Hexose Phosphates Can Be Converted to Storage Polysaccharides

When fructose-6-phosphate is generated by gluconeogenesis or photosynthesis (see chapter 15), an equimolar amount of glucose-1-phosphate is usually removed from the hexose monophosphate pool by conversion to storage polysaccharide (glycogen in animals and many kinds of microorganisms starch in green plants). 

The first step in the conversion of glucose-1-phosphate to starch or glycogen is activation at the expense of a nucleoside triphosphate. In plants the activated form is ADP-glucose, formed by transfer of an AMP moiety from ATP to the phosphate of glucose-1-phosphate by the en- 

The elongation step in starch synthesis, (a) The activated form of glucose in starch synthesis is ADP-glucose. This is formed from glucose and ATP as shown. (b) The ADP-glucose does not react 

Assuming that the pyrophosphate formed is hydrolyzed, the equation for the incorporation of the glucosyl residue of glucose-1 -phosphate into starch is the reverse of the equation for the phosphorolysis of a polysaccharide, except for the conversion of ATP to ADP and phosphate ion. 

In this equation, Starch +1 represents the starch molecule after addition of a glucosyl residue. The reactions in this conversion, which include cleavage of both of the pyrophosphate bonds of ATP and the formation of a new pyrophosphate bond, are a bit more complex than in the case of a simple kinase reaction, but the thermodynamic effect is merely that of adding an ATP-to-ADP conversion in the direction of polysaccharide synthesis. Thus, the pseudocycle that connects glucose-1 -phosphate and starch is energetically equivalent to any other in which two oppositely directed conversions differ by one ATP-to-ADP conversion. 

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Hexose Phosphates Can Be Converted to Storage Polysaccharides Summary of Gluconeogenesis Regulation of Glycolysis and Gluconeogenesis... 

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