Proton translocation coupled to the oxidation of carbon monoxide to CO2 and H2 in Methanosarcina barkeri

Authors


Correspondence to R. K. Thauer, Laboratorium für Mikrobiologie, Fachbereich Biologie, Philipps-Universität Marburg, Karlvon-Frisch-Straβe, D-3550 Marburg/Lahn, Federal Republic of Germany

Abstract

Cell suspensions of acetate-grown Methanosarcina barkeri mediate the conversion of CO and H2O to CO2 and H2. The reaction is coupled with the phosphorylation of ADP. Evidence is presented that CO oxidation by the cells is associated with the transient acidification of the suspension medium. Up to 2 mol vectorial protons were measured/mol CO oxidized when the transmembrane electrical gradient was kept low by the addition of valinomycin (20 nmol/mg protein) and KCl (200 mM) or of KSCN (50 mM). No transient acidification was observed in the presence of the protonophore tetrachlorosalicylanilide which stimulated rather than inhibited CO oxidation. Proton extrusion remained unaltered when the proton-translocating ATPase was specifically inhibited by dicyclohexylcarbodiimide. The latter finding indicates that proton translocation is associated with CO conversion to CO2 and H2 rather than with ATP hydrolysis in the cells. The data substantiate that the coupling of CO oxidation with ADP phosphorylation in M. barkeri occurs via a chemiosmotic mechanism.

Abbreviations
Ph4P+

tetraphenylphosphonium cation

ΔΦ=-(RT/F)ln ([Ph4P+]in/[Ph4P+]out)

transmembrane electrical gradient

ΔpH = (pHin -pHout)

transmembrane chemical gradient of H+

Δp (=ΔΦ -0.062 ×ΔpH)

proton motive force in volts

TCS

3,5,3′,4′-tetrachlorosalicylanilide

→ H+

vectorial protons

Enzymes
 

Carbon monoxide dehydrogenase (EC 1.2.99.2)

 

hydrogenase (EC 1.12.–.–)

 

ATPase or ATP synthase, proton translocating (EC 3.6.1.34)

 

methyl-coenzyme M reductase (EC 1.8.–.–)

Ancillary