EVIDENCE FOR A NOVEL TYPE OF IRON CLUSTER IN THE RESPIRATORY-CHAIN OFTHE ARCHAEON SULFOLOBUS-METALLICUS

A new type of metal centre was detected in the membranes of the thermo acidophilic archaeon Sulfolobus metallicus. This centre has an S=1/2 g round state in the oxidised form, yielding an axial EPR signal with g values at 2.035 (g(parallel to)) and 1.97 (g(perpendicular to)), optim ally 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 pote ntial of + 350 mV, at pH 6.5, The physiological role of this novel cen tre was probed through a general characterisation of S, metallicus res piratory chain: this archaeon contains NADH and succinate dehydrogenas e activities, and cytochromes b(562), a(586) and a(600) on the oxygen reductase system. Since it is reduced in the presence of succinate, an d taking into account its high reduction potential, this centre is pro posed to be a functional analogue of the Rieske centres, (C) 1998 Fede ration of European Biochemical Societies.