Cytochrome Cg 417 



Kamen: Even if one accepts the possibility that the sulphate reducers represent a primitive 

 tyf)c of haematin system the question still remains how the haematin compounds 

 could be produced in the absence of oxygen. We do not know as yet how strict 

 anaerobes synthesize the haem moiety. The glycine-succinic-porphobilinogen sequence 

 appears to be based on an oxygen requirement. The possibility that some sort of 

 primitive haem existed in a pre-formed state prior to the development of the cyto- 

 chromes seems very far-fetched but cannot, of course, be ruled out. 



Lemberg : It appears likely from geochemical considerations that organisms using sulphate 

 as oxidant may have been evolved early. One may perhaps put up here an interesting 

 speculation. Cytochromes c appear to be extraordinarily complex haemoproteins 

 and still are found in these (probably) early organisms. One may perhaps bring 

 together the facts that oxygen was absent in the early earth atmosphere, that oxygen 

 is known to be required for the oxidative decarboxylation of propionic acid to vinyl 

 side chains in protohaem formation, and that cysteine-thioether linkages are found in 

 cytochromes c, by assuming that oxidative decarboxylation of the propionic acid side 

 chains by oxygen was preceded by one involving protein-cystine, leading simul- 

 taneously to thioether linkage formation, thus: 



— CH2CH2CO2H + R— S— S— R -> — CH— CH3 + CO2 + RSH 



SR 



Are these catalase-free organisms sensitive to oxygen, and is anything known of 

 their pyruvate metabolism ? 



Postgate: The organism is totally deficient in catalase; its metabolism is stopped by 

 oxygen but it is not killed even after several hours in air. Its pyruvate metabolism is 

 phosphoclastic, yielding acetyl phosphate, CO2 and either free hydrogen or ethanol. 



Falk : The question of the synthesis of protohaem by some anaerobes is a very interesting 

 one. In animal tissues, oxygen is required \n the biosynthetic pathway for the formation 

 of succinyl-coenzyme A, and again for the conversion of two propionic acid groups 

 to vinyl groups : all the remaining steps can be performed anaerobically. It has been 

 found recently (Falk et al.. Nature, Land. 184, 1217, 1959) that for protoporphyrin 

 synthesis in chicken erythrocytes in vitro there is a sharp optimal oxygen tension at 

 about 0-07 atmospheres ; oxygen tensions higher than this are increasingly inhibitory 

 and at the 0-2 atm. of sea-level air the inhibition is already some 25 %. We have found 

 that this inhibition by oxygen operates somewhere after porphobilinogen has been 

 formed, and before uroporphyrin appears, and we are currently looking for the 

 sensitive reaction. We feel that this inhibition is directly related to the adaptation of 

 haemopoiesis to oxygen tension (e.g. altitude) and to such adaptations as those found 

 in Daphnia and other Crustacea and invertebrates, though other factors also are no 

 doubt involved. 



It appears possible that this control is one of the factors in adaptive cytochrome 

 synthesis in micro-organisms also; in fact it was the finding of Moss {Aust. J. exp. Biol, 

 med. Sci. 34, 395, 1956) that the synthesis of cytochromes by Aerobactor aerogenes 

 shows an optimum against oxygen tension which led us to these studies. To return to 

 the less common but definite anaerobic formation of protohaem by micro-organisms, 

 the requirements, as different from animal tissues, are that the formation of succinyl 

 coenzyme A and of vinyl-groups must occur anaerobically. As far as the latter is 

 concerned, it is easy to postulate a dehydrogenase with a terminal acceptor other than 

 oxygen. In a way, we have an analogy for this, though in a reductive, not an oxidative, 

 dehydrogenase-type reaction, in the enzymic, non photo-catalytic conversion of 

 protochlorophyll to chlorophyll in certain plants. 



Kaziro: In connexion with the problem now under discussion, I would like to mention that 

 there are some bacteria, not strictly anaerobic, which produce hydrogen peroxide as a 

 metabolite. Since they have no catalase, the hydrogen peroxide accumulates in the 

 medium as they proliferate. 



It might be of some interest, as suggested now by Lemberg, to refer to a peculiar 

 disturbance observed in the cases of congenital deficiency of catalase in man. I have 



