells and can pass through cell membranes under certain condition. An essential property of these catalysts is speeding up the rate of chemical reactions and while doing so they remain unchanged without loss of catalytic activity. Many enzymes produced by the cell carry out metabolic reaction within the cell are hence called endoenzymes. Some enzymes are liberated by the cell and catalyze reaction in the vicinity of the cell. These are called exoenzymes.
Most of the enzymes endowed with endowed with digestive function are included in this category.
Many enzymes contain a non-protein component which could be either an organic compound or a metal ion. The non-protein component is called a coenzyme, while the metal ion forms a cofactor. The enzyme and the prosthetic group together form a holoenzyme. If the cofactor is dissociated from the enzyme molecule, it loses its catalytic function and is called apoenzyme.
The substance on which the enzyme acts is called substrate. Most enzymes have specific substrates whose specificity is determined by the protein or apoenzyme. The specificity of enzymes towards their substrates is important because it exerts biological control over metabolic function some of the properties that can be attributed to enzymes are:
1.    Enzymes are present in the cell in small amount only.
2.    During the course of catalytic activity, their chemical nature is not altered irreversibly, hence they can participate in many individual reactions over and over again.
3.    The chemical equilibrium of an enzyme catalyzed reaction remains unchanged. Usually enzymes favour a chemical reaction by accelerating it. However, they can reaction in a reverse direction provided it is favoured by an increase in the ratio of products to reactants.

holoenzyme is composed of factor coenzyme and an apoenzyme molecule

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