Phosphorolysis

Phosphorolysis

Structure of glycogen phosphorylase, an enzyme involved in phosphorolysis.

Phosphorolysis is a biochemical process that involves the cleavage of a compound by the addition of orthophosphate (Pi). This process is distinct from hydrolysis, where water is used to break chemical bonds. Phosphorolysis is a critical reaction in the metabolism of carbohydrates, particularly in the breakdown of glycogen and starch.

Mechanism

In phosphorolysis, a glycosidic bond is cleaved by the addition of a phosphate group. This reaction is catalyzed by enzymes known as phosphorylases. The general reaction can be represented as:

Glycogen (n residues) + Pi _ Glycogen (n-1 residues) + Glucose-1-phosphate

The enzyme glycogen phosphorylase plays a crucial role in this process, facilitating the release of glucose-1-phosphate from glycogen. This reaction is essential for maintaining blood sugar levels during fasting or between meals.

Role in Glycogen Metabolism

Phosphorolysis is a key step in glycogenolysis, the metabolic pathway that breaks down glycogen into glucose-1-phosphate. This pathway is activated in response to hormonal signals such as glucagon and epinephrine, which indicate a need for increased blood glucose levels.

Glycogen phosphorylase in action.

Glycogen phosphorylase is regulated by allosteric interactions and covalent modification. It is activated by phosphorylation, which is mediated by phosphorylase kinase, and inhibited by dephosphorylation, which is mediated by protein phosphatase 1.

Comparison with Hydrolysis

While both phosphorolysis and hydrolysis involve the cleavage of bonds, they differ in the molecule that is added to the substrate. In hydrolysis, water is added, whereas in phosphorolysis, a phosphate group is added. This distinction is important in metabolic pathways, as phosphorolysis conserves energy by producing phosphorylated products that can enter glycolysis directly.

Related Pages

• Glycogenolysis
• Glycogen
• Phosphorylase
• Metabolism
• Enzyme