Enzymes are catalytic proteins; therefore they change the rate of a reaction without being consumed. This means that once and enzyme does its job it can still perform the same function as it did before. Organic catalysts change the rate of a reaction without being permanently changed. Enzymes are polypeptides that are made up of amino acids. Enzyme variable groups that are exposed are the places in which biological processes take place. These side chains, commonly called "R groups," make up the active site and catalyze the conversion of the substrate to make a product. These side chains are often called variable groups because they are often what determines the kind of enzyme it is, therefore determining what substrates it will bond with.
A change in temperature or a fluctuation in pH can alter the enzyme's structure. Anent temperature the alteration of the enzymes occurs when the temperature is very high and the enzyme denatures and is unable to perform the desired task. The temperature is so high that the active site of the enzyme changes and it is unable to bond with substrates. The pH at which different enzymes denature differs from enzyme to enzyme. Similar to too high a temperature, if a specific enzyme is at a pH at which the active site changes, the enzyme is unable to function. This illustrates how the structure of an enzyme is vital to its proper functioning. Allosteric enzymes differ in structure than the previous enzymes addressed. The extreme temperature and pH rules,