58 OXIDATION-REDUCTION POTENTIALS 



The oxidation of reduced coenzyme by cytochrome is stated to be catalysed 

 always by flavoprotein but Dickens and Mcllwain have shown that phenazine dyes 

 react directly with coenzymes in the hexose monophosphate system and are active 

 oxygen catalysts in the absence of fiavoproteins. It is perhaps significant that 

 phenazines and quinones share with flavoprotein the property of forming semi- 

 quinones. In a later section, a fuller description of semiquinone pigments is given. 



By indirect comparison with the ^-hydroxybutyric acid system the normal 

 potential of cozymase I has been estimated to be — 0-28 v. at pH 7 (Borsook, 1940 ; 

 Dewan and Green, 1937) ; direct potentiometric determination of the potential 

 was not found to be possible. Comparison with the lactate-pyruvate system gives a 

 value of —0-325 v. at pH 7-4 (Clark, 1938). 



Hemophilus influenzce cannot synthesise the Coenzyme I necessary for growth 

 processes and the coenzyme must be supj)lied (V factor of Lwoff and Lwoff ). On the 

 other hand some micro-organisms can synthesise it so that symbiotic growth of 

 H. influenzw with some other organisms can occur in culture medium not containing 

 any of the coenzyme. 



ANEURINE, THIAMINE, VITAMIN B^, COCARBOXYLASE 



Cocarboxylase is the pyrophosphoric ester of Vitamin B^ (Aneurin or Thiamin). 

 It functions as coenzyme for carboxylase which catalyses the conversion of pyruvic 

 acid to acetaldehyde in the final stages of alcoholic fermentation (see next chapter) : — 



CH3CO.COOH -> CH3CHO -f CO2 



When dried yeast is washed with phosphate it loses its power of decarboxylation 

 owing to the removal of cocarboxylase, which has been isolated also from blood, liver, 

 kidney, etc. Aneurin contains a thiazole ring, but appears to behave as a thiol 

 compound capable of reversible oxidation-reduction to the disulphide. 



NH2 HO 

 Thiazole N— CH 



N=iC tH-S 



CH3C /C-CHj-r 



N-CH 



Thiol N— CH C=C-C2H40H 



CH, 



'3 



Thiamin 



