AUTHOR=Dale-Evans Alister R. , Robinson Martin J. , Lloyd-Laney Henry O. , Gavaghan David J. , Bond Alan M. , Parkin Alison 

TITLE=A Voltammetric Perspective of Multi-Electron and Proton Transfer in Protein Redox Chemistry: Insights From Computational Analysis of Escherichia coli HypD Fourier Transformed Alternating Current Voltammetry

JOURNAL=Frontiers in Chemistry

VOLUME=9

YEAR=2021

URL=https://www.frontiersin.org/journals/chemistry/articles/10.3389/fchem.2021.672831

DOI=10.3389/fchem.2021.672831

ISSN=2296-2646

ABSTRACT=<p>This paper explores the impact of pH on the mechanism of reversible disulfide bond (CysS-SCys) reductive breaking and oxidative formation in <italic>Escherichia coli</italic> hydrogenase maturation factor HypD, a protein which forms a highly stable adsorbed film on a graphite electrode. To achieve this, low frequency (8.96 Hz) Fourier transformed alternating current voltammetric (FTACV) experimental data was used in combination with modelling approaches based on Butler-Volmer theory with a dual polynomial capacitance model, utilizing an automated two-step fitting process conducted within a Bayesian framework. We previously showed that at pH 6.0 the protein data is best modelled by a redox reaction of two separate, stepwise one-electron, one-proton transfers with slightly “crossed” apparent reduction potentials that incorporate electron and proton transfer terms (<inline-formula id="inf1"><mml:math id="m1" xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msubsup><mml:mi>E</mml:mi><mml:mrow><mml:mtext>app</mml:mtext><mml:mn>2</mml:mn></mml:mrow><mml:mn>0</mml:mn></mml:msubsup></mml:mrow></mml:math></inline-formula> &gt; <inline-formula id="inf2"><mml:math id="m2" xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msubsup><mml:mi>E</mml:mi><mml:mrow><mml:mtext>app</mml:mtext><mml:mn>1</mml:mn></mml:mrow><mml:mn>0</mml:mn></mml:msubsup></mml:mrow></mml:math></inline-formula>). Remarkably, rather than collapsing to a concerted two-electron redox reaction at more extreme pH, the same two-stepwise one-electron transfer model with <inline-formula id="inf3"><mml:math id="m3" xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msubsup><mml:mi>E</mml:mi><mml:mrow><mml:mtext>app</mml:mtext><mml:mn>2</mml:mn></mml:mrow><mml:mn>0</mml:mn></mml:msubsup></mml:mrow></mml:math></inline-formula> &gt; <inline-formula id="inf4"><mml:math id="m4" xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msubsup><mml:mi>E</mml:mi><mml:mrow><mml:mtext>app</mml:mtext><mml:mn>1</mml:mn></mml:mrow><mml:mn>0</mml:mn></mml:msubsup></mml:mrow></mml:math></inline-formula> continues to provide the best fit to FTACV data measured across a proton concentration range from pH 4.0 to pH 9.0. A similar, small level of crossover in reversible potentials is also displayed in overall two-electron transitions in other proteins and enzymes, and this provides access to a small but finite amount of the one electron reduced intermediate state.</p>