In the fed state, glycogen is synthesized from glucose in both liver and muscle. The reaction is a stepwise addition of glucose units onto the glycogen that is already present.
As shown in Figure 5.30, glycogen synthesis involves the intermediate formation of UDP-glucose (uridine diphosphate glucose) by reaction between glucose 1-phosphate and UTP (uridine triphosphate). As each glucose unit is added to the growing glycogen
chain, so UDP is released, and must be rephosphorylated to UTP by reaction with ATP There is thus a significant cost of ATP in the synthesis of glycogen: 2 mol of ATP is converted to ADP plus phosphate for each glucose unit added, and overall the energy cost of glycogen synthesis may account for 5% of the energy yield of the carbohydrate stored.
Glycogen synthetase forms only the a 1^4 links that form the straight chains of glycogen. The branch points are introduced by the transfer of 6—10 glucose units in a chain from carbon-4 to carbon-6 of the glucose unit at the branch point.
The branched structure of glycogen means that it traps a considerable amount of water within the molecule. As discussed in section 5.2, in the early stages of food restriction there is depletion of muscle and liver glycogen, with the release and excretion of this trapped water. This leads to an initial rate of weight loss that is very much greater than can be accounted for by catabolism of adipose tissue, and, of course, it cannot be sustained — once glycogen has been depleted the rapid loss of water (and weight) will cease.
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