How Do Electric Bacteria Breathe Without Oxygen or Soluble Electron Acceptors? Protein Nanowires: Structures, Functions, and Ultrafast Electron Transfer Mechanisms

Deep in the ocean or underground, where there is no oxygen, <i>Geobacter </i>“breathe” by projecting tiny hair-like protein filaments called "nanowires" into the soil, to dispose of excess electrons resulting from the conversion of nutrients to energy, cleaning up radioactive sites. Although it is long known that <i>Geobacter</i> use filaments for electron transfer (<i>Nature</i> 2002, 2005), it was not clear what they are actually made of and why they are conductive.<br/>Our studies have revealed a surprise: the nanowires have a core of hemes lining up to create a continuous path along which electrons travel (<i>Cell</i>’19, <i>Nature Chem.Bio.</i>’20, <i>Nature Micro '2</i>3) and and can be engineered with atomic precision using recombinant DNA technology, making for remarkably versatile electronic components.<br/>We have further found that <i>Geobacter</i> pili remain hidden inside the cell and serve as a piston to secrete nanowire-forming cytochromes (<i>Nature</i> 2021) rather than functioning as a nanowire themselves as previously thought (<i>Current Opinion </i>2020).<br/>These studies solve a longstanding mystery to explain our previous findings that these bacteria transport electrons via nanowires (<i>Nature Nano.</i> 2014) over 100-times their size to electron acceptors (<i>Nature Nano</i><i>. </i>2011) and partner cells (<i>Science</i> 2010) and store electrons when acceptors are absent akin to how humans use their lungs (<i>ChemPhysChem </i>2012) .<br/>Our contact-free measurements of intrinsic electron conductivity in individual protein nanowires reveals how energetics and proximity of proton acceptors modulate conductivity by 100-fold (<i>PNAS</i> 2021, <i>Biochem. Journal</i> 2021). We have also developed synthetic protein nanowires with tunable conductivity and programmable self-assembly using non-natural click chemistry functionality (<i>Nature Comm.</i> 2022).<br/>In this talk I will present our efforts to identify the physical and molecular mechanism of high conductivity of microbial cytochrome nanowires. Our conducting-probe AFM measurements show one of the highest electronic conductivity ever reported in proteins (&gt; 100 S/cm) (<i>Nature Chem.Bio.</i>’20). Femtosecond transient absorption spectroscopy and quantum dynamics simulations reveal ultrafast (&lt;200 fs) electron transfer between nanowire hemes upon photoexcitation, enhancing carrier density and mobility. Photoconductive atomic force microscopy shows up to 100-fold increase in photocurrent in purified individual nanowires. Photocurrents respond rapidly (&lt;100 ms) to the excitation and persist reversibly for hours (<i>Nature Comm.</i>’22). Furthermore, nanowires and biofilms show non-classical temperature dependence of conductivity with cooling accelerating electron transport by 300-fold.<br/>Our efforts to computationally model the heme redox potential and conductivity of nanowires yielded up to a billion-fold lower conductivity than experiments (<i>JPCB’21</i>, <i>JPCB’22</i>), illustrating that the existing computational models based on electron hopping assumption fail to capture electron transfer in biological nanowires. This raises the possibility that biological nanowires employ a fundamentally different, currently unknown mechanism. Thus, existing models predict the same conductivity for all nanowires with computed timescales for heme-to-heme electron transfers (100 ns), million-fold lower than that measured using transient absorption in excited-state (<i>Nature Comm.</i>) and conductivity in ground-state for fully hydrated (<i>Nature</i> & <i>Cell</i>) or air-dried nanowires (<i>Science Adv.</i>).<br/><br/>Notably, multiple computational studies have predicted that invoking quantum effects could account for the high conductivity of these nanowires (<i>Nanotechology</i>’20, <i>IEEE</i>’21, <i>ACS Nano</i>’23). I will present our efforts to experimentally assess these computational predictions using multiple probes such as light, temperature, electric and magnetic fields. I will also discuss how our studies are helping to understand, predict and control extracellular electron transfer by nanowires used by diverse environmental microbes to capture, convert and store energy.