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Date Submitted: 11/15/2012 11:25 AM
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...