THE CHEMISTRY OF PHOTOSYNTHESIS 11 



In the presence of Mg ions, the enzyme enolase catalyses the conversion of 

 2-phosphogIyceric acid into phospho-^«o/-pyruvic acid 



COOH COOH 



I -HoO I 



HCOPOsH. ^ > CO~PO:,H.. 



I enolase II 



HoCOH CHo 



2-phosphoglyceric phospho-enol- 



acid pyruvic acid 



The energy-rich phosphate bond produced is transferred by pyruvic phos- 

 phokinase to ADP and pyruvic acid is formed. Mg ions and K ions are in- 

 dispensable to this reaction. 



COOH COOH 



I pyruvic phosphokinase \ 

 C0~P0.3Hi + ADP > CO + ATP 



II K ions Mg ions I 

 CHo CH3 



phospho-enol- pyruvic 



pyruvic acid acid 



In glycolysis there are two main reactions producing energy-rich phosphate 

 bonds: 



1. the conversion of 3-phosphoglyceraldehyde into 3-phosphoglyceric acid. 



2. the conversion of 2-phosphoglyceric acid into pyruvic acid. 



All the reactions discussed are reversible. Under appropriate conditions 

 both pyruvic acid and the intermediate products can be converted back to 

 glucose. The only irreversible reaction is the formation of glucose-6-phos- 

 phate from glucose. However, this reaction can be reversed by a special phos- 

 phatase. 



Pyruvic acid is reduced to lactic acid by lactic acid dehydrogenase (War- 

 burg's "reducing fermentation enzyme"). 



CH3.COCOOH -f DPNH + H+-> CHrCHOHCOOH + DPN + 



It is to be noted that DPNH produced during the oxidation of 3-phospho- 

 glyceraldehyde is oxidized during the reduction of pyruvic acid. Thus, there 

 exists an oxidoreduction reaction between 3-phosphoglyceraldehyde and py- 

 ruvic acid. Glycolysis is therefore neither an oxidation nor a reduction re- 

 action. This is shown by the over-all reaction of glycolysis 



C6H12O6 «=i 2 CsHeOs 

 glucose lactic acid 



The whole reaction mechanism can be represented as follows: 



oxidizing fermen- 

 1. 3-phosphoglyceraldehyde + DPN+ + H:iP04 ^- — ^ 



tation enzyme 



1,3-diphosphoglyceric acid + DPNH + H + 



