3.D.7 The H₂:Heterodisulfide Oxidoreductase (HHO) Family

A single multicomponent system, the membranous H₂:heterodisulfide oxidoreductase system of the methanogenic archaeon, Methanosarcina mazei Gö1 catalyzes (1) the H₂-dependent reduction of 2-hydroxyphenazine by 2-hydroxyphenazine-dependent hydrogenase and (2) the dihydro-2-hydroxyphenazine-dependent reduction of the heterodisulfide of coenzyme M-SH and coenzyme B-SH (CoM-S-S-CoB) by heterodisulfide reductase. Washed inverted vesicles of M. mazei couple both processes with the transfer of H⁺ across the cytoplasmic membrane. The H⁺/2e- ratio measured was 0.9 for each reaction, and the electrochemical proton gradient generated could drive ATP synthesis via the H⁺-translocating ATP synthase.

M. mazei can grow on (1) H₂ + CO₂, (2) methanol, (3) methylamines and (4) acetate. In the pathways of methanogenesis, all substrates are converted to methyl-S-CoM (2-methylthioethane-sulfonate) either by reduction of CO₂ or by demethylation of methanol, methylamine or acetate. Methane is formed from methyl-S-CoM by a two electron reduction reaction catalyzed by methyl-S-CoM reductase which uses HS-CoB (7-mercaptoheptanoylthreonine phosphate) as the electron donor. This reaction produces the heterodisulfide, CoM-S-S-CoB, which is the terminal electron acceptor of the membraneous electron transport system of M. mazei. If H₂ is present, a membranous F₄₂₀ [(N-L-lactyl-γ-L-glutamyl)-L-glutamic acid phosphodiester of 7,8-didemethyl-8-hydroxy-5-deazariboflavin-5&146;-phosphate]-nonreducing hydrogenase channels electrons via b-type cytochromes to the heterodisulfide reductase which reduces the terminal electron acceptor. This electron transport system, referred to as H₂:heterodisulfide oxidoreductase, is coupled to proton translocation across the cytoplasmic membrane. When cells are grown on methanol, part of the methyl groups are oxidized to CO₂, and reducing equivalents are transferred to coenzyme F₄₂₀. In Methanosarcina strains, reduced F₄₂₀ (F₄₂₀H₂) is reoxidized in a reaction catalyzed by the membrane-bound F₄₂₀H₂ dehydrogenase which is part of the F₄₂₀H₂:heterodisulfide oxidoreductase. Electrons are channeled to the heterodisulfide reductase, resulting in both the reduction of CoM-S-S-CoB and the formation of an electrochemical proton gradient. The resulting ΔμH⁺ (transmembrane electrochemical gradient of H⁺) is the driving force for ATP synthesis from ADP plus P_i as catalyzed by an A₁A₀-type ATP synthase (TC #3.2). The system is probably subject to respiratory control.

A proposed energy-coupling mechanism is presented in Figure 3 of Ide et al. (1999). Five gene products are involved where electrons are passed from H₂ to the heterodimeric hydrogenase, VhoGA, then to a cytochrome b₁, VhoC, then to methanophenazine (which may serve as the transmembrane proton carrier), and finally to the heterodisulfide oxidoreductase, HdrDE, which reduces CoB-S-S-CoM.

Homologues of VhoAG included numerous hydrogenases of bacteria and archaea; VhoC is distantly related to a quinone-reactive Ni²⁺/Fe²⁺-hydrogenase cytochrome b in Wolinella succinogenes; HtrD is homologous to numerous bacterial and archaeal enzymes including fumarate reductase and glycerol-3-P dehydrogenase, and HtrE, a second b-type cytochrome with five putative transmembrane spanners, has distant homologues in bacteria, other archaea and eukaryotes. In spite of the apparent mosaic origin of the components of the system, the complete system may be specific to methanogenic archaea. The H₂:heterodisulfide oxidoreductase may be related to F₄₂₀H₂ and CO:heterodisulfide oxidoreductases which are also believed to generate a proton motive force by redox potential-driven H⁺ translocation.

The two reactions of the H₂:heterodisulfide oxidoreductase which are coupled to H⁺ translocation (extrusion) are:

(1) H₂ + 2-hydroxyphenazine + H⁺ (in) → dihydro-2-hydroxyphenazine + H⁺ (out)

(2) dihydro-2-hydroxyphenazine + CoM-S-S-CoB + H⁺ (in) →
2-hydroxyphenazine + HS-CoM + HS-CoB + H⁺ (out).

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