CHARACTERISTICS AND NOMENCLATURE

Specificity and nomenclature

Enzymes are nature's biological catalysts possessing the ability to promote specific chemical reactions under the mild conditions that prevail in most living organisms. They are all proteins but range widely in their size from as few as 60–70 amino acid residues as in RNase to as many as several thousand. Generally they are much larger than their substrates and bind with them by means of active sites created by the specific three-dimensional folding of the protein. Interaction of specific functional groups in a small number of amino acid residues lining the active site with the substrate results in the formation of a transition state for which the activation energy barrier is significantly reduced relative to the non-enzyme-catalysed reaction. As a result, the reaction rate is increased by a factor of many millions relative to the uncatalysed reaction. Enzymes do not alter the position of equilibrium of reversible reactions that they catalyse but they do accelerate the establishment of the position of equilibrium for the reaction.

Many enzymes are members of coordinated metabolic or signalling pathways that collectively are responsible for maintaining a cell's metabolic needs under varying physiological conditions (Sections 15.5 and 17.4.5). The over- or under-expression of an enzyme can lead to cell dysfunction which we may recognise as a particular disease state. Enzyme inhibitors are widely used as therapeutic agents for the treatment of such conditions (Sections 15.2 and 18.1).