The Molecular Basis for Binding of an Electron Transfer Protein to a Metal Oxide Surface

Abstract

Achieving fast electron transfer between a material and protein is a long-standing challenge confronting applications in bioelectronics, bioelectrocatalysis, and optobioelectronics. Interestingly, naturally occurring extracellular electron transfer proteins bind to and reduce metal oxides fast enough to enable cell growth, and thus could offer insight into solving this coupling problem. While structures of several extracellular electron transfer proteins are known, an understanding of how these proteins bind to their metal oxide substrates has remained elusive because this abiotic-biotic interface is inaccessible to traditional structural methods. Here, we use advanced footprinting techniques to investigate binding between the Shewanella oneidensis MR-1 extracellular electron transfer protein MtrF and one of its substrates, α-FeO nanoparticles, at the molecular level. We find that MtrF binds α-FeO specifically, but not tightly. Nanoparticle binding does not induce significant conformational changes in MtrF, but instead protects specific residues on the face of MtrF likely to be involved in electron transfer. Surprisingly, these residues are separated in primary sequence, but cluster into a small 3D putative binding site. This binding site is located near a local pocket of positive charge that is complementary to the negatively charged α-FeO surface, and mutational analysis indicates that electrostatic interactions in this 3D pocket modulate MtrF-nanoparticle binding. Strikingly, these results show that binding of MtrF to α-FeO follows a strategy to connect proteins to materials that resembles the binding between donor-acceptor electron transfer proteins. Thus, by developing a new methodology to probe protein-nanoparticle binding at the molecular level, this work reveals one of nature's strategies for achieving fast, efficient electron transfer between proteins and materials.

In SituOptical Quantification of Extracellular Electron Transfer using Plasmonic Metal Oxide Nanocrystals

“…It is notable that even the fully induced mtrC complement did not completely recapture the wild-type EET phenotype, exhibiting ∼4.5-fold less activity compared to the empty vector control. This may highlight the importance of other missing EET components in this strain, specifically omcA and mtrF , in binding and electron transfer to metal oxide substrates (53, 54). Tiered electron flux based on varying gene expression was also quantified as current output, ranging from 0.12 ± 0.04 (uninduced) to 0.68 ± 0.08 (fully induced) fA • cell −1 at steady-state (Table 2).…”

Extracellular Electron Transfer by Shewanella oneidensis Controls Pd Nanoparticle Phenotype

“…Altering the redox-potential and catalytic activity of heme-containing proteins is well-precedented 41 , and key mutations could be applied to identify/tune the Pd(II) reduction activity of MtrC hemes 42 . Additionally, a binding-site for iron oxide nanoparticles to the outer membrane cytochrome, MtrF, has recently been elucidated using protease footprinting 43 , and similar methods could be used to identify and affect residues involved in particle nucleation by MtrC.…”

Nanomaterials Facilitating Microbial Extracellular Electron Transfer at Interfaces

“…Conformational change was rarely observed before and after binding, and complementary electrostatic interactions were speculated to mediate the binding ( Figure A,B). [ 108 ] Besides metal compounds, certain functional groups may also interact with cytochromes. Our studies on bread‐derived 3D macroporous carbon anode showed that pyrrolic nitrogen and graphitic nitrogen are beneficial for electron transfer from c‐type cytochromes.…”

“…A,B) Reproduced with permission. [ 108 ] Copyright 2017, American Chemical Society. C–F) Reproduced with permission.…”