126 PROVISION OF energy: fermentation 



(ATP) to the 6-position in the glucose molecule. Glucose-6- 

 phosphate is then altered to fructose-6-phosphate by the 

 action of the enzyme phosphohexose-isomerase (sometimes 

 called oxoisomerase) and a second phosphate enters the 

 molecule in the 1 -position, the second phosphate again being 

 transferred from ATP, but under the action of the enzyme 

 phosphohexokinase in this case. This series of reactions 

 results in the formation of hexosediphosphate from glucose, two 

 molecules of phosphate being taken up from two molecules of 

 ATP with the formation of adenosine-di-phosphate in each case. 



Hexosediphosphate then splits into an equilibrium mixture 

 of triosephosphates under the action of the enzyme zymo- 

 hexase (also called aldolase). The two triosephosphates are 

 glyceraldehyde-phosphate and dihydroxyacetone-phosphate, 

 and their interconversion is catalysed by the enzyme isomerase. 

 We are mainly concerned with the breakdown of glyceralde- 

 hyde-phosphate in fermentation reactions and, as this is 

 removed, dihydroxyacetone-phosphate isomerises to form 

 more glyceraldehyde-phosphate, so that eventually the whole 

 of the hexosediphosphate that is broken down by zymohexase 

 can pass through the series of reactions starting with glycer- 

 aldehyde-phosphate. 



Glyceraldehyde-phosphate is oxidised by the enzyme 

 triosephosphate dehydrogenase (or glyceraldehyde-phosphate 

 dehydrogenase). This enzyme catalyses the transfer of 

 hydrogen from its substrate to coenzyme I, and is only active 

 in the presence of inorganic phosphate. The immediate 

 product of the oxidation is l.S.diphosphoglyceric acid: 



CHO COOPO3H2 



I Triosephosphate I 



CHOH -f- HgO + coenzyme I + phosphate > CHOH + reduced 



I dehydrogenase I coenzyme I 



CH2OPO3H2 CH2OPO3H2 



and the reduced coenzyme I is available as H-donator for 

 other reactions. The diphosphogly eerie acid can give up 

 the second phosphate to either adenyhc acid or adenosine-di- 

 phosphate resynthesising ATP in the presence of the necessary 



