Enzymes are highly specific, and a particular enzyme will catalyse only a single reaction. However, enzymes are not rigid, and the relationship is better conceptualized as a hand fitting into a glove. Many enzymes also require a variety of nonprotein cofactors for their activity.
Enzymes function within narrow limits of temperature and pH .
Enzyme action is further influenced by various inhibitors which compete with the natural substrate for the active site, or by metal ions such as copper or lead, which can cause irreversible damage to active sites and are thus toxic. Control of enzyme activity in cells is essential for the normal growth, development and functioning of organisms, since ultimately all reactions in cells are mediated by enzymes. Since enzymes are proteins, they are the direct products of genes , and their synthesis and structure is determined by DNA . Their action is also regulated by a variety of negative feedback mechanisms governed by the relative concentrations of substrates and products. In some metabolic pathways the end product of the pathway acts as an inhibitor of the first enzyme in the sequence, thus exercising a fine-control mechanism.