/ Glucose 1-phosphate hexokinase

Glucose 6-phosphate

/ Glucose 1-phosphate


UDP-glucose glycogen synthase


FIGURE 15-36 Control of glycogen synthesis from blood glucose in myocytes. Insulin affects three of the five steps in this pathway, but it is the effects on transport and hexokinase activity, not the change in glycogen synthase activity, that increase the flux toward glycogen.

(Fig. 15-36) in rat and human muscle. They found that the flux control coefficient for glycogen synthase was smaller than that for the steps catalyzed by the glucose transporter and hexokinase. This finding, too, contradicts the conventional wisdom that glycogen synthase is the locus of flux control and suggests that the importance of the phosphorylation/dephosphorylation of glycogen synthase is related instead to the maintenance of metabolite homeostasis—that is, regulation, not control. Two metabolites in this pathway, glucose and glucose 6-phosphate, are key intermediates in other pathways, including glycolysis, the pentose phosphate pathway, and the synthesis of glucosamine. Metabolic control analysis suggests that when the blood glucose level rises, insulin acts in muscle to (1) increase glucose transport into cells by bringing GLUT4 to the plasma membrane, (2) induce the synthesis of hexokinase, and (3) activate glycogen synthase by covalent alteration (Fig. 15-29). The first two effects of insulin increase glucose flux through the pathway (control), and the third serves to adapt the activity of glycogen synthase so that metabolite levels (glucose 6-phosphate, for example) will not change dramatically with the increased flux (regulation).

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