title: The heme complex of Hmu O, a bacterial heme degradation enzyme from Corynebacterium diphtheriae. Structure of the catalytic site.
abstract: Hmu O, a heme degradation enzyme in Corynebacterium diphtheriae, forms a stoichiometric complex with iron protoporphyrin IX and catalyzes the oxygen-dependent conversion of hemin to biliverdin, carbon monoxide, and free iron. Using a multitude of spectroscopic techniques, we have determined the axial ligand coordination of the heme-Hmu O complex. The ferric complex shows a pH-dependent reversible transition between a water-bound hexacoordinate high spin neutral pH form and an alkaline form, having high spin and low spin states, with a pK(a) of 9. (1)H NMR, EPR, and resonance Raman of the heme-Hmu O complex establish that a neutral imidazole of a histidine residue is the proximal ligand of the complex, similar to mammalian heme oxygenase. EPR of the deoxy cobalt porphyrin IX-Hmu O complex confirms this proximal histidine coordination. Oxy cobalt-Hmu O EPR reveals a hydrogen-bonding interaction between the O(2) and an exchangeable proton in the Hmu O distal pocket and two distinct orientations for the bound O(2). Mammalian heme oxygenase has only one O(2) orientation. This difference and the mixed spin states at alkaline pH indicate structural differences in the distal environment between Hmu O and its mammalian counterpart.
authors.affiliation: Department of Physiology and Biophysics, Case Western Reserve University School of Medicine, Cleveland, Ohio 44106-4970, USA.
authors.forename: G C
authors.initials: GC
authors.lastname: Chu
authors.forename: T
authors.initials: T
authors.lastname: Tomita
authors.forename: F D
authors.initials: FD
authors.lastname: Sönnichsen
authors.forename: T
authors.initials: T
authors.lastname: Yoshida
authors.forename: M
authors.initials: M
authors.lastname: Ikeda-Saito
mesh_terms: D001426:Bacterial Proteins / Q000737:chemistry*; D001665:Binding Sites; D003035:Cobalt / Q000737:chemistry; D003353:Corynebacterium diphtheriae / Q000201:enzymology*; D004578:Electron Spin Resonance Spectroscopy; D006419:Heme Oxygenase (Decyclizing); D006427:Hemin / Q000737:chemistry; D006639:Histidine / Q000737:chemistry; D006863:Hydrogen-Ion Concentration; D007501:Iron / Q000737:chemistry; D009682:Magnetic Resonance Spectroscopy; D009097:Multienzyme Complexes; D011524:Protoporphyrins / Q000737:chemistry; D013053:Spectrophotometry; D013059:Spectrum Analysis, Raman
chemical_list: D001426:Bacterial Proteins; D009097:Multienzyme Complexes; D011524:Protoporphyrins; D003035:Cobalt; D006639:Histidine; D006427:Hemin; C028025:protoporphyrin IX; D007501:Iron; D006419:Heme Oxygenase (Decyclizing); C104107:HmuO protein, bacteria