EPR Characterization of an Archaeal Succinate Dehydrogenase in the Membrane‐Bound State

The membrane‐bound succinate dehydrogenase from the thermoacidophilic archaeon Thermoplasma acidophilum was characterized by EPR spectroscopy and its functional properties were determined. The highest turnover values of succinate dehydrogenase activity were observed at pH 7.4, which is somewhat above the internal pH value of T. acidophilum. The temperature optimum of the reaction was determined as 78 °C and the K_m value for succinate using phenazine methosulfate as the electron acceptor at 53 °C was 0.32 mM. The membrane‐bound enzyme was able to reduce the artificial electron acceptors phenazine methosulfate, N,N,N′,N′‐tetramethyl‐p‐phenylenediamine, and 2,6‐dichloroindophenol. Succinate oxidation was coupled to oxygen consumption in a completely 2‐n‐heptyl‐4‐hydroxyquinoline‐N‐oxide‐sensitive manner. In the oxidized state, T. acidophilum membranes exhibited an almost isotropic EPR spectrum with g‐values at g_z= 2.017, g_y= 2.000, and g_x= 1.968 that were assigned to a [3Fe‐4S]¹⁺ cluster (S3). Upon reduction with succinate, the membranes displayed a spectrum characteristic of 2Fe‐2S clusters (S1), with g‐values at g_z= 2.029, g_y= 1.935, and g_x= 1.915. In the dithionite‐reduced state, additional resonances can be observed. An axial component, with g‐values at g_z= 2.057, g_y= 1.917, and g_x= 1.917 was assigned to a [4Fe‐4S]¹⁺ cluster. The saturation behaviour of the S1 cluster was strongly altered in the dithionite‐reduced form, thus indicating spin‐spin interaction between the S1 center and another paramagnetic center, possibly cluster S2. In both the succinate and the dithionite‐reduced membranes, parallel‐mode EPR spectra displayed a resonance at g= 14, which may be due to a transition of the S= 2 multiplet of the reduced 3Fe‐4S cluster. Spin quantitation yielded a relative stoichiometry of cluster S1 to cluster S3 of 1:1. The results obtained by EPR spectroscopy indicated that the characteristic iron‐sulfur cluster S1 [2Fe‐2S], S2 [4Fe‐4S], and S3 [3Fe‐4S], were also present in this archaeal succinate dehydrogenase. EPR redox titrations of T. acidophilum membranes at pH 5.5 yielded a reduction potential of +60±20 mV for cluster S3 and of +68±20 mV for cluster S1. The axial [4Fe‐4S]^2+/1+ center had a reduction potential of −210±20 mV.