Enzyme

Regulatory enzymes are characterized by increased or decreased activity in response to chemical signals. Metabolic pathways are regulated by controlling the activity of one or more enzymatic steps along that path. Regulatory control allows cells to meet changing demands for energy and biomolecules. The scheme shown in Figure 5 illustrates the concept of a regulatory feedback mechanism.

Enzymatic activity is regulated in four general ways:

1. Allosteric control. Allosteric enzymes have distinct binding sites for effector molecules which control their rates of reaction.
2. Control proteins participate in cellular regulatory processes. For example, calmodulin is a Ca²⁺ binding protein which binds with high affinity and modulates the activity of many Ca²⁺-regulated enzymes. In this way, Ca²⁺ acts as "second messenger" to allow crosstalk and feedback control in metabolic pathways.
3. Enzyme activity controlled by reversible covalent modification is carried out with a variety of chemical groups. A common example is the transfer of phosphate groups, catalyzed by enzymes known as kinases (Figure 6). Glucose metabolism is modulated by phosphate transfer.
4. Proteolytic activation. The above example is reversible in nature. Irreversible hydrolytic cleavage of peptide fragments from inactive forms of enzymes known as zymogens converts them into fully active forms. Proteolytic enzymes are controlled by this mechanism.