Structural insights into the electron/proton transfer pathways in the quinol:fumarate reductase from Desulfovibrio gigas

Hong-Hsiang Guan,Yen-Chieh Huang,Chun-Jung Chen,Masato Yoshimura,Phimonphan Chuankhayan,Li-Ying Chen,Shao-Kang Chen,Nai-Chi Chen,Yin-Cheng Hsieh,Atsushi Nakagawa,Chien-Chih Lin,Sunney I Chan,Pei-Ju Lin

doi:10.1038/s41598-018-33193-5

Abstract

The membrane-embedded quinol:fumarate reductase (QFR) in anaerobic bacteria catalyzes the reduction of fumarate to succinate by quinol in the anaerobic respiratory chain. The electron/proton-transfer pathways in QFRs remain controversial. Here we report the crystal structure of QFR from the anaerobic sulphate-reducing bacterium Desulfovibrio gigas (D. gigas) at 3.6 Å resolution. The structure of the D. gigas QFR is a homo-dimer, each protomer comprising two hydrophilic subunits, A and B, and one transmembrane subunit C, together with six redox cofactors including two b-hemes. One menaquinone molecule is bound near heme bL in the hydrophobic subunit C. This location of the menaquinone-binding site differs from the menaquinol-binding cavity proposed previously for QFR from Wolinella succinogenes. The observed bound menaquinone might serve as an additional redox cofactor to mediate the proton-coupled electron transport across the membrane. Armed with these structural insights, we propose electron/proton-transfer pathways in the quinol reduction of fumarate to succinate in the D. gigas QFR.

Highlights

Desufovibrio gigas (D. gigas), an anaerobic sulphate-reducing bacteria (SRB) with versatile anaerobic respiratory mechanisms[1], requires specific enzymes to mediate the anaerobic respiratory processes that catalyze the sequential reduction reactions to obtain energy
The two-electron transfer through hemes and Fe-S clusters was previously proposed based on the locations of the redox cofactors in the quinol:fumarate reductase (QFR) structure of W. succinogenes[5]
Native succinate:quinone reductase (SQR) of E. coli contains one heme; an E. coli SQR mutant lacking this heme retains the activity of ubiquinol reductase[27], indicating that the exact role of the heme in this electron-transport protein is not fully understood

Summary

Introduction

Desufovibrio gigas (D. gigas), an anaerobic sulphate-reducing bacteria (SRB) with versatile anaerobic respiratory mechanisms[1], requires specific enzymes to mediate the anaerobic respiratory processes that catalyze the sequential reduction reactions to obtain energy. The terminal electron acceptors in these reactions are moderate oxidants, such as sulphate, sulphite, other sulphur compounds, and fumarate, rather than the strong oxidants, e.g. dioxygen, utilized in aerobic respiration. One of these crucial enzymes is quinol:fumarate reductase (QFR), which is an integral membrane protein with three subunits: a flavoprotein (subunit A), an iron-sulphur protein (subunit B), and a membrane-embedded subunit (subunit C). QD is apparently not redox-active in E. coli QFR10,11 Based on these structural findings, electron transfer mechanisms have been proposed to account for the quinol reduction of fumarate to succinate to enable the anaerobic microorganisms to grow on fumarate[4,5,7]. Detailed comparison of the structures and redox cofactors among the various QFRs has allowed us to propose the pathways of electron and proton transfers in QFR from D. gigas during turnover

Methods

Results

Conclusion