Ruffling of metalloporphyrins bound to IsdG and IsdI, two heme-degrading enzymes in Staphylococcus aureus

Abstract

IsdG and IsdI are paralogous proteins that are intracellular components of a complex heme uptake system in Staphylococcus aureus. IsdG and IsdI were shown previously to reductively degrade hemin. Crystal structures of the apoproteins show that these proteins belong to a newly identified heme degradation family distinct from canonical eukaryotic and prokaryotic heme oxygenases. Here we report the crystal structures of an inactive N7A variant of IsdG in complex with Fe3+-protoporphyrin IX (IsdG-hemin) and of IsdI in complex with cobalt protoporphyrin IX (IsdI-CoPPIX) to 1.8 A? or better resolution. These structures show that the metalloporphyrins are buried into similar deep clefts such that the propionic acids form salt bridges to two Arg residues. His 77 (IsdG) or His76 (IsdI), a critical residue required for activity, is coordinated to the Fe3+ or Co3+ atoms, respectively. The bound porphyrin rings form extensive steric interactions in the binding cleft such that the rings are highly distorted from the plane. This distortion is best described as ruffled and places the β- and δ-meso carbons proximal to the distal oxygen-binding site. In the IsdG-hemin structure, Fe3+ is pentacoordinate, and the distal side is occluded by the side chain of Ile55. However, in the structure of IsdI-CoPPIX, the distal side of the CoPPIX accommodates a chloride ion in a cavity formed through a conformational change in Ile55. The chloride ion participates in a hydrogen bond to the side chain amide of Asn6. Together the structures suggest a reaction mechanism in which a reactive peroxide intermediate proceeds with nucleophilic oxidation at the β- or δ-meso carbon of the hemin. © 2008 by The American Society for Biochemistry and Molecular Biology, Inc.