Adsorption and electron transfer of metal-reducing decaheme cytochrome protein MtrF on iron oxide nanoparticle surfaces

Abstract

Dissimilatory metal-reducing bacteria (DMRB) transfer electrons to extracellular metal oxides via a multiheme cytochrome network. Coupling DMRB with iron oxide nanoparticles (NPs) enables continuous redox processes for various applications such as bioremediation and bioenergy. The conformation of the terminal decaheme cytochrome MtrF on the surface critically influences electron transfer (ET) efficiency. In this work, we used molecular dynamics simulations and master equations to study MtrF adsorption on 3.6 and 6.0 nm α-Fe₂O₃ NPs and its steady-state ET in water. Our study shows that the heme cofactors can have strong electrostatic interactions with iron oxide NP surfaces, promoting protein adsorption and interfacial ET, while a small number of hydration water molecules in the first hydration shell of the iron oxide NP form hydrogen bonds with protein residues, stabilizing them near the NP surface. The NP adsorption sites, which are favorable for the interfacial ET, are located at the heme groups near the terminals of two intersecting heme chains. Among these sites, the region around hemes 4 and 5, near the terminal of the long heme chain, along with heme 7 at the terminal of the staggered cross short chain, is found to be relatively energetically favorable and ET-efficient, anchoring MtrF in a lie-down orientation on the NP. As the NP size increases, more protein residues adsorb onto the NP, potentially hindering heme attachment. The MtrF adsorption on the NP distorts its heme network and affects ET, but has a negligible effect on the protein's secondary structure. The kinetic behavior of ET across MtrF and the rate-limiting step are governed by heme–NP contacts, the ratio of electron injection to ejection rate constants, and the direction of ET. Our study of protein–NP interactions is important for the development of bionanotechnologies.