There continues to be widespread interest in the applications of hydrogen as a clean fuel and its potential for local electricity production and use in transport. Recent years have seen a variety of breakthroughs in our understanding of the nature, structure, and biosynthesis of hydrogenases. This book is a timely description of these developments,

The utilization of enzymes as bioelectrocatalysts is of increasing interest due to their advantages in chemical specificity and catalytic rates. Emphasis has been placed on hydrogen producing biocatalysts to overcome drawbacks of current heterogeneous catalysts, including limited availability and poor selectivity. Native enzymes, such as the [NiFe] hydrogenase, have demonstrated extreme proficiency as bi-directional catalysts for proton reduction and hydrogen oxidation, inspiring a variety of small molecule and protein mimics. The utilization of a robust and stable protein scaffold with a similar primary coordination environment as the native enzyme can result in similar activity. Previous reports have demonstrated that nickel-substituted rubredoxin (NiRd) serves as a structural, functional and mechanistic model of the [NiFe] hydrogenase, active towards proton reduction electrochemically and in solution, with an identical primary coordination sphere at the nickel center as the native enzyme. While the mechanism of proton reduction has been experimentally and computationally modeled to be similar to that of the native enzyme, key catalytic intermediates have not yet been isolated and characterized. This project aims to address some of the limitations of this current model, such as significant overpotential and lack of spectroscopic characterization of catalytically competent intermediates. Further, correlation between redox activity and protein structure are investigated by modification of the protein primary sphere coordination site. Primary sphere mutants demonstrate changes in the redox and catalytic behavior dependent on the cysteine site being modified. Drastic changes to the primary coordination sphere are explored using electrochemistry along with optical, multiwavelength resonance Raman, X-ray and electron paramagnetic resonance spectroscopies. This study demonstrates the ability to keep and shut off catalysis, aiding in the understanding of enzyme selectivity and its correlations to active site structure and geometry. Expanding beyond the primary coordination sphere, changes to the secondary sphere have resulted in drastic changes to the turnover frequencies of the enzyme, with modest changes in overpotential. Previous work using electrochemical analysis of a suite of secondary sphere mutants have shown dramatic variation in the catalytic rates and redox activity of NiRd, with some mutants demonstrating a change in the proton reduction mechanism. The observed differences in catalytic and redox activity are hypothesized to arise from changes in the protein structure, dynamics and H-bonding networks. A structural study of a suite of secondary and primary sphere mutants utilizing Nuclear Magnetic Resonance (NMR) spectroscopy directly provides solution-phase information on protein dynamics. The small size of Rd (5.2 kDa) makes it an ideal candidate for NMR interrogations. Additionally, due to the high spin (S=1) active site of the NiIIRd artificial enzyme, and the favorable relaxation properties of pseudo-tetrahedral four coordinate NiII, paramagnetic NMR spectroscopy can be used to gain further insight into the metal electronic structure and the H-bonding network. The information gathered will identify key factors that affect catalysis within a model protein scaffold, allowing for not only rational catalyst design and catalyst optimization, but the development of paramagnetic NMR methodology that allows to probe structural changes by NiRd under catalytically relevant conditions.

The energy crisis of the last decade has led to new and innovative approaches to combat this pressing issue. One leading candidate as an alternative fuel is hydrogen due to its derivability from water, its high energy density, and its status as a clean fuel. However, the current methods for hydrogen production are not economically feasible to replace fossil fuels because of a reliance on precious metal catalysts. A promising alternative for hydrogen catalysis is through the use of the enzymes hydrogenases, which utilize earth-abundant metals such as nickel and iron. Hydrogenases, however, are limited by their intolerance to oxygen, changes in temperature and pH, and the complexity of their biosynthesis. These factors, among others, leave little potential for industrial application, but hydrogenases can serve as inspiration for structural and functional models. One such model is the metalloprotein rubredoxin. Rubredoxin is a robust, electron-transfer protein with a promiscuous, tetrathiolate active site that binds a variety of metals. When the native iron is replaced with nickel, it mimics the primary coordination of the nickel site in the [NiFe] hydrogenase cofactor. We have demonstrated this construct not only acts as a structural model of hydrogenase, but also a functional mimic as it produces molecular hydrogen through the reduction of protons. The construct’s catalytic capabilities were probed with protein film electrochemistry (PFE), confirming the presence of a proton-coupled electron transfer process and inhibition from carbon monoxide, characteristics shared by the native hydrogenase. Through spectroscopic studies coupled to density functional theory calculations, a theoretical model of the Ni(II)Rd resting state was constructed and validated by resonance Raman spectroscopy. As typical PFE experiments were insufficient to investigate NiRd by standard enzymatic studies, a new quantitative PFE (qPFE) method that utilizes an internal protein film standard was created and validated on Rd. This technique allowed for the observation of mechanistic constraints that suggest a particular mechanism that matches that of the [NiFe] hydrogenase catalysis. This mechanism was validated through electrochemical simulations and DFT calculations. Lastly, this thesis presents preliminary data on a library of secondary sphere mutant variants around the NiRd active site. A wide range of characteristics is observed for the mutants, including an increase in activity by 10-fold and shifts in the catalytic onset potential over an 86 mV range. It has been previously demonstrated that these types of interactions greatly affect catalysis in the native hydrogenase as well, establishing NiRd as a mimic for the secondary sphere interactions of [NiFe] hydrogenase.

This book provides an overview of bioinspired metal-sulfur catalysis by covering structures, activities and model complexes of enzymes exhibiting metal sulphur moieties in their active center.