46 BACTERIAL ENZYMES 



reaction is AHg + Og = A + HgOg, resembling some of the 

 oxidase mechanisms discussed above. The distribution of 

 catalase is not universal amongst bacterial species, and those 

 organisms which are devoid of cytochrome are often devoid 

 also of catalase. It has been suggested that organisms such 

 as the Clostridia, the Streptococci, etc., owe their sensitivity 

 to the presence of oxygen to the fact that, being devoid of 

 catalase, they become poisoned by the formation of H2O2 

 under aerobic conditions. The amounts of HgOg which would 

 be formed and which might be toxic are so small as to be 

 beyond present methods of detection, and this point has yet 

 to be satisfactorily investigated. It is suggestive, however, 

 that Pneumococci can be protected in the presence of air by 

 pyruvic acid, and it is known that pyruvic acid and H2O2 

 react together chemically in such a way as to destroy the 

 H2O2. 



(c) Coenzyme systems: substrate types S3 and S4, Table 

 III. In Esch. coll we have coenzyme systems which are 

 apparently identical with those in animal- and yeast-cells; 

 thus L-malic acid dehydrogenase: 



CH2.COOH Malic CH2.COOH 



I + Coenzyme I ^^ | 



CHOH . COOH dehydrogenase qq cqOH 



L-Malic acid Oxalacetic acid 



+ Keduced coenzyme I 



and alcohol dehydrogenase: 



Alcohol 

 CH3.CH2OH -f Coenzyme I ^ -^ CH3.CHO + 



dehydrogenase 



Eeduced coenzyme 1. 



The coenzyme I of Esch. coli has never been isolated in 

 sufficient quantity and purity for its chemical structure to 

 be determined, but we know that (1) the alcohol and L-malic 

 dehydrogenases of Esch. coli will not reduce cytochrome or 

 methylene blue in the absence of a coenzyme which can be 



