Every living cell needs to get rid of leftover electrons when metabolism extracts energy through the oxidation of nutrients. Common soil microbes such as Geobacter sulfurreducens live in harsh environments that do not afford the luxury of soluble, ingestible electron acceptors like oxygen. Instead of resorting to fermentation, which requires the export of reduced compounds (e.g. ethanol or lactate derived from pyruvate) from the cell, these organisms have evolved a means to anaerobically respire by using nanowires to export electrons to extracellular acceptors in a process called extracellular electron transfer (EET) [ 1]. Since 2005, these nanowires were thought to be pili filaments [ 2]. But recent studies have revealed that nanowires are composed of multiheme cytochromes OmcS [ 3, 4] and OmcZ [ 5] whereas pili remain inside the cell during EET and are required for the secretion of nanowires [ 6]. However, how electrons are passed to these nanowires remains a mystery ( Figure 1A). Periplasmic cytochromes (Ppc) called PpcA-E could be doing the job, but only two of them (PpcA and PpcD) can couple electron/proton transfer — a necessary condition for energy generation. In a recent study, Salgueiro and co-workers selectively replaced an aromatic with an aliphatic residue to couple electron/proton transfer in PpcB and PpcE (Biochem. J. 2021, 478 (14): 2871–2887). This significant in vitro success of their protein engineering strategy may enable the optimization of bioenergetic machinery for bioenergy, biofuels, and bioelectronics applications.
Making protons tag along with electrons: For efficient electron transfer in proteins, it is very important that protons can move along with electrons. In this issue, Guberman-Pfeffer and Malvankar (pp. 4093–4097) discuss strategies for proton-coupled electron transfer in bacterial proteins building on their work using crystalline amyloid proteins (green) that can perform proton-coupled electron transport over micrometre distances via stacked tyrosine residues. Intrinsic (contact-free) electron transfer was measured by placing amyloid protein crystal between gold electrodes shown at the sides. Image credit: Catharine Shipps.
Making protons tag along with electrons
Matthew J. Guberman-Pfeffer, Nikhil S. Malvankar; Making protons tag along with electrons. Biochem J 10 December 2021; 478 (23): 4093–4097. doi: https://doi.org/10.1042/BCJ20210592
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