Frequency and potential dependence of reversible electrocatalytic hydrogen interconversion by [FeFe]-hydrogenases

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⁺/H₂ potential, 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 alga Chlamydomonas reinhardtii, which contains only the active-site "H cluster," and CpI from the fermentative anaerobe Clostridium 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⁺/H₂ potential, at 0 °C) of 157 electrons (78 molecules H₂) per second for CpI and 25 electrons (12 molecules H₂) per second for CrHydA1. 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 in CpI and a low reorganization energy for electron transfer on/off the H cluster.