Enzymes are proteins that consist of chains of amino acids connected together by peptide bonds. An enzyme molecule may have one or more of these polypeptide chains.
The sequence of amino acids within the polypeptide chains is distinct in each enzyme and this is what determines the unique three-dimensional shape in which the chains are folded. It is this three-dimensional structure of an enzyme that determines its activities.
Many enzymes also contain an extra non-protein component called a coenzyme or cofactor. This may be an organic molecule or a metal ion. Some enzymes have coenzymes that are closely attached to the protein, whereas others contain coenzymes that dissociate quickly. These coenzymes are required to make an enzyme work.
Most enzymes can be unfolded or inactivated. The unfolding of peptide chains is possible if they are exposed to high temperatures, or acid or alkaline conditions. This is referred to as denaturation. Enzyme denaturation may be reversible or irreversible; however, this depends on the enzyme.
Heating an enzyme beyond a certain temperature will destroy the enzyme permanently. Most enzymes work well at the right temperature and pH. For example, the enzyme pepsin works best when the pH is around 1.5. However, most enzymes will not work unless the temperature and pH is right.