Phosphorylase


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

Function

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 to an acceptor, not to be confused with a phosphatase or a kinase. A phosphatase removes a phosphate group from a donor using water, whereas a kinase transfers a phosphate group from a donor to an acceptor.

Types

The phosphorylases fall into the following categories:
  • Glycosyltransferases
  • *Enzymes that break down glucans by removing a glucose residue
  • **glycogen phosphorylase
  • **starch phosphorylase
  • **maltodextrin phosphorylase
  • *Enzymes that break down nucleosides into their constituent bases and sugars
  • **Purine nucleoside phosphorylase
  • Nucleotidyltransferases
  • *Enzymes that have phosphorolytic 3' to 5' exoribonuclease activity
  • **RNase PH
  • **Polynucleotide Phosphorylase
All known phosphorylases share catalytic and structural properties.

Activation

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.

Pathology

Some disorders are related to phosphorylases: