In biochemistry, phosphorylases are enzymes that catalyze the addition of a phosphate group from an inorganic phosphate (phosphate+hydrogen) to an acceptor.

A-B + P ⇌ A + P-B

They include allosteric enzymes that catalyze the production of glucose-1-phosphate from a glucan such as glycogen, starch or maltodextrin.

Phosphorylase is also a common name used for glycogen phosphorylase in honor of Earl W. Sutherland Jr., who in the late 1930s discovered it as the first phosphorylase.^[1]

Phosphorylases should not be confused with phosphatases, which remove phosphate groups. In more general terms, phosphorylases are enzymes that catalyze the addition of a phosphate group from an inorganic phosphate (phosphate + hydrogen) to an acceptor, not to be confused with a phosphatase (a hydrolase) or a kinase (a phosphotransferase). A phosphatase removes a phosphate group from a donor using water, whereas a kinase transfers a phosphate group from a donor (usually ATP) to an acceptor.

The phosphorylases fall into the following categories:

• Glycosyltransferases (EC 2.4)
  • Enzymes that break down glucans by removing a glucose residue (break O-glycosidic bond)
    • glycogen phosphorylase
    • starch phosphorylase
    • maltodextrin phosphorylase
  • Enzymes that break down nucleosides into their constituent bases and sugars (break N-glycosidic bond)
    • Purine nucleoside phosphorylase (PNPase)
• Nucleotidyltransferases (EC 2.7.7)
  • Enzymes that have phosphorolytic 3' to 5' exoribonuclease activity (break phosphodiester bond)
    • RNase PH
    • Polynucleotide Phosphorylase (PNPase)

All known phosphorylases share catalytic and structural properties.^[2]

Phosphorylase a is the more active R form of glycogen phosphorylase that is derived from the phosphorylation of the less active R form, phosphorylase b with associated AMP. The inactive T form is either phosphorylated by phosphoylase kinase and inhibited by glucose, or dephosphorylated by phosphoprotein phosphatase with inhibition by ATP and/or glucose 6-phosphate. Phosphorylation requires ATP but dephosphorylation releases free inorganic phosphate ions.

Some disorders are related to phosphorylases:

• Glycogen storage disease type V - muscle glycogen
• Glycogen storage disease type VI - liver glycogen

• Hydrolase