Electron transport across the cell envelope via multiheme c-type cytochromes in Geobacter sulfurreducens

Abstract

Extracellular electron transfer (EET) enables certain microorganisms to respire using soluble and insoluble extracellular electron acceptors by transporting electrons across the cell envelope. Among these, G. sulfurreducens serves as a model organism for understanding direct EET pathways, where multiheme c-type cytochromes mediate electron transport from intracellular redox carriers to extracellular acceptors such as Fe(III) oxides and electrodes. This review focuses on heme-dependent electron transfer in Geobacter sulfurreducens, detailing the roles of inner membrane cytochromes, periplasmic carriers, outer membrane conduits, and recently characterized extracellular nanowires formed by polymerized multiheme c-type cytochromes, including OmcS, OmcE, and OmcZ. We examine the state of understanding of their physiological function, their structural features, expression patterns, and essentiality under various respiratory conditions. These insights advance our understanding of microbial anaerobic respiration and have implications for biogeochemical cycling, bioenergy generation, and bioremediation. The molecular architecture, assembly mechanisms, and secretion pathways of multiheme c-type cytochrome nanowires remain active areas of investigation, offering promising directions for future research and biotechnological innovation in engineered microbial systems.

Keywords: cytochrome nanowire; electron transport; extracellular electron transfer; geobacter sulfurreducens; outer membrane cytochromes.

FIGURE 1

All (c-cyts) and multiheme (MHCs) c-type cytochromes as a fraction of total unique proteins encoded in the G. sulfurreducens genome as compared to other organisms. Inset: number of MHCs in G. sulfurreducens and other model organisms. Proteomes were downloaded June 2025 from UniProt (proteome ID Gs: UP000000577; Ec: UP000000625 Sc: UP000002311; Hs: UP000005640).

FIGURE 2

Components of the electron transfer pathway in G. sulfurreducens, from intracellular acetate oxidation to outer membrane and extracellular cytochromes. IM = inner membrane, P = periplasm, OM = outer membrane.