Class of enzymes
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]
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 (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.
| Enzyme name
|
Enzymes class
|
Reaction
|
Notes
|
| Phosphorylase |
Transferase
(EC 2.4 and EC 2.7.7)
|
A-B + H-OP ⇌ A-OP + H-B
|
transfer group = A = glycosyl- group or
nucleotidyl- group
|
| Phosphatase |
Hydrolase
(EC 3)
|
P-B + H-OH ⇌ P-OH + H-B
|
|
| Kinase |
Transferase
(EC 2.7.1-2.7.4)
|
P-B + H-A ⇌ P-A + H-B
|
transfer group = P
|
| P = phosphonate group, OP = phosphate group, H-OP or P-OH = inorganic phosphate
|
Types
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)
- Enzymes that break down nucleosides into their constituent bases and sugars (break N-glycosidic bond)
- Nucleotidyltransferases (EC 2.7.7)
- Enzymes that have phosphorolytic 3' to 5' exoribonuclease activity (break phosphodiester bond)
All known phosphorylases share catalytic and structural properties.[2]
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:
