Electronic Structure of the H Cluster in Fe-Hydrogenases

Fe-Hydrogenase II isolated from C. pasteurianum contains 14 Fe which are distributed among the so-called H cluster (the catalytic center) and two 4Fe-4S clusters. Insights gained from Mössbauer studies of M-4Fe-4S 2+ cluster assemblies (M is a paramagnetic center) in sulfite reductase and carbon monoxide dehydrogenase have suggested that the H cluster contains a 4Fe-4S 2+ cluster covalently linked to a smaller Fe-containing cluster. Recent X-ray studies of two Fe-hydrogenases, combined with the results of FTIR studies, have revealed that the H cluster contains a novel binuclear Fe cluster, 2Fe H, that is linked by a cysteinyl sulfur to a 4Fe-4S cluster; 2Fe H was found to have CO, CN -, and thiolate ligands. The analysis of the Mössbauer spectra of Hydrogenase II in the oxidized, reduced, and the CO-inhibited states has enabled us to assign the 57 Fe magnetic hyperfine tensors observed by ENDOR and Mössbauer spectroscopy to the two subclusters. Thus, A I = +25.3 MHz and A II = −28.4 MHz of H ox -CO can be assigned to the two delocalized pairs of 4Fe − 4S . In our coupling model these A-values result for j ≈ 100 cm - 1 where j describes the exchange interaction between [4Fe − 4S and [2Fe H . The 18 MHz A-value of H ox obtained by ENDOR must result from one Fe site of 2Fe H, while the 7.5 MHz ENDOR A-value seems to be associated with 4Fe-4S H . Analysis of the Mössbauer spectra of H red shows that the 4Fe cluster is in the 2+ state and that 2Fe H contains presumably two low-spin Fe II sites with Δ E Q ≈ 0.85 mm/s and δ ≈ 0.08 mm/s. The observation that the 4Fe-4S cluster is in the 2+ state in H ox, H ox -CO, and H red suggests that the 2Fe H subcluster is in the mixed-valent Fe III Fe II state in H ox and H ox -CO. Given the environment of strong-field ligands in 2Fe H, the Fe III site must have low-spin configuration. While such an assignment is compatible with the EPR g-values, low-spin Fe III sites with g ≈ 2 commonly exhibit very anisotropic 57 Fe A-tensors (due to spin-dipolar interactions) and thus the isotropic A-values of H ox and H ox -CO observed by ENDOR are difficult to explain. This point is discussed in some detail.