Question

A second year student decides to do research in a nutrition laboratory that is studying the effect of caffeine on glycogen metabolism. Caffeine inhibits cAMP phosphodiesterase. If the caffeine is added to cells, which enzymes are affected? Would you expect the rate of glycogen degradation process to decrease or increase? What about the rate of glycogen synthesis? Please explain. (10 points)

Answer 1

Answer

If the caffeine is added to cells, stimulating the production of NO, NO is synthesized by nitric oxide synthase,

The rate of glycogen degradation process is decreases because glycogen breakdown regulatory pathway can be reversed at several levels.

• First, the ligand can leave the receptor.
• Second, the G proteins have an inherent GTPase activity that serves to turn them off over time.
• Third, cells have phosphodiesterase (inhibited by caffeine) for breaking down cAMP.
• Fourth, an enzyme known as protein phosphatase (also called phosphoprotein phosphatase) can remove phosphates from phosphorylase kinase (inactivating it) AND from GPa, converting it to the much less active GPb.

The anabolic pathway contrasting with glycogen breakdown is that of glycogen synthesis.

• Glycogen synthesis from glucose involves phosphorylation to form G6P, and isomerization to form G1P
• The second step of glycogen synthesis is catalyzed by the enzyme Glycogen Synthase.