Cysteine SH and Glutamate COOH Contributions to [NiFe] Hydrogenase Proton Transfer Revealed by Highly Sensitive FTIR Spectroscopy

• Published in: Angewandte Chemie International Edition 2019-09, Vol.58 (38), p.13285-13290
• Main Authors: Tai, Hulin, Nishikawa, Koji, Higuchi, Yoshiki, Mao, Zong‐wan, Hirota, Shun
• Format: Article
• Language: English
• Subjects: biocatalysis; Catalysis; Electron transfer; hydrogen; Hydrogenase; Infrared spectra; Intermetallic compounds; IR spectroscopy; Iron compounds; Nickel compounds; Oxidation; proton transfer; Protonation; Protons; Sensitivity; Spectral sensitivity; Spectrum analysis; Stretching; Water chemistry
• ISSN: 1433-7851; 1521-3773
• DOI: 10.1002/anie.201904472

A [NiFe] hydrogenase (H2ase) is a proton‐coupled electron transfer enzyme that catalyses reversible H2 oxidation; however, its fundamental proton transfer pathway remains unknown. Herein, we observed the protonation of Cys546‐SH and Glu34‐COOH near the Ni–Fe site with high‐sensitivity infrared difference spectra by utilizing Ni‐C‐to‐Ni‐L and Ni‐C‐to‐Ni‐SIa photoconversions. Protonated Cys546‐SH in the Ni‐L state was verified by the observed SH stretching frequency (2505 cm−1), whereas Cys546 was deprotonated in the Ni‐C and Ni‐SIa states. Glu34‐COOH was double H‐bonded in the Ni‐L state, as determined by the COOH stretching frequency (1700 cm−1), and single H‐bonded in the Ni‐C and Ni‐SIa states. Additionally, a stretching mode of an ordered water molecule was observed in the Ni‐L and Ni‐C states. These results elucidate the organized proton transfer pathway during the catalytic reaction of a [NiFe] H2ase, which is regulated by the H‐bond network of Cys546, Glu34, and an ordered water molecule. Catalytic contributions: Protonated Cys546‐SH, H‐bonded Glu34‐COOH, and ordered water frequencies were observed for the Ni‐C, Ni‐L, and Ni‐SIa states of [NiFe] hydrogenase, showing that the organized proton transfer pathway during the catalytic reaction is regulated by the H‐bond network of Cys546, Glu34, and an ordered water molecule.