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Spectroscopic and Computational Evidence that [FeFe] Hydrogenases Operate Exclusively with CO-Bridged Intermediates

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posted on 2019-12-26, 15:03 authored by James A. Birrell, Vladimir Pelmenschikov, Nakul Mishra, Hongxin Wang, Yoshitaka Yoda, Kenji Tamasaku, Thomas B. Rauchfuss, Stephen P. Cramer, Wolfgang Lubitz, Serena DeBeer
[FeFe] hydrogenases are extremely active H2-converting enzymes. Their mechanism remains highly controversial, in particular, the nature of the one-electron and two-electron reduced intermediates called HredH+ and HsredH+. In one model, the HredH+ and HsredH+ states contain a semibridging CO, while in the other model, the bridging CO is replaced by a bridging hydride. Using low-temperature IR spectroscopy and nuclear resonance vibrational spectroscopy, together with density functional theory calculations, we show that the bridging CO is retained in the HsredH+ and HredH+ states in the [FeFe] hydrogenases from Chlamydomonas reinhardtii and Desulfovibrio desulfuricans, respectively. Furthermore, there is no evidence for a bridging hydride in either state. These results agree with a model of the catalytic cycle in which the HredH+ and HsredH+ states are integral, catalytically competent components. We conclude that proton-coupled electron transfer between the two subclusters is crucial to catalysis and allows these enzymes to operate in a highly efficient and reversible manner.