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  Enzymes are proteins derived from living cells, the function of which is to mediate the chemical reactions which occur in the metabolism of living organisms. Enzymes perform this function by acting as catalysts; catalysts need not be made of protein, but enzymes make extremely efficient catalysts and are also very specific in that they will only catalyse certain reactions. Enzymes greatly increase the rate of chemical reactions which would often not proceed at all without catalysis. They achieve their function by providing a ‘rendezvous’ for the chemicals (called substrates) which are the starting point for the reaction. The enzyme binds the substrate(s) in such a way that the reaction can readily proceed; the chance of these conditions being met without the intervention of an enzyme are greatly reduced. This concept of enzyme function has been called the ‘lock and key hypothesis’.

Enzymes are sensitive to their environment, particularly temperature and acidity, and will not function optimally if their environmental conditions are not tightly regulated, as they are within living cells and organisms. They are sensitive to such factors because they are composed largely of protein, which is a chain-like molecule. Protein is made from subunits (amino acids) which are linked in a specific order to form a chain which folds up in a pattern dependent upon the sequence of amino acids in the chain. This 3-dimensional structure is, as the term ‘lock and key’ suggests, essential to the function of the enzyme but is not rigid. Changes in temperature or in other conditions can cause shifts in the structure of the enzyme so that it can no longer bind its substrates. Substrates are bound in a binding site, and the range of structures which are bound is very limited because of the 3-dimensional shape of the binding site (analogous to a keyhole). This is the explanation for enzymes specificity, but chemicals with a molecular structure which is very similar to that of the substrate may also bind to the enzyme, sometimes more strongly than the substrate itself. This blocks the enzyme\'s function and is the mechanism by which many drugs and poisons work, modifying the metabolism of the substrate by blocking enzyme function. Studies of enzyme structure in 3-dimensions present the possibility of designing specific enzyme inhibitors for use as drugs; currently, most drugs are discovered by serendipity.

Enzyme-mediated reactions were first recognized by Eduard Buchner who, in 1897, showed that a cell-free extract of yeast could catalyse the fermentation which was previously thought to require the presence of living yeast cells. The first enzyme was purified as a crystal of protein in 1926, and all enzymes subsequently purified have been found to be based upon protein molecules. Enzymes, which are the product of the genes, are thus the mechanism by which the DNA controls the function of the organism; the direction taken by metabolism is dependant upon the enzymes present. RB

See also biochemistry; biotechnology; fermentation; pharmacology.Further reading Robert Murray, Biochemistry; , John Smith and , Hywell Williams, Drug Design.



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