Glucose is stored as the polymer glycogen. Glucose polymerization, as a stand-alone reaction, has a positive AG and will not occur. The synthesis of glycogen is therefore driven by the hydrolysis of nucleoside triphosphates—not only ATP but also UTP. If phosphorylated glucose is not available, then it is made by hexokinase using ATP (page 284). Glucose-1-phosphate then reacts with UTP to make UDP-glucose (Fig. 13.10). Glycogen synthase then transfers glucose from the UDP to the growing glycogen chain (Fig. 13.11). Other enzymes insert the a(1^6) branches at intervals.
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