EPR spectroscopy of the iron-sulphur cluster and sirohaem in the dissimilatory sulphite reductase (desulphoviridin) from Desulphovibrio gigas

Desulphoviridin in the oxidized state showed EPR signals around g = 6, consistent with the sirohaem being in the high-spin ferric state. This was unreactive with sulphite, sulphide or cyanide; but readily reduced by methyl viologen. When the enzyme was treated with Na 2 S 2 O 4 the sirohaem was slowly reduced and a spectrum of a reduced iron-sulphur cluster at g = 2.07, 1.93, 1.91 appeared over the course of an hour. An intermediate in this reaction was indicated by a free radical signal which appeared within seconds and then gradually disappeared. On treatment with nitrite and reduced methyl viologen, the enzyme gave a spectrum of a nitroxide derivative similar to that seen with plant nitrite reductase. The midpoint reduction potential of the haem was estimated to be −310 mV or less. The iron-sulphur cluster has a very low potential, being only reduced in the presence of free Na 2 S 2 O 4 around −560 mV. Desulphoviridin can be classed with sirohaem-containing iron-sulphur proteins.