Applications of Photogating and Time Resolved Spectroscopy to Mechanistic Studies of Hydrogenases.

Rapid and facile redox chemistry is exemplified in nature by the oxidoreductases, the class of enzymes that catalyze electron transfer (ET) from a donor to an acceptor. The key role of oxidoreductases in metabolism and biosynthesis has imposed evolutionary pressure to enhance enzyme efficiency, pushing some toward the diffusion limit. Understanding the detailed molecular mechanisms of these highly optimized enzymes would provide an important foundation for the rational design of catalysts for multielectron chemistry, including fuel production. The hydrogenases (H2 ases) are the oxidoreductases that catalyze the most basic electron and proton transfer reactions relevant to fuel production, the interconversion of protons and hydrogen, with kcat > 103 s-1 . Thus, they provide a model system for studying the efficiency exhibited by oxidoreductases. Because of the extraordinarily fast catalytic rates of these enzymes, their mechanisms have been difficult to study directly but instead have been inferred from structural and steady-state measurements. Although informative, the kinetic competency of observed equilibrium steps can only be suggested by these methods, not demonstrated, because the fundamental (fast) catalytic steps remain unresolved, resulting in minimal insight regarding the underlying ET and proton transfer (PT) events. Motivated by this gap in understanding, we developed an approach capable of observing elementary ET and PT during such fast enzyme turnover by combining a laser-induced potential jump with time-resolved spectroscopy. The potential jump initiates enzyme turnover by utilizing a short-pulsed laser to release a "caged" electron from a nanomaterial or NAD(P)H, which is then captured by a mediator such as methyl viologen. The subsequent enzyme reduction and turnover are monitored by transient absorption spectroscopy in the visible or mid-IR spectral regions. The method is completely general and in principle can be applied to any catalytic redox reaction. In the case of hydrogenases, time-resolved infrared spectroscopy of the active site CO ligands is particularly informative since the IR frequencies are exquisitely sensitive to the redox and protonation states. Using this methodology, we have developed a description of the catalytic mechanism of the Pyrococcus furiosus [NiFe]-hydrogenase by demonstrating the kinetic and chemical competency of equilibrium states and by invoking new intermediates. Additionally, the pre-steady-state kinetics revealed a distinct role of proton tunneling in concerted electron-proton transfer (EPT) modulated by a conserved glutamic acid residue. Similar multisite EPT processes have been implicated in numerous enzymes but have not been demonstrated explicitly. These methods have also been successfully applied to an electron bifurcating [FeFe]-H2 ase from Thermotoga maritima, establishing the kinetic competency of the Hox , Hred , and Hsred intermediates of the [FeFe] enzyme. These results provide fundamental insight on the factors that control low barrier proton and electron flow in enzymes and thus provide a foundation for the rational design of reversible biomimetic catalysts.