THE PRISMANE PROTEIN RESOLVED - X-RAY STRUCTURE AT 1.7 ANGSTROM AND MULTIPLE SPECTROSCOPY OF 2 NOVEL 4FE CLUSTERS

The three-dimensional structure of the native ''putative prismane'' pr otein from Desulfovibrio vulgaris (Hildenborough) has been solved by X -ray crystallography to a resolution of 1.72 Angstrom. The molecule do es not contain a [6Fe-6S] prismane cluster, but rather two 4Fe cluster s some 12 Angstrom apart and situated close to the interfaces formed b y the three domains of the protein. Cluster 1 is a conventional [4Fe-4 S] cubane bound, however, near the N-terminus by an unusual, sequentia l arrangement of four cysteine residues (Cys 3, 6, 15, 21). Cluster 2 is a novel 4Fe structure with two mu(2)-sulfido bridges, two mu(2)-oxo bridges, and a partially occupied, unidentified mu(2) bridge X. The p rotein ligands of cluster 2 are widely scattered through the second ha lf of the sequence and include three cysteine residues (Cys 312, 434, 459), one persulfido-cysteine (Cys 406), two glutamates (Glu 268, 494) , and one histidine (His 244). With this unusual mixture of bridging a nd external type of ligands, cluster 2 is named the ''hybrid'' cluster , and its asymmetric, open structure suggests that it could be the sit e of a catalytic activity. X-ray absorption spectroscopy at the Fe K-e dge is readily interpretable in terms of the crystallographic model wh en allowance is made for volume contraction at 10 K; no Fe .. Fe dista nces beyond 3.1 Angstrom could be identified, EPR, Mossbauer and MCD s pectroscopy have been used to define the oxidation states and the magn etism of the clusters in relation to the crystallographic structure. R educed cluster 1 is a [4Fe-4S](1+) cubane with S = 3/2; it is the firs t biological example of a ''spin-admixed'' iron-sulfur cluster. The hy brid cluster 2 has four oxidation states from (formally) all Fe-III to three Fe-II plus one Fe-III. The four iron ions are exchange coupled resulting in the system spins S = 0, 9/2, 0 (and 4), 1/2, respectively , for the four redox states. Resonance Raman spectroscopy suggests tha t the bridging ligand X which could not be identified unambiguously in the crystal structure is a solvent-exchangeable oxygen.