328 BIOLOGICAL ACTIVITIES OF YEASTS 



This diphosphorylated triose is oxidized to 1,3-diphosphoglyceric 

 acid by the removal of two hydrogen atoms. Prior to the introduc- 

 tion of the concept of phosphorylated intermediates, the explanation 

 of the mechanism of dehydrogenation (oxidation) of an aldehyde 

 group was to assume that the aldehyde group was first hydrated and 

 then dehydrogenated. An examination of the oxidation of diphos- 

 phoglyceraldehyde to diphosphoglyceric acid obviates the necessity 

 for such an assumption. In this oxidation step, one of the energy- 

 poor phosphate bonds, § a low potential phosphate ester bond, is con- 

 verted to an energy-rich bond by the energy generated by the oxida- 

 tion. The two hydrogen atoms are accepted by Coenzyme I or 

 diphosphopyridine nucleotide, DPN, which is converted to the re- 

 duced form, H2-DPN. 



OH O 



HCO— PO3H2 Triose CO—POsHa 



phosphate 



dehydrogenase 



HCOH + DPN , HCOH + H2-DPN 



CH2O— PO3H2 CH2O— PO3H2 (7) 



1,3-Diphospho- 1,3-Diphospho- 



glyceraldehyde glyceric acid 



The work of Meyerhof and Junowicz-Kocholaty,^^ on the other 

 hand, suggests that the formation of 1,3-diphosphoglyceraldehyde 

 does not occur. According to these investigators, 3-phosphoglycer- 

 aldehyde and inorganic phosphate are simultaneously adsorbed onto 

 the surface of the oxidative enzyme, triose phosphate dehydrogenase. 

 DPN then removes one hydrogen from the aldehyde and one from the 

 inorganic phosphate, thereby effecting in one reaction the simultane- 

 ous oxidation of the triose phosphate and the formation of 1,3-di- 

 phosphoglyceric acid. 



Ho-DPN, the reduced form of Coenzyme I, may reduce a mole- 

 cule of phosphorylated triose to glycerophosphate (which on hy- 

 drolysis yields glycerol) and itself be regenerated to its oxidized 

 form, DPN. However, by far the greater load of regenerating the 

 oxidized form, DPN, is borne by the reaction involving the reduc- 

 tion of acetaldehyde to ethyl alcohol with the concomitant oxidation 



§ Energy-poor phosphate bonds: linkages which when cleaved yield small 

 amounts of energy, ca. 3000 calories (or actually require energy) ; those linkages 

 where the phosphate residue is linked to an alcoholic hydroxyl group, e.g., 

 hexose phosphate, triose phosphate, glycerol phosphate, etc. Energy-poor 

 phosphate bonds will be indicated in the text as : — PO3H2. 



