While enzymes are essential to metabolic function they are not all needed at the same time, in the same amounts nor in the same cell. Therefore biological systems must regulate them so that they are available when and where they are needed. Several mechanisms are responsible for enzymatic activity.
Enzymes may be controlled by competitive and noncompetitive inhibition. In competitive inhibition, the end product of the enzymatic reaction serves as an inhibitor, binding the active site of he enzyme, preventing it from binding its normal substrate. In noncompetitive inhibition, the end product of the metabolic pathway binds to a site on the enzyme called the allosteric site which is different from the active site. Binding thus, shuts down the metabolic pathway.
Reversible Covalent Modification
This is the making and breaking of a covalent bond between a non-protein group and an enzyme. An example is the addition and removal of phosphate groups (phosphorylation and dephosphorization) on an enzyme which can either activate or deactivate it.
Some enzymes are made in their active form, i.e. zymogen or proenzyme form. In order to activate them, they are cleaved by proteases at specific sites on the polypeptide chain of the enzyme. Examples of zymogens include trypsinogen, chymotrypsinogen, and pepsin which are all involved in protein digestion.
The amount of enzyme in a cell can be determined by regulating transcription and translation of specific genes responsible for their production.
Isoenzyme (or Isozyme)
Isozymes differ in their amino acid sequence and three-dimensional structure, but they are used to carry out the same function. The kinetics of the enzyme are different, depending on which form of isozyme is present.