title: Electron paramagnetic resonance investigations of exogenous ligand complexes of low-spin ferric chloroperoxidase: further support for endogenous thiolate ligation to the heme iron.
abstract: Previous spectroscopic studies of chloroperoxidase have provided evidence for endogenous thiolate sulfur donor ligation to the central heme iron of the enzyme. This conclusion is further supported by recent DNA sequence data which revealed the existence of a third cysteine residue (in addition to a disulfide pair detected earlier) in the protein available for coordination to the heme iron. Thus, chloroperoxidase shares many spectroscopic properties with cytochrome P-450, the only other known thiolate-ligated heme protein. Surprisingly, a previous electron paramagnetic resonance (EPR) study of low-spin ferric chloroperoxidase-ligand complexes (Hollenberg, P.F., Hager, L.P., Blumberg, W.E. and Peisach, J. (1980) J. Biol. Chem. 255, 4801-4807) was unable to provide clear support for the presence of a thiolate ligand, although sulfur coordination was implicated. This was, in part, because an insufficient number of complexes was examined. In this work, we have significantly expanded upon the previous EPR study by using an extensive variety of over twenty exogenous ligands including carbon, nitrogen, oxygen, phosphorus and sulfur donors. Crystal field analysis, using the procedure of Blumberg and Peisach, of the present data in comparison with data for analogous complexes of cytochrome P-450-CAM, thiolate-ligated heme model systems, and myoglobin, is clearly indicative of endogenous thiolate ligation for chloroperoxidase. In addition, the UV-visible absorption and EPR spectral data suggest that a carboxylate ligand is a possible candidate for the endogenous sixth ligand to the heme iron that is responsible for the reversible conversion of ferric chloroperoxidase from high-spin to low-spin at low temperatures (less than 200 K).
authors.affiliation: Department of Chemistry, University of South Carolina, Columbia 29208.
authors.forename: M
authors.initials: M
authors.lastname: Sono
authors.forename: L P
authors.initials: LP
authors.lastname: Hager
authors.forename: J H
authors.initials: JH
authors.lastname: Dawson
mesh_terms: D002711:Chloride Peroxidase / Q000737:chemistry*; D003432:Cross-Linking Reagents; D004578:Electron Spin Resonance Spectroscopy; D006418:Heme / Q000737:chemistry*; D007501:Iron / Q000737:chemistry*; D046911:Macromolecular Substances; D003904:Mitosporic Fungi / Q000201:enzymology; D013438:Sulfhydryl Compounds; D014961:X-Ray Diffraction
chemical_list: D003432:Cross-Linking Reagents; D046911:Macromolecular Substances; D013438:Sulfhydryl Compounds; D006418:Heme; D007501:Iron; D002711:Chloride Peroxidase