Glycogen metabolism is tightly regulated by the enzymes glycogen phosphorylase and glycogen synthase, which control glycogen breakdown and synthesis, respectively. A key regulatory mechanism involves reversible phosphorylation that alters enzyme activity in response to cellular signals.

This article clarifies how phosphorylation affects the activities of these two enzymes.

Effects of Phosphorylation on Glycogen Phosphorylase

• Phosphorylation activates glycogen phosphorylase.

• Glycogen phosphorylase exists in two forms:

  • Phosphorylase b: less active, dephosphorylated form.

  • Phosphorylase a: active, phosphorylated form.

• Phosphorylation by phosphorylase kinase converts phosphorylase b into phosphorylase a, increasing its activity and promoting glycogen breakdown.

• This activation is part of the hormonal response to glucagon and epinephrine signaling, which triggers glycogenolysis.

Effects of Phosphorylation on Glycogen Synthase

• Phosphorylation inhibits glycogen synthase.

• Glycogen synthase also exists in two forms:

  • Active (dephosphorylated) form: promotes glycogen synthesis.

  • Inactive (phosphorylated) form: reduced activity.

• Protein kinases, including protein kinase A (PKA), phosphorylate glycogen synthase, decreasing its activity and thus slowing glycogen synthesis during energy-demanding conditions.

Summary of Statements

Correct Combination

The combination of correct statements is:

(4) A and D

Conclusion

Phosphorylation activates glycogen phosphorylase, promoting glycogen breakdown, and inhibits glycogen synthase, reducing glycogen synthesis. This coordinated regulation ensures efficient control of glycogen metabolism according to cellular energy needs.