INTERCONVERSION OF SUGARS 



tion, and introduce amino groups. This is what we know, and 

 is probably only a small part of what cells can do. 



Transfer of Phosphate Groups 



The study of phosphoric esters and of the corresponding 

 enzymes has been pursued intensely ever since Harden and 

 Young discovered the action of inorganic phosphate in yeast 

 juice fermentation. This discovery was followed by the patient 

 investigations of Robison and co-workers, which led to the iso- 

 lation of several sugar esters. The mechanism of synthesis of 

 these compounds was partly elucidated by the discovery of hexo- 

 kinase by Meyerhof. This ubiquitous enzyme is reponsible for 

 the introduction of phosphate in position 6 of glucose, fructose, 

 or mannose using adenosine- triphosphate as donor. More recent 

 work has disclosed the presence of similar enzymes which lead to 

 phosphorylation in position 1 of fructose or galactose. 



The specificity of these enzymes is not as stringent as was 

 believed. Thus, the enzyme which acts on glucose also acts on 

 glucosamine, and galactokinase appears to be able to lead to the 

 phosphorylation not only of galactose but also of galactosamine. 

 This fact raises the problem as to whether such reactions are only 

 side effects due to a lack of specificity, or whether they occur 

 normally in cell metabolism. Intriguing reactions of the same 

 type have been observed with phosphoglucomutase. This 

 enzyme catalyzes the transfer of phosphate from position 1 to 

 position 6 of glucose. The reaction is dependent on the presence 

 of catalytic amounts of glucose-l,6-diphosphate and consists in 

 a transfer of phosphate from position 1 of glucose diphosphate 

 to position 6 of glucose- 1 -phosphate, probably mediated by a 

 phospho enzyme complex. The products are glucose-6-phos- 

 phate and glucose-l,6-diphosphate. The latter is, therefore, re- 

 generated at each step. A wider specificity of phosphoglucomu- 

 tase became apparent when it was found that it acted on man- 

 nose- 1 -phosphate, and that glucose-l,6-diphosphate also acted 

 catalytically in this case (13). The reaction has been formulated 

 in two steps. The first is: 



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