title: Frequency and potential dependence of reversible electrocatalytic hydrogen interconversion by [FeFe]-hydrogenases.
abstract: The kinetics of hydrogen oxidation and evolution by [FeFe]-hydrogenases have been investigated by electrochemical impedance spectroscopy-resolving factors that determine the exceptional activity of these enzymes, and introducing an unusual and powerful way of analyzing their catalytic electron transport properties. Attached to an electrode, hydrogenases display reversible electrocatalytic behavior close to the 2H+/H2potential, making them paradigms for efficiency: the electrocatalytic "exchange" rate (measured around zero driving force) is therefore an unusual parameter with theoretical and practical significance. Experiments were carried out on two [FeFe]-hydrogenases,CrHydA1 from the green algaChlamydomonas reinhardtii, which contains only the active-site "H cluster," andCpI from the fermentative anaerobeClostridium pasteurianum, which contains four low-potential FeS clusters that serve as an electron relay in addition to the H cluster. Data analysis yields catalytic exchange rates (at the formal 2H+/H2potential, at 0 °C) of 157 electrons (78 molecules H2) per second forCpI and 25 electrons (12 molecules H2) per second forCrHydA1. The experiments show how the potential dependence of catalytic electron flow comprises frequency-dependent and frequency-independent terms that reflect the proficiencies of the catalytic site and the electron transfer pathway in each enzyme. The results highlight the "wire-like" behavior of the Fe-S electron relay inCpI and a low reorganization energy for electron transfer on/off the H cluster.
authors.affiliation: Inorganic Chemistry Laboratory, Department of Chemistry, University of Oxford, Oxford OX1 3QR, United Kingdom.|School of Solar Energy, Pandit Deendayal Petroleum University, Gandhinagar 382007, Gujarat, India.
authors.forename: Kavita
authors.initials: K
authors.lastname: Pandey
authors.affiliation: Inorganic Chemistry Laboratory, Department of Chemistry, University of Oxford, Oxford OX1 3QR, United Kingdom.
authors.forename: Shams T A
authors.initials: ST
authors.lastname: Islam
authors.affiliation: AG Photobiotechnologie Ruhr-Universität Bochum, 44801 Bochum, Germany.
authors.forename: Thomas
authors.initials: T
authors.lastname: Happe
authors.affiliation: Inorganic Chemistry Laboratory, Department of Chemistry, University of Oxford, Oxford OX1 3QR, United Kingdom; fraser.armstrong@chem.ox.ac.uk.
authors.forename: Fraser A
authors.initials: FA
authors.lastname: Armstrong
mesh_terms: D002384:Catalysis; D020134:Catalytic Domain; D016825:Chlamydomonas reinhardtii / Q000201:enzymology; D003013:Clostridium / Q000201:enzymology; D004578:Electron Spin Resonance Spectroscopy; D004579:Electron Transport; D006859:Hydrogen / Q000737:chemistry*; D006864:Hydrogenase / Q000378:metabolism*; D007700:Kinetics; D008958:Models, Molecular; D011487:Protein Conformation
chemical_list: D006859:Hydrogen; D006864:Hydrogenase
keywords: electrocatalysis; electron transfer; hydrogen; hydrogenase; impedance spectroscopy