416 Discussion 



compounds for reduction to sulphide is insufficient for production of adenosine 

 triphosphate (ATP) from adenosine diphosphate (ADP) and inorganic phosphate 

 with transfer of one electron. Even with the most extreme assumptions about con- 

 centration of redox complex involved, there is barely sufficient energy available. 

 However, two electrons must be transferred to reduce SO4 to SO3 . Perhaps the 

 double haem character of cytochrome c^ is attached to this requirement. There are no 

 haematin compounds other than cytochrome C3 present, whereas in other systems 

 there are at least two and usually more species of haematin compounds present. 

 Perhaps the necessity of multiple haem groups is satisfied in Desulphovibrio by putting 

 two haems in one protein. 



In relation to the use of potential difference for these calculations, one can state 

 that standard potentials cannot be applied directly but within a range of ± 100 mV 

 concentration effects only partially invalidate considerations based on such potentials. 

 An excellent discussion of this matter is given by Hill in his review of oxidation- 

 reduction potentials (Handbuch der Pflanzenphysiologie 5, 1960) 



Function of Cytochrome Cg in Hydroxylamine Reduction 



Egami: In relation to Postgate's findings, we found that hydroxylamine reduction by 



cytochrome Cg was enhanced by addition of Mn++, in the same way as was the reduction 



by the bacterial hydroxylamine reductase. 

 As to be expected from thermodynamical data, our experiments show that there is 



no requirement for activation of sulphite by adenosine triphosphate or other activating 



compounds before reduction. 

 Postgate: It is quite possible that our sulphite contained some sulphate, and that the 



apparent effect of ATP was due to its permitting reduction of sulphate in addition to 



the sulphite. 



The Occurrence of Cytochrome C3 in Desulphovibrio Desulphuricans 



Estabrook: It has been repeatedly stated that cytochrome c^ is the only cytochrome in 

 this organism. What is the evidence to support this claim? 



Postgate : The spectrum of intact organism shows no splitting in liquid nitrogen, nor does 

 the spectrum of extracted material. All extraction procedures yield cytochrome C3 in 

 the supernatant and residue and nothing else; the total haematin present as pyridine 

 haemochrome is quantitatively similar to the ordinary cytochrome C3 content. 



Kamen: Repeated efforts to extract haem compounds other than cytochrome C3 have 

 failed. Residues from all extraction procedures show only bound cytochrome C3. 



Evolutionary Aspects of the Sulphate-reducing Bacteria and of Cytochrome C3 



Legge: Could Postgate comment on the possible evolutionary position occupied by the 

 sulphate reducing bacteria ? 



Postgate: Thode and his colleagues in Canada {Geochim. cosmochim. Acta 3, 235, 1953) 

 have shown that the microbial sulphur cycle leads to partial fractionation of the sulphur 

 isotopes. Minerals such as sulphur or metal sulphides formed by way of microbial 

 action are enriched with the light natural isotope and the residual sulphate becomes 

 enriched with the heavier natural isotope. The major contribution to this fractionation 

 is made by the sulphate-reducing bacteria. 



By examining sulphur-bearing deposits of various geological ages they have detected 

 such fractionation as far back as 8 x 10* years ago, from which it seems likely that 

 massive sulphate-reduction occurred as far back as then. It seems that all dissimilatory 

 sulphate-reducers today are obligate anaerobes, and this is perfectly consistent with 

 the current belief that 8x10^ years ago the earth's atmosphere was anaerobic. If at 

 that time they required haematins of the cytochrome type, as they do today, it seems 

 reasonable to suppose that these cytochromes represent the evolutionary precursors 

 of the more complex aerobic cytochrome systems that are widespread today. 



