Phosphorolysis

Phosphorolysis is cleavage of a chemical bond by reaction with inorganic phosphate, with a phosphate group becoming part of a product. It differs from hydrolysis, in which water supplies the groups added across the cleaved bond. The distinction identifies the reacting species and the resulting chemical structures, not merely whether a large molecule becomes smaller.

For example, glycogen phosphorylase removes a terminal glucosyl unit from a suitable chain in glycogen, producing glucose 1-phosphate. Suppressing protonation details, writewhere $G_n$ denotes the glucan chain and $P_i$ inorganic phosphate. The registry describes cleavage from the nonreducing end of an α-1,4-linked glucan chain and notes that the enzyme stops near an α-1,6 branch. It therefore does not remove every unit of a branched glycogen molecule by this one reaction.^[ref]

As an original stoichiometry example, ten permitted cleavage events consume ten inorganic phosphate units and release ten glucose 1-phosphate molecules. No adenosine triphosphate is required by this written reaction. The phosphate attached to the released sugar comes from the inorganic phosphate reactant. Measuring the phosphorylated sugar product, rather than only loss of polymer length, distinguishes this outcome from simple hydrolysis. Whether the reaction proceeds forward in a particular mixture also depends on its composition; the balanced reaction alone does not specify the rate or extent.