Evidence for a novel type of iron cluster in the respiratory chain of the archaeon Sulfolobus metallicus

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Abstract
A new type of metal centre was detected in the membranes of the thermoacidophilic archaeon Sulfolobus metallicus. This centre has an S=1/2 ground state in the oxidised form, yielding an axial EPR signal with g values at 2.035 (g ||) and 1.97 (g ⊥), optimally detected at 4.6–10 K; in the reduced form it is EPR silent (even spin). These magnetic properties point to a spin‐coupled iron cluster, with a minimum of two iron ions. The centre has a high reduction potential of +350 mV, at pH 6.5. The physiological role of this novel centre was probed through a general characterisation of S. metallicus respiratory chain: this archaeon contains NADH and succinate dehydrogenase activities, and cytochromes b 562, a 586 and a 600 on the oxygen reductase system. Since it is reduced in the presence of succinate, and taking into account its high reduction potential, this centre is proposed to be a functional analogue of the Rieske centres.