Reaction rates of
enzymes are continuously changing within the body and can occur through alterations in enzyme concentration or enzyme activity. Concentration of an enzyme can be altered by rates of synthesis, rates of release, and stability (degradation or inactivation). Alternatively, the activity of an enzyme can be altered. Certain enzymes have another site (other than the active site) where molecules can bind and alter reaction rates. This is the regulatory site where modulators can bind and thereby alter reaction rates. This type of regulation is called allosteric regulation, since the modulator alters the structure of the enzyme in a way that affects its activity (either to increase or decrease). Binding to that allosteric site is reversible, which means that the activity of the enzyme can be altered by the concentration of the modulator. Alternately, enzymes can be controlled via covalent regulation, where changes in enzyme activity are brought about by covalent bonding of specific chemical groups to enzymes. These covalent bonds are usually created by another enzyme. A common form of
covalent regulation involves the addition or removal of a phosphate group. Addition is called phosphorylation, whereas removal is called dephosphorylation. Protein kinases are enzymes that phosphorylate enzymes while
phosphatases
dephosphorylate enzymes. Interestingly, enzymes can also be regulated by feedback loops involving the product of a series of reactions. This type of feedback inhibition is quite common in metabolic pathways where it has been called end-product inhibition. The product of one reaction will alter the structure of an earlier enzyme, thereby decreasing its activity. Thus, feedback inhibition is a specific form of
allosteric regulation. Finally, some reactions are controlled through a feedforward activation. In a series of enzyme-mediated reactions, products from an earlier reaction will activate the enzyme for a reaction that is downstream from the product in question.
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