Heterodisulfide reductase

Murakami E, U Deppenmeier, SW Ragsdale (2001) Characterization of the intramolecular electron transfer pathway from 2-hydroxyphenazine to the heterodisulfide reductase from Methanosarcina thermophila. J Biol Chem 276 2432-2439. 

In the first step of the reductive branch of this metabolic pathway three out of four methyl groups are transferred from methanol to CoM-SH (13) by methyl transferases, with formation of methyl-S-CoM (21) (Scheme 1) [21]. The transformation of 21 and CoB-SH (15) into methane and CoB-S-S-CoM (22) is catalyzed by the methyl-CoM reductase. Again, reductive cleavage of 22 is mediated by the heterodisulfide reductase [22]. The oxidative part involves oxidation of... 

Methanogenesis from acetate in extracts of Methanosarcina does not require membrane addition [260]. However, this does not exclude a function for cytochromes in acetoclastic methanogenesis by whole cells. Rather, the role of H2 in cell extracts, the ability of cytochrome b from Methanosarcina species to react with CO, and the observation that membrane-bound cytochromeb of M. barkeri is reduced by H2, and is oxidized by CH3-CoM -I- ATP or CH3-C0M + acetyl-phosphate, all point to the participation of cytochromes in Methanosarcina. A role of cytochromes in transport of electrons generated from carboxyl-group oxidation to the heterodisulfide reductase is a logical hypothesis. 

The MR from M. thermophila does not require A proteins in vitro [268]. The enzyme is reductively activated by Ti(IlI) (a process stimulated by, but not requiring, ATP), to give a specific activity of 0.22 pmol/minmg. The minimal components needed for activity are MR, Ti(III) and HSHTP, but physiologically it is likely that ferredoxin, cytochrome b, and heterodisulfide reductase are involved. The MR from M voltae, M. jannaschii, and M. barkeri 227 have also been purified [386], and it was shown with these enzymes that crude A components were active when used with MR from other methanogens. 

Elucidation of the structure of component B (HSHTP) and the discovery of heterodisulfide reductase are two recent successes in work on the biochemistry of methanogens [131,174,178,197,423]. The enzyme heterodisulfide reductase (HR) carries out Reaction (22), regenerating CoM and HSHTP [131,174,423]. HR is found in all methanogens examined [55], and has been proposed to be involved in energy conservation. Two approaches have been used to study this enzyme characterization of the purified enzyme, and studies with membranes and everted vesicles. 

Several portions of the pathway are still unclear. How the methyl of methanol enters the reversed methanogenic pathway is unknown, since some suggest it may not proceed via methyl-CoM nonetheless, evidence is clear that most of the reversed H2-CO2 path is used. Also, our understanding of methyl-transfer reactions at several portions of the pathways remains incomplete, e.g. the way in which methanol is reduced to methane, the enzymes involved in methyl transfer in CO2 methanogenesis, and routes of nonmethanol methyl-substrate entry into the path. In several cases, the source of electrons for a reductive step is unknown, e.g. the heterodisulfide reductase and formyl-methanofuran dehydrogenase steps. 

The final step in methanogenesis is the reductive demethylation of CH3-S-CoM to CH4. This reduction involves two reactions CH3-S-C0M is reduced with jV-7-mercaptoheptanoylthreonine phosphate (H-S-HTP) (Fig. 2B) as electron donor to yield CH4 and a heterodisulfide of H-S-CoM and H-S-HTP (CoM-S-S-HTP) (Reaction 7, Table 2). This reaction is catalyzed by CH3-S-C0M reductase [70-72] which contains a nickel porphinoid, factorF430, as prosthetic group (Fig. 2C) (for a recent review see Friedmann et al. [73]). The subsequent reduction of the heterodisulfide with H2 to yield H-S-HTP and H-S—CoM (Reaction 8, Table 2) is catalyzed by CoM-S-S-HTP-dependent heterodisulfide reductase. The enzyme is an iron-sulfur protein containing FAD as prosthetic group [74]. The physiological electron donor for the heterodisulfide reductase is not known. 

Fig. 5. Proposed mechanism of ATP synthesis coupled to methyl-coenzyme M (CH3-S-C0M) reduction to CH4 The reduction of the heterodisulfide (CoM-S-S-HTP) as a site for primary translocation. ATP is synthesized via membrane-bound -translocating ATP synthase. CoM-S-S-HTP, heterodisulfide of coenzyme M (H-S-CoM) and 7-mercaptoheptanoylthreonine phosphate (H-S-HTP) numbers in circles, membrane-associated enzymes (1) CH3-S-C0M reductase (2) dehydrogenase (3) heterodisulfide reductase 2[H] can be either H2, reduced coenzymeF420 F420H2) or carbon monoxide the hatched box indicates an electron transport chain catalyzing primary translocation the stoichiometry of translocation (2H /2e , determined in everted vesicles) was taken from ref. [117] z is the unknown If /ATP stoichiometry A/iH, transmembrane electrochemical...

---