Control of Glycogen Metabolism

Control of Glycogen Metabolism

Different pathways for breakdown and synthesis of glycogen allow fine control Control is mainly directed at two opposing enzymes • Glycogen phosphorylase (catabolism) • Glycogen synthase (anabolism)

Opposing pathways of glycogen synthesis and degradation

Fig:16.6 edugen.wileyplus.com

Control of Glycogen Metabolism

Two different types of control operate 1.Allosteric (binding of cofactor at noncatalytic site affecting enzyme conformation hence activity) 2.Covalent modification (formation of covalent bonds that alter the chemistry of the enzyme)

Allosteric Control

Glycogen phosphorylase: –activated by AMP (AMP signals need for glucose breakdown) –inhibited by ATP and G6P (ATP & G6P indicate high energy) Glycogen synthase: –activated by G6P (excess G6P signals storage for future need)

Covalent modification

Phosphorylation and dephosphorylation provides a sensitive control mechanism: – Glycogen phosphorylase: activated by phosphorylation; inhibited by dephosphorylation – Glycogen synthase: activated dephosphorylation; inhibited by phosphorylation

Reciprocal Control

Glycogen phosphorylase

Activated by phosphorylation; inhibited by dephosphorylation

Inactive

Glycogen phosphorylase b

Active

+ ATP

Glycogen phosphorylase a

P

+ ADP

Glycogen synthase

Activated by dephosphorylation; inhibited by phosphorylation

Active

Glycogen synthase

Inactive

+ ATP

Glycogen synthase

P

+ ADP

Control Cascade

Cascade of reactions involving phosphorylations controls glycogen metabolism; set in motion by hormonal changes. Hormones bind to membrane receptor

Receptor

ATP

Cyclic AMP

Glycogen phosphorylase interconversions

Fig: 16.10 edugen.wileyplus.com

Hormonal signals re: glycogen metabolism

Liver: Low blood sugar: •Signal = Glucagon released by pancreas (1st messenger) •Attaches receptors on liver cells modifies Gprotein •G-protein activates adenylate cyclase converts ATP to cAMP...