Characterization of succinate dehydrogenase from Micrococcus luteus (lysodeikticus) by electron‐spin‐resonance spectroscopy

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Abstract
Low-temperature electron spin resonance spectroscopy has been used to study the biophysical properties of succinate dehydrogenase from the gram-positive bacterium Micrococcus luteus. The paramagnetic redox centres of the enzyme were identified in a succinate-dehydrogenase–antigen complex, which had been purified with the aid of monospecific serum from membranes solubilized with Triton X-100. The centres were characterized in further detail using the membrane-bound and Triton-solubilized forms of the enzyme. These studies distinguished two types of iron-sulphur centres, viz. a [4Fe-4S]3+ cluster displaying a narrow signal at g= 2.01 in the oxidized state (conventionally termed centre S-3) and a [2Fe–2S] cluster with an axial signal at g= 2.03 and 1.93 in the reduced state (conventionally termed centre S-1). Centre S-3 had a mid-point redox potential of + 10 mV, a comparatively low value for this type of cluster. The behaviour of the g= 1.93 signal of centre S-1 was a complex function of the redox potential, microwave power and temperature of measurement. When measured at low power (i.e. non-saturating conditions), the intensities observed for the g= 1.93 signal poised at various critical potentials in the redox titration were similar. However, the corresponding intensities differed markedly at high power, where conditions were saturating. It is proposed that under saturating conditions the spin-lattice relaxation of the [2Fe–2S] cluster S-1 (mid-point potential +70 mV) is enhanced by centre S-3 between the potential range +10 –+ 70 mV and by an ESR-silent centre, termed centre S-2, with a mid-point potential of −295 mV.