A NOVEL IRON CENTER IN DESULFOFERRODOXIN FROM D.DESULFURICANS ATCC 27774: CRYSTAL STRUCTURE AT 1.8 RESOLUTION. Pedro M. Matias¹, Ana Coelho^1,2, Maria A. Carrondo^1,3, Vilmos Flp⁴, Ana Gonzalez⁵ and Andy Thompson⁶, ¹ITQB, Universidade Nova de Lisboa, 2780 OEIRAS, Portugal; ²Universidade de vora, 7000 VORA, Portugal; ³IST, Universidade Tcnica de Lisboa, 1000 LISBOA, Portugal; ⁴ LMB and OCMS, University of Oxford, Oxford OX1 3QU, UK ⁵ESRF, BP-220, 38043 GRENOBLE CEDEX France; ⁶EMBL Grenoble Outstation, BP-156, 38042 GRENOBLE CEDEX France

A novel iron centre in desulfoferrodoxin from sulfate reducing bacteria D.desulfuricans ATCC 27774 (DFX) has been structurally characterized: this center contains a single Fe atom octahedrally coordinated to four equatorial histidines, one axial cysteine and one axial water molecule, and unlike most other iron centres in proteins, it is exposed to the solvent, rather than buried within the bulk of the polypeptide chain. The three dimensional structure of DFX contains two domains linked by a short stretch of aminoacid residues. The larger domain is associated with the novel Fe centre. The smaller domain contains a desulforedoxin-like Fe-S₄ centre (DX). This presentation will focus on the structural details that can be perceived from the current model and will include a comparison between DX and the DX-like domain of DFX. The crystal structure of DFX has been determined to 2.5 by the MAD method using data measured at the ESRF. The current R-factor value is 26.0 % (R_free 34.0 %). Refinement against 1.8 data measured at SRS-Daresbury is in progress.