174 



UNITY AND DIVERSITY IN BIOCHEMISTRY 



postulate that the reaction begins by the fixation of lactic acid and the 

 coenzyme DPN+ on the enzyme protein molecule (apoenzyme). 



Lactic acid 



DPN 



Apoenzyme 



A proton is then lost by the lactic acid to the solution and, simul- 

 taneously, an electron passes from the lactic acid to the apoenzyme. 



The electron travels along the apoenzyme until it reaches the coenzyme 

 where it combines with a proton furnished by the solution and the DPN+ 

 is reduced to DPNH. 



I 



O 



CH, CH COOH 



H+ 



I 



DPN 



O 



I 



CH, CH COOH 



H-DPN 



Apoenzyme 



Apoenzyme 



The role of pyridoxal phosphate as a coenzyme in the decarboxylation 

 of amino acids can, according to Mendeles, Koppelman and Hanke (1954), 

 be explained by the following scheme : 



. coo- 

 * I 



H-C=0 HsN-C-H 



HO 



HsC 



sC t^Vj. 



CHjOPOaHz 



+H2O 



COO" 

 I 

 H— C=N— C-H 

 I 



-H,0 J^ R 



H 



-CO, 



+COa 



H-C=N-C— H 

 I 

 R 



H 



H 



H— C=0+H8N -C— H 



— HoO 



+H2O 



H 



H-C=N.-C— H 



N 

 H 



-H* 



+H* 



H-C-N=C— H 



A k 



N' 

 H 



