METABOLIC FUNCTIONS OF B VITAMINS 219 



starch), a series of reactions occur which eventually produce pyruvic 

 acid. Although other ways in which hexoses can be degraded are known, 3 

 the mechanism employed by almost all organisms is the one represented 

 by the classical scheme of fermentation. 1 The individual reactions involve 

 the formation and cleavage of phosphate esters, isomerizations, the 

 cleavage of the hexose diphosphate into two triose phosphates (a reaction 

 which is the reverse of an aldol condensation), the dehydration of a 

 /^-hydroxy acid, and one dehydrogcnation: 



1 . Transesterification : 



glucose-1-phosphate ^ "* glucose-6-phosphate 



2. Phosphorylation: 



glucose-6-phosphate+ATP > glueose-l,6-diphosphate+ADP 



3. Isomerization: 



glucose-l,6-diphosphate ^ ^ fructose-l,6-diphosphate 



4. Aldol cleavage or formation: 



fructose-l,6-diphosphate ^ "*• glyceraldehyde-3-phosphate + 



dihydroxyacetone phosphate 



5. Isomerization: 



dihydroxyacetone phosphate ^ *" glyceraldehyde-3-phosphate 



6. Nonenzymalic formation and disintegration of a carbonyl-phosphoric acid addition 

 product (see p. 140): 



2(glyceraldehyde-3-phosphate) +2H 3 P0 4 =^=^ 



2 (glyceraldehyde-3-phosphate) -phosphoric acid addition product 



7. Hydrogenation-dehydrogenation: 



2(glyceraldehyde-3-phosphate) -phosphoric acid addition product + 



2DPN =^= 2(3-phosphoglyceroyl phosphate) +2(DPN-2H) 



8. Phosphorylation: 



2(3-phosphoglvceroyl phosphate) +2ADP -v ^ 



2(3-phosphoglyceric acid) +2 ATP 



9. Transesterification: 



2(3-phosphoglyceric acid) =^=^= 2(2-phosphoglyceric acid) 



10. Dehydration or hydration (involving /3-hydroxy acid): 



2(2-phosphoglyceric acid =^=*= 2(phospho(enol)pyruvic acid)+2H 2 



1 1 . Phosphorylation: 



2(phospho(enol)pyruvic acid)+2ADP =5=^ 2(pyruvic acid)+2ATP 



Of the eleven reactions, only one, the dehydrogenation of the diphosphate 

 derivative of glyceraldehyde, is of the type which requires a B vitamin 

 coenzyme. The hydrogen atoms from this reaction are accepted by the 

 diphosphopyridine nucleotide. The net transformation of the organic 

 molecules brought about by the process and the B vitamin coenzymes 

 involved may be summed up thus: 



hexose unit of polysaccharide 

 DPN 



2 [2H] DPN 2 pyruvic acid 



