Enzymes are protein structures that act as the body’s catalysts. The substrate is bound to the enzyme to form an enzyme-substrate complex, which then undergoes reaction to form the enzyme-bound product. Ionic binding interactions must be strong enough to hold the substrate long enough for the enzyme-catalyzed reaction to take place, but weak enough to allow the product to depart once it is formed. Nucleophilic amino acids, such as serine or cysteine are commonly involved in enzyme-catalyzed mechanisms and will form a temporary covalent bond with the substrate as part of the reaction mechanism. When levels of the final product are low, the allosteric site is unoccupied and the enzyme is active. If the intermolecular interactions are extremely strong, the equilibrium could be so far over to the enzyme–inhibitor complex that inhibition is effectively irreversible. The catalytic reaction is identical, but the amino acid composition between isozymes is slightly different.