Transport in methanogens

The uptake of essential trace elements, e.g. Na, K, Ni, Co, Mg , POj , MoOj and, in the case of auxotrophic mutants, of coenzymes, vitamins, and amino acids requires transport systems. In addition to Na transport (see previous sections), active transport systems for Ni, K, PO3, coenzyme M (H-S-CoM), methyl-coenzyme M (CH3-S-C0M), isoleucine and other branched-chain amino acids have been described in methanogens (for recent literature on ion transport in prokaryotes see refs. [251,252]). 

Growth of Methanobrevibacter (formerly Methanobacterium ruminantium) on H2/CO2 is dependent on exogenous coenzyme M (H-S-CoM) as a vitamin [253]. The uptake of 

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Bioenergetics and transport in methanogens and related thermophilic archaea... 

In summary, methanophenazine (10) is the first phenazine whose involvement in the electron transport of biologic systems could be established. The experiments indicate that its role in the energy metabolism of methanogens corresponds to that of ubiquinones in mitochondria and bacteria. 

What should be emphasized is that the redox potentials measured for 10 and 22 allow for both the reduction of 10 to dihydro-10 via F420H2 and H2, and the oxidation of dihydro-10 to 10 by 22. This finding, supported by electrochemical experiments, also strongly corroborates the hypothesis that 10 plays a prominent role as an electron carrier in the electron transport system of methanogens. 

The sodium gradient in the methanogen Methanococcus voltae is exploited in the transport of isoleucine as a positively charged complex,74 where concentration gradients over 100 can be achieved. It is noteworthy that the methanogens represent one of the few cases where a growth requirement for Na+ can be shown.75,76... 

In acetate-catabohzing methanogens, enzyme-bound methylcorrinoids are also indicated to serve as intermediates in methyl-group transport. There, methane and carbon dioxide are formed from acetic acid (via acetyl coenzyme A, whose acetyl group is spht into carbon monoxide and a corrin-bound methyl group). Accordingly, the methyl-group... 

The findings clearly indicate that the last step of the CO2 reduction pathway, the reduction of CoM-S-S-HTP by H2, is a coupling site for ATP synthesis. It is concluded that CoM-S-S-HTP is the terminal electron acceptor ( = —200 mV) of a membrane-bound electron transport chain, with molecular H2 ( ° = -414mV) being the electron donor in hydrogenotrophic methanogens [115]. The physiological electron donor for CoM-S-S-HTP reduction is not known (Fig. 5). 

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