Spectroscopic characterization of the nickel and iron‐sulphur clusters of hydrogenase from the purple photosynthetic bacterium Thiocapasa roseopersicina 2. Electron spin‐echo spectroscopy

Abstract

Pulsed electron‐spin‐resonance techniques were applied to the hydrogenase of the purple photosynthetic bacterium Thiocapsa roseopersicina, an enzyme which contains nickel and iron‐sulphur clusters but no flavin. The linear electric field effect profile of the spectrum in the region of g= 2.01 indicated that the strong ESR signal in the oxidized protein is due to a [3Fe–4S] cluster. The electron spin‐echo envelope of this spectrum was modulated by hyperfine interactions with 1H and 14N nuclei, probably from the polypeptide chain. The ESR spectrum of this species shows a complex pattern arising from spin‐spin interaction with another paramagnet. When the protein was partially reduced by ascorbate plus phenazine methosulphate, the complexity of the spectrum was abolished but the form of the electron spin‐echo envelope modulation (ESEEM) pattern was unchanged. This indicates that the reversible disappearance of the spin‐spin interaction pattern on partial reduction is not due to cluster interconversion to a [4Fe–4S] cluster. In the ESR spectrum of nickel(III), weak hyperfine interactions with 1H and 14N were also observed by ESEEM. The nature of the interacting nuclei is discussed. Copyright © 1989, Wiley Blackwell. All rights reserved