Molecular basis of maintaining an oxidizing environment under anaerobiosis by soluble fumarate reductase

Sunghwan Kim,Chris A Kaiser,Hara Kang,Chang Min Kim,Jaeyoung Song,Jae Young Choi,Young-Jin Son,Hyun Ho Park,Rahel K Siegenthaler,Tae-Ho Jang,Younho Lee

doi:10.1038/s41467-018-07285-9

Abstract

Osm1 and Frd1 are soluble fumarate reductases from yeast that are critical for allowing survival under anaerobic conditions. Although they maintain redox balance during anaerobiosis, the underlying mechanism is not understood. Here, we report the crystal structure of a eukaryotic soluble fumarate reductase, which is unique among soluble fumarate reductases as it lacks a heme domain. Structural and enzymatic analyses indicate that Osm1 has a specific binding pocket for flavin molecules, including FAD, FMN, and riboflavin, catalyzing their oxidation while reducing fumarate to succinate. Moreover, ER-resident Osm1 can transfer electrons from the Ero1 FAD cofactor to fumarate either by free FAD or by a direct interaction, allowing de novo disulfide bond formation in the absence of oxygen. We conclude that soluble eukaryotic fumarate reductases can maintain an oxidizing environment under anaerobic conditions, either by oxidizing cellular flavin cofactors or by a direct interaction with flavoenzymes such as Ero1.

Highlights

Osm[1] and Frd[1] are soluble fumarate reductases from yeast that are critical for allowing survival under anaerobic conditions
The single gene deletion of OSM1 significantly restricts cell growth under oxygen-deficient conditions, whereas the double gene deletion of FRD1 and OSM1 is lethal under the same conditions, suggesting that soluble fumarate reductases are critical for survival under anaerobic conditions[4,12]
These findings indicate that Osm[1] and Frd[1] might function differently than the remainder of the Fcc[3] family of fumarate reductase, possibly by utilizing a distinct electron transport mechanism

Summary

Introduction

Osm[1] and Frd[1] are soluble fumarate reductases from yeast that are critical for allowing survival under anaerobic conditions. Fcc[3] is organized into three distinct domains, the N-terminal cytochrome domain that contains four heme groups, which are critical for electron transfer, the flavin domain containing the non-covalently bound FAD and the substrate-binding active site, and the clamp domain, which is believed to be involved in controlling substrate access to the active site via structural movement This family of enzymes uses four heme centers in the cytochrome domain to allow for appropriate electron transfer to occur. A sequence comparison within the Fcc[3] family has suggested that both Osm[1] and Frd[1] contain only a flavin domain and a clamp domain, and lack the cytochrome domain, which is critical for electron transfer (Fig. 1b) These findings indicate that Osm[1] and Frd[1] might function differently than the remainder of the Fcc[3] family of fumarate reductase, possibly by utilizing a distinct electron transport mechanism. Genetic studies have suggested that Osm[1] functionally co-operates with an ER-resident enzyme, Ero[113–15]

Methods

Results

Conclusion