Regulatory enzymes refer to enzymes along a pathway that are specifically targeted for the regulation of that pathway.

Allosteric enzymes rely on inhibitor molecules that bind to an enzyme at the allosteric site. Their binding induces a conformational change that reduces the affinity of the enzyme’s active site for its substrate. The binding of this allosteric inhibitor changes the conformation of the enzyme and its active site, so the substrate is not able to bind. Allosteric enzymes usually rely on control loops and feedback mechanisms where the allosteric inhibitor is a product from downstream in the metabolic pathway. This reduces the rate of the enzyme and concentration of product made at the end of the pathway.

Covalently-modified enzymes are modifications made using covalent bonds. Enzymes structures are modified to either activate or inactivate the enzyme molecule. The functional groups on amino acids form covalent bonds to molecules that are often catalyzed by other enzymes. A common example is when a phosphate molecule forming a covalent bond with an enzyme via phosphorylation. Phosphorylation can be used to activate and inactivate an enzyme as needed by the cell. It works by changing the conformation of the enzyme between its active or inactive form and also can be used to modify the affinity of the enzyme for the substrate.

Zymogens are examples of inactive enzymes. Zymogens are enzymes that begin as an inactive precursor and are activated by a biochemical change. An example of this is in the pancreas where the enzyme pepsin is secreted in its inactive form pepsinogen and is activated in the stomach via a hydrolysis reaction with stomach acid.

Key Points

• Enzymes can be modified to reduce or enhance their activity.

• Allosteric enzymes are enzymes that change their conformational ensemble upon binding of an effector which results in an apparent change in binding affinity at a different ligand binding site.

• Covalently-modified enzymes can be activated or deactivated through the addition or removal of a modifier using a reversible covalent bond.

• Zymogens are an inactive substance which is converted into an enzyme when activated by another enzyme.

Key Terms

Allosteric Site: A site other than the active site on an enzyme.

Active site: The active site is the part of an enzyme to which substrates bind and where a reaction is catalyzed.

Substrate: A reactant in a chemical reaction is called a substrate when acted upon by an enzyme.