STRUCTURE OF ``PRISMANE'' PROTEIN REVEALS TWO NOVEL 4Fe CLUSTERS
S.J.Cooper1, S.Bailey1, D.Collison2, W.R.Hagen3, P.F.Lindley4
1Daresbury Lab, CCLRC, Warrington WA4
4AD, Cheshire, England
2Univ Manchester, Dept Chem,
Manchester M13 9PL, England
3Agr Univ Wageningen, Dept Biochem,
NL-6703 HA Wageningen, Netherlands
4European Synchrotron Radiation
Facility, BP 220, F-38043 Grenoble, France
Metalloproteins are abundant in nature and fulfil a wide range of biological functions. However the basic building blocks of these structures are few, all being variants of three archetypal patterns: [Fe-O-Fe], [2Fe-2S] and [4Fe-4s]. In 1989 an unknown protein was discovered that co-purified with the soluble hydrogenase of a sulphate reducing anaerobic bacterium Desulfovibrio vulgaris. On the basic of EPR spectrascopic analysis it was proposed that the iron sulphur prosthetic group of this protein was unprecidented in its structure and redox chemistry: [6Fe-6S] prismane core with four stable oxidation states within a relatively narrow redox-potential window of ca 0.5V [1]. The results of subsequent extensive biochemical, molecular biological and spectrascopic studies were consistent with the original hypothesis.
The crystal structure of this so called 'prismane protein' from Desulfovibrio desulfuricans has now been determined to 1.72 A [2] and does \textit{not} contain and 6Fe cluster. it contains two 4Fe clusters, cluster 1 is conventional [4Fe-4S] cubane, the other, cluster 2, being a novel 4Fe cluster. The two metal clusters are 12 Aapart situated close to the interface formed by the three domains of the protein. Cluster 1 is bound near the N-terminus by an unusual sequential arrangement of four cystein residues. This differs significantly from typical linkage in high-potential iron proteins, aconitase and bacterial ferredoxins. This cluster is bound near the N-terminus of the molecule close to the exterior of the protein and may participate in electron transfer.
Cluster 2 appears to be unique among the iron sulphur containing protein whose structures have so far been elucidated. The cluster comprises both O and S bridges between the 4 iron atoms. Two iron atoms (Fe5, Fe6) have what could be described a tetrahedral local environment whilst the other two (Fe7,Fe8) are trigonal bipyramidal. The absence of an atom between Fe5 and Fe7 leaves incomplete coordination for both these ions.