| REGULATION OF ENZYME ACTIVITY
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In multistep metabolic pathways, the slowest step limits the overall rate of the reaction. It is therefore most efficient to
regulate the metabolic pathway by controlling key enzymes that are involved in this 'rate-limiting' step. Generally, five independent mechanisms are involved in these processes:
- The expression of the enzyme protein from the corresponding gene changes in response to the cell's changing environment or metabolic demands.
- Enzymes may be irreversibly activated or inactivated by proteolytic enzymes.
- Enzymes may be reversibly activated or inactivated by covalent modification, such as phosphorylation.
- Allosteric regulation modulates the activity of key enzymes through reversible binding of small molecules at sites distinct from the active site in a process that is relatively rapid and, hence, the first response of cells to changing conditions.
- The degradation of enzymes by intracellular proteases in the lysosome or by proteasomes in the cytosol also determines the lifetimes of the enzymes and consequently enzyme activity over a much longer period of time.
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