enzyme regulation by covalent modification

Meaning: A covalent bond is made or broken on the enzyme itself, changing its shape and so its activity. The most tested case is phosphorylation. A kinase transfers a phosphate from ATP to the hydroxyl group of a serine, threonine or tyrosine side chain, and a phosphatase removes it by hydrolysis. The added negative charge shifts the enzyme's conformation and its active site. Other modifications include acetylation and glycosylation. Proteolytic cleavage of a zymogen (trypsinogen → trypsin) is also covalent, but it is irreversible.

On the MCAT: Passages show hormone signaling, such as glucagon or epinephrine → cAMP → protein kinase A, and ask whether a target enzyme is more or less active afterward. Glycogen phosphorylase is activated by phosphorylation, while glycogen synthase is inhibited, so one signal turns one pathway on and the opposing pathway off. Expect questions on what happens when a kinase or phosphatase is inhibited, and on timing: this control works in seconds to minutes, faster than changing gene expression.

Common confusion: Phosphorylation does not always activate an enzyme; the direction depends on the enzyme. It is also not allosteric regulation. An allosteric effector binds noncovalently and can dissociate on its own, whereas a covalent modification stays until a separate enzyme, such as a phosphatase, removes it.