The proton/electron ration of the menaquinone-dependent electron transport from dihydrogen to tetrachloroethene in "Dehalobacter restrictus"

Abstract

In the anaerobic respiration chain of "Dehalobacter restrictus," dihydrogen functioned as the electron donor and tetrachloroethene (PCE) functioned as the electron acceptor. The hydrogenase faced the periplasm, and the PCE reductase faced the cytoplasmic side of the membrane. Both activities were associated with the cytoplasmic membrane. UV spectroscopy showed that membrane-bound menaquinone (MQ) was reduced by oxidation of H2 and reoxidized by reduction of PCE, indicating that MQ functions as an electron mediator. Fast proton liberation (t1/2 = 6 +/- 2 s) during electron transport from H2 to PCE and to trichloroethene (TCE) after addition of either PCE or TCE to H2-saturated cells resulted in an extrapolated H+/e- ratio of 1.25 +/- 0.2. This ratio indicated that besides the formation of protons upon oxidation of H2, vectorial translocation of protons from the inside to the outside could also occur. Proton liberation was inhibited by carbonylcyanide m-chlorophenylhydrazone (CCCP), 2-n-heptyl-4-hydroxyquinoline N-oxide (HOQNO), and CuCl2. Fast proton liberation with an H+/e- ratio of 0.65 +/- 0.1 was obtained after addition of the MQ analog 2,3-dimethyl-1,4-naphthoquinone (DMN) as an oxidant pulse. This acidification was also inhibited by CCCP, HOQNO, and CuCl2. Oxidation of reduced DMN by PCE was not associated with fast acidification. The results with DMN indicate that the consumption and release of protons associated with redox reactions of MQ during electron transfer from H2 to PCE both occurred at the cytoplasmic side of the membrane. The PCE reductase was photoreversibly inactivated by 1-iodopropane, indicating that a corrinoid was involved in the PCE reduction.