86 OXIDATION-REDUCTION POTENTIALS 



CH2O— ph.CHOH.CH(OH)0— ph+C0.I->CH20— pli.CH0H.C00^ph+C0.I.2H 



The accumulation of reduced coenzyme is prevented because this reaction is 

 coupled with a later reaction (No. 13) in the chain, for when pyruvic acid is 

 reduced to lactic acid the reduced coenzyme is reconverted to the oxidised form. 



(9) A phosphokinase now transfers the r-^ph group to adenosine diphosphate 

 and 3-phosphoglycerate is formed. 



(10) Phosphoglyceromutase now transfers the ester phosphate group forming 

 2-phosphogly cerate . 



(11) Enolase (which is activated by Mg and inhibited by fluoride) converts 

 2-phosphoglycerate to phosphoenolpyruvate which contains one r-^ph. 



CH2OH.CHO— ph.COOH -> CH2 = COr^ph.COOH + H^O 



(12) A phosphokinase enzyme transfers one ^ph group to adenosine di- 

 phosphate and by a keto-enol transformation pyruvic acid is formed. 



CH2 = CO ^ph.COOH + ADP->CH2 = COH.COOH + ATP -> CH3CO.COOH 



(13) The second oxidation-reduction reaction of the chain now occurs. The 

 reduced coenzyme I formed in reaction 8 together with lactic dehydrogenase 

 reduces pyruvic acid to lactic acid. 



CH3CO.COOH + C0.I.2H -> CH3 CHOH.COOH + CO.I 



(See Baldwin, 1948 for a discussion.) 



FREE ENERGY CHANGES OF GLYCOLYSIS 



The net result of the anaerobic glycolysis chain is that in the conversion of one 

 molecule of glucose (from glycogen) to two molecules of lactic acid, one r-'ph bond is 

 absorbed in phosphorylation and four r-'ph bonds are made available. Thus the free 

 energy of three r^ph bonds or nearly 36,000 calories are accounted for. By calcula- 

 tion of the difference in the free energy contents of glucose and lactic acid there 

 should be 58,000 calories. It is not clear where the missing free energy is to be traced. 

 In the two oxidation-reduction reactions the coenzyme is both reduced and oxidised 

 so that no free energy is involved, and since the oxidation of diphosphoglyceraldehyde 

 and reduction of pyru^^c acid occur at similar Eo^ levels there should be no free 

 energy change here. 



GLYCOGEN AND STARCH SYNTHESIS 



In animals carbohydrate is stored as glycogen and in plants as starch : in each 

 case sugar can be brought into the reaction cycle immediately it is required by the 

 agency of phosphorylase : — 



Phosphorylase 

 Glycogen + H3PO4 > d-oc-glucopyranose-1-phosphate 



The reaction is reversible and equilibrium is established between divalent phosphate 

 ions of the inorganic phosphate and glucose-1-phospliate. In the case of starch the 

 long chained (amylose fraction) is formed and probably some other enzyme is required 

 for the formation of the branched amylopectin structure. 



