The existence of separate anabolic and catabolic pathways is an important motif in biochemistry. One example is the synthesis and breakdown of glycogen, which occur by different reaction pathways. The advantage of having separate anabolic and catabolic pathways is much greater control. For example, in this case, the free energy of the synthesis of glycogen, does not depend on the concentration of phosphate, which is involved in the degradation. Glycogen synthesis occurs by the activity of the glycosyl donor, uridine diphosphate glucose, or UDP-glucose (we are a step past MCAT level here). Glycogen breakdown occurs by phosphorolysis (cleavage with phosphate) rather than hydrolysis, promoted by the enzyme phosphorylase. There are several advantages of phosphorolysis over hydrolysis. The glucose produced can enter glycolysis already phosphorylated. Furthermore, the ionized phosphate group decreases the rate of diffusion of glucose through membranes.

As a control factor, cyclic AMP is centrally important in the hormonally coordinated control of glycogen synthesis and breakdown. Present significantly when ATP is depleted (a favorite MCAT fact), cyclic AMP activates phosphorylase. Low cyclic AMP leads to an increase in blood glucose by, among other effects, triggering glycogenolysis.

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