phosphoenzyme

A phosphoenzyme is an enzyme that forms a covalent bond with a phosphate group during catalysis, creating a phosphoenzyme intermediate. This transient enzyme-bound phosphate allows the transfer of the phosphate to a substrate or to another intermediate in the reaction sequence, enabling turnover through covalent catalysis.

In many cases the phosphate attaches to a specific amino acid residue in the active site, most commonly histidine, serine, or cysteine, though aspartate and other residues can participate in certain systems. The phosphate donor can be ATP, inorganic phosphate, or a phosphorylated substrate. The phosphoenzyme state is typically short-lived, serving as an intermediate rather than a final resting form of the enzyme.

Examples and contexts where phosphoenzyme intermediates are important include two-component signal transduction systems, where sensor histidine kinases autophosphorylate on a histidine residue to form a phosphoenzyme, which then transfers the phosphate to a response regulator. In metabolism, phosphoglycerate mutase operates via a phosphohistidine intermediate, formed when the enzyme accepts a phosphate from a donor and subsequently transfers it to the substrate.

The term distinguishes enzymes that rely on covalent enzyme phosphorylation as part of their catalytic cycle from those that transfer phosphate without forming a covalent, enzyme-bound intermediate. Studying phosphoenzyme intermediates often involves rapid-quench methods and analyses of acid-lability to identify phospho-enzyme species.