Time-Resolved Infrared Spectroscopy Reveals the pH-Independence of the First Electron Transfer Step in the [FeFe] Hydrogenase Catalytic Cycle

Abstract

[FeFe] hydrogenases are highly active catalysts for hydrogen conversion. Their active site has two components: a [4Fe−4S] electron relay covalently attached to the H 2 binding site and a diiron cluster ligated by CO, CN – , and 2-azapropane-1,3-dithiolate (ADT) ligands. Reduction of the [4Fe−4S] site was proposed to be coupled with protonation of one of its cysteine ligands. Here, we used time-resolved infrared (TRIR) spectroscopy on the [FeFe] hydrogenase from Chlamydomonas reinhardtii ( Cr HydA1) containing a propane-1,3-dithiolate (PDT) ligand instead of the native ADT ligand. The PDT modification does not affect the electron transfer step to [4Fe−4S] H but prevents the enzyme from proceeding further through the catalytic cycle. We show that the rate of the first electron transfer step is independent of the pH, supporting a simple electron transfer rather than a proton-coupled event. These results have important implications for our understanding of the catalytic mechanism of [FeFe] hydrogenases and highlight the utility of TRIR.

Electronic Structure and Excited State Dynamics of Cadmium Chalcogenide Nanorods

“…Once an enzyme is bound, when the NR is photoexcited, the electron can transfer from the NR to the enzyme which can then use the transferred electron to catalyze a redox reaction. The quantum efficiency of ET from NRs to the enzyme (Φ ET ) defines the upper limit on the QY of the overall product formation. ,,, Other schemes for enzyme catalysis utilize redox mediators which accept electrons from NCs before transferring to the enzyme. ,,, Because of the critical role of ET in the overall photochemistry, like in the case of metal catalysts, the dynamics of ET have been studied in some detail, primarily for the systems involving direct and redox shuttle-mediated electron injection from CdS NRs to H 2 ase. ,,,,, Enzyme sizes are on the order of the dimensions of NCs, so unlike the case of small molecules, the numbers of enzymes that can bind on each NC are relatively small. In the cases of direct ET from NRs to enzymes, it is particularly important to characterize the population distribution of NR-enzyme complexes because a NR that transfers electrons to one enzyme will have drastically different excited state decay than a NR with no enzymes or with multiple ones.…”

“…280,286,332,451 Other schemes for enzyme catalysis utilize redox mediators which accept electrons from NCs before transferring to the enzyme. 435,443,452,453 Because of the critical role of ET in the overall photochemistry, like in the case of metal catalysts, the dynamics of ET have been studied in some detail, primarily for the systems involving direct and redox shuttle-mediated electron injection from CdS NRs to H 2 ase. 280,286,332,431,443,451 Enzyme sizes are on the order of the dimensions of NCs, so unlike the case of small molecules, the numbers of enzymes that can bind on each NC are relatively small.…”

“…NR-H 2 ase systems that are mediated by a redox shuttle have different limiting factors to enzyme catalytic efficiency. ,,, When the ET to the enzyme is mediated by a redox shuttle, the ET from NRs to the mediator, transport of the mediator to the enzyme, and ET from the mediator to the enzyme all effect the overall efficiency of the system. A study investigating CdSe/CdS DiRs with H 2 ase, with ET mediated by MV 2+ and propyl-bridged 2,2′-bipyridinium (PDQ 2+ ), found that the identity of the redox shuttle and its redox potential, along with the ability to form long-lived charge-separated states, were crucial for achieving high efficiencies of H 2 generation .…”