Mind the gap: cytochrome interactions reveal electron pathways across the periplasm of Shewanella oneidensis MR-1

Abstract

Extracellular electron transfer is the key metabolic trait that enables some bacteria to play a significant role in the biogeochemical cycling of metals and in bioelectrochemical devices such as microbial fuel cells. In Shewanella oneidensis MR-1, electrons generated in the cytoplasm by catabolic processes must cross the periplasmic space to reach terminal oxidoreductases found at the cell surface. Lack of knowledge on how these electrons flow across the periplasmic space is one of the unresolved issues related with extracellular electron transfer. Using NMR to probe protein-protein interactions, kinetic measurements of electron transfer and electrostatic calculations, we were able to identify protein partners and their docking sites, and determine the dissociation constants. The results showed that both STC (small tetrahaem cytochrome c) and FccA (flavocytochrome c) interact with their redox partners, CymA and MtrA, through a single haem, avoiding the establishment of stable redox complexes capable of spanning the periplasmic space. Furthermore, we verified that the most abundant periplasmic cytochromes STC, FccA and ScyA (monohaem cytochrome c5) do not interact with each other and this is likely to be the consequence of negative surface charges in these proteins. This reveals the co-existence of two non-mixing redox pathways that lead to extracellular electron transfer in S. oneidensis MR-1 established through transient protein interactions.

Genomic, Proteomic, and Biochemical Analysis of the Organohalide Respiratory Pathway in Desulfitobacterium dehalogenans

“…Among these are the FAD binding C terminus of the Fe 3ϩ -induced flavocytochrome c fumarate reductase, Ifc3, from Shewanella frigidimarina NCIMB400 (Table 2), or a cytochrome c-dependent methacrylate reductase from Geobacter sulfurreducens AM-1. However, these predicted flavoproteins from D. dehalogenans, unlike Ifc3, do not contain any N-terminal heme binding CxxCH motifs (data not shown) (44)(45)(46)(47). Two of these flavoproteins are annotated as flavocytochrome c, and the remaining seven are annotated as succinate dehydrogenase/fumarate reductases.…”

Multi-haem cytochromes inShewanella oneidensisMR-1: structures, functions and opportunities

“…As for the case of cytochrome -cytochrome interactions discussed above [57], NMR studies in solution can reveal information on interactions between soluble substrates and multi-haem cytochromes close to the haem cofactors as well as dissociation constants. In [84], the interaction of several OM cytochromes of S. oneidensis with several redox shuttles was thus investigated, yielding only in some cases interactions close to a haem.…”

“…In the case of multi-haem cytochromes specifically, only complexes that bring haems close together so as to enable interprotein ET are of interest, and thus it is reasonable to focus on chemical shift perturbations of haem methyl groups. This has made it possible to propose, or exclude, ET-relevant interactions among the periplasm-associated cytochromes CymA, FccA, STC, ScyA and MtrA in [57] and for FccA and STC even the haem involved in interprotein contact could be determined. Interestingly, the latter two cytochromes use the same haem to contact both CymA and MtrA-thus, they could not possibly form a ternary complex to bridge the periplasm between CymA at the IM and MtrA at the OM.…”

“…Employment of the NMR methods mentioned in §4.2.1 is also conceivable in principle. However, the NMR measurements in [57] were done at most on tetra-haem cytochromes, and trying such measurements for the larger OM deca-haem cytochromes might encounter additional complications.…”

Towards patterned bioelectronics: facilitated immobilization of exoelectrogenic Escherichia coli with heterologous pili

“…Menaquinol is in turn oxidized by the inner membrane‐bound quinol dehydrogenase CymA (Marritt et al ., 2012), or in its absence, the enzyme complex SirCD (Cordova et al ., 2011). Next, electrons are transferred across the periplasmic space by diffusible c ‐cytochromes STC and FccA (Schuetz et al ., 2009; Fonseca et al ., 2013) and transferred to terminal reductases at the outer membrane such as MtrA and MtrC which are associated with the MtrCAB complex (Fig. 1) (Coursolle and Gralnick, 2010; Breuer et al ., 2015).…”