CYTOCHROME c^ 



By J. POSTGATE 

 Microbiological Research Establishment, Porton, Wiltshire 



The cytochrome known as c^ is a pigment of one of the sulphate-reducing 

 bacteria. It is one of the few bacterial cytochromes to have been obtained 

 as yet as a single pure substance, and is unusual in being among the first 

 cytochromes to be observed in obligately anaerobic organisms. The present 

 contribution is a review of knowledge of its character and function, and is 

 preceded by a brief account of the relevant characters of the bacteria in which 

 it is found. 



THE SULPHATE-REDUCING BACTERIA 

 The writer has recently reviewed several aspects of the physiology and 

 taxonomy of these bacteria (Postgate, 1958, 1959) and the reader is referred 

 to these reviews for documentation of most of the statements made herein. 



These bacteria constitute a biochemical group of microbes wliich use the 

 oxygen atoms of the sulphate ion as terminal electron acceptors for respiration 

 in place of the free oxygen gas used by ordinary aerobic organisms. Sulphide 

 is formed as a by-product of respiration; a typical metabolic reaction is: 



2CH3CHOHCOO--f SO4— ^2CH3.COO- + 2H20-f2C02-f S - (1) 



Reaction (1) is almost universal throughout the group. Though species of 

 sulphate-reducing bacteria able to utilize acetate have been reported, they 

 are not well authenticated, and all the better-known species oxidize their 

 carbon compounds down to a fatty acid level of oxidation only. In this 

 character they might be regarded as anaerobic acetic acid bacteria. 



They have also biochemical analogies to the nitrate-reducing bacteria, 

 wliich can utilize the oxygen atoms of nitrate in place of free oxygen for 

 respiration, but are distinguished from this group in being very exacting 

 anaerobes. None of them grows in the presence of oxygen, and its absence 

 is usually insufficient to ensure growth; they require an environment of 

 negative Ej^ to start multiplication. In spite of their highly anaerobic charac- 

 ter, their metabolism resembles that of an aerobe or a nitrate-reducing 

 organism rather than that of an ordinary anaerobe because it is oxidative 

 rather than fermentative. This point has been amplified by the author 

 elsewhere (Postgate, 1958). 



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