Phosphorylation of Carbohydrates 



CARL F. CORI 



Washington University School of Medicine, St. Louis 



THE METABOLISM of Carbohydrate in animal tissues is made up 

 of a series of enzymatic reactions in which phosphate plays an 

 essential role. What is usually referred to as the phosphate cycle 

 can be divided into four parts: the uptake of inorganic phosphate, 

 the intramolecular migration of phosphate groups, the transfer of 

 phosphate groups from one molecule to another (transphosphoryla- 

 tion), and the regeneration of inorganic phosphate. 



Uptake of Inorganic Phosphate 



The only reaction leading to the uptake of inorganic phosphate 

 that is definitely known to be enzymatic is the phosphorylation of 

 glycogen and starch. The uptake of inorganic phosphate which is 

 associated with the oxidation of phosphoglyceraldehyde is presum- 

 ably non-enzymatic, and the same may be true of the uptake of 

 inorganic phosphate associated with the oxidation of pyruvate. 



In the phosphorylation of glycogen the C— O— C bond of the 1—4 

 glucosidic chain is replaced by the C— O— P bond of glucose-1- 

 phosphate. This reaction is reversible, and from the position of the 

 equilibrium it may be calculated that the change in free energy is 

 very small. This may be interpreted to mean that the ester linkage 

 in glucose-1-phosphate is nearly equivalent to the glucosidic linkage 

 in the large polysaccharide molecule. 



When phosphate is replaced by water, as in the hydrolysis of 

 glycogen or starch by diastase, the reaction seems to be largely 

 irreversible; that is, the end products of diastatic activity, maltose 

 and glucose, even when added to diastase in high concentrations, are 

 not polymerized to glycogen or starch. Glucose, in order to undergo 

 enzymatic polymerization, must first be phosphorylated. Reversi- 

 bility is thus clearly connected with the introduction of a phosphate 

 group into the polysaccharide molecule. The position of the equi- 

 librium at physiological pH is about 77 per cent to the glycogen 

 side (see Table 2), and is determined by the concentration of the 

 divalent ions of orthophosphate and glucose-1-phosphate. Since the 



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