Cells tightly regulate their enzymatic reactions, allowing necessary reactions to proceed while inhibiting unnecessary or damaging ones. One means of regulating enzyme action is allosteric regulation. In allosteric regulation, the covalent modification or noncovalent binding of a regulator to an enzyme can cause an enzyme to change shape and expose an active site. Negative regulation can work this way as well, with the active site becoming hidden.
The change in enzyme shape from the binding of a noncompetitive inhibitor is an example of allostery. Allosteric regulation occurs when a non-substrate molecule binds or modifies an allosteric site, which is a site other than the active site of an enzyme, inducing the enzyme to change its shape. When the inhibitor releases, the enzyme returns to its original conformation, allowing the substrate to bind to the active site and convert into products.
Allosteric regulators can be inhibitors or activators. Activators also bind to allosteric sites noncovalently. The binding essentially opens up the active site, making it more accessible to the substrate, increasing the affinity between the two. Allosteric regulators bind reversibly, and when they dissociate from the enzyme, the enzyme reverts back to its original activity level.
Allosteric regulation can also occur by covalent modification, which typically occurs by the addition of a phosphate group to one or more of the enzyme's amino acids. If the phosphate is added in a hydrophobic region of the enzyme, it makes that region hydrophilic, because phosphate carries a negative charge. The protein twists, and this can expose or hide the active site.
Enzymes speed up chemical reactions, making reactions occur at the right time and place in a cell. Although enzymes are essential for life, they must be tightly regulated so that cells focus their energy and resources on appropriate cellular reactions. Allosteric regulation occurs when a non-substrate molecule binds or modifies a site other than the active site of an enzyme (called the allosteric site), thereby inducing the enzyme to change its shape. The shape change can result in the activation or inactivation of an enzyme.