Direct Electron Transfer between the frhAGB-Encoded Hydrogenase and Thioredoxin Reductase in the Nonmethanogenic Archaeon Thermococcus onnurineus NA1

Abstract

To date, NAD(P)H, ferredoxin, and coenzyme F-420 have been identified as electron donors for thioredoxin reductase (TrxR). In this study, we present a novel electron source for TrxR. In the hyperthermophilic archaeon Thermococcus onnurineus NA1, the frhAGB-encoded hydrogenase, a homolog of the F-420- reducing hydrogenase of methanogens, was demonstrated to interact with TrxR in coimmuno-precipitation experiments and in vitro pulldown assays. Electrons derived from H-2 oxidation by the frhAGB-encoded hydrogenase were transferred to TrxR and reduced Pdo, a redox partner of TrxR. Interaction and electron transfer were observed between TrxR and the heterodimeric hydrogenase complex (FrhAG) as well as the heterotrimeric complex (FrhAGB). Hydrogen-dependent reduction of TrxR was 7-fold less efficient than when NADPH was the electron donor. This study not only presents a different type of electron donor for TrxR but also reveals new functionality of the frhAGB-encoded hydrogenase utilizing a protein as an electron acceptor. IMPORTANCE This study has importance in that TrxR can use H-2 as an electron donor with the aid of the frhAGB-encoded hydrogenase as well as NAD(P)H in T. onnurineus NA1. Further studies are needed to explore the physiological significance of this protein. This study also has importance as a significant step toward understanding the functionality of the frhAGB-encoded hydrogenase in a nonmethanogen; the hydrogenase can transfer electrons derived from oxidation of H-2 to a protein target by direct contact without the involvement of an electron carrier, which is distinct from the mechanism of its homologs, F-420-reducing hydrogenases of methanogens.