LAUSR

The Thermus thermophilus Rieske protein (TtRP) contains a 2Fe-2S cluster with one iron (Fe-Cys) coordinated by four sulfur atoms (2xS2− and 2xCys) and one iron (Fe-His) by two sulfur and two nitrogen atoms (2xS2−, His134 and His154). Here, the protein is investigated at three pH values (6.0, 8.5 and 10.5) in order to elucidate the protonation states of the His-ligands. Examination of the effect of protonation on the electronic structure of the cluster via Mössbauer spectroscopy gives a deeper understanding of the coupling of electron transfer to the protonation state of the His-ligands. Two components (1 referring to Fe-Cys and 2 to Fe-His) with parameters typical for a diamagnetic [2Fe-2S]2+ cluster are detected. The Mössbauer parameters and the protonation state clearly correlate: while δ remains almost pH-independent with δ1 (pH6.0) = 0.23 (± 0.01) mms− 1 and δ1 (pH10.5) = 0.24 (± 0.01) mms− 1 for Fe-Cys, it decreases for Fe-His from δ2 (pH6.0) = 0.34 (± 0.01) mms− 1 to δ2 (pH10.5) = 0.28 (± 0.01) mms− 1. ΔEQ changes from ΔEQ1 (pH6.0) = 0.57 (± 0.01) mms− 1 to ΔEQ1 (pH10.5) = 0.45 (± 0.01) mms− 1 and from ΔEQ2 (pH6.0) = 1.05 (± 0.01) mms− 1 to ΔEQ2 (pH10.5) = 0.71 (± 0.01) mms− 1. Density functional theory (DFT)-calculations based on the crystal structure (pdb 1NYK) (Hunsicker-Wang et al. Biochemistry 42, 7303, 2003) have been performed for the Rieske-cluster with different His-ligand protonation states, reproducing the experimentally observed trend.