Histidine 20, the crucial proximal axial heme ligand of bacterial heme oxygenase Hmu O from Corynebacterium diphtheriae

Grace C. Chu, Koki Katakura, Takeshi Tomita, Xuhong Zhang, Danyu Sun, Michihiko Sato, Masanao Sasahara, Takamasa Kayama, Masao Ikeda-Saito, Tadashi Yoshida

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28 Citations (Scopus)


The hemin complex of Hmu O, a 24-kDa soluble heme degradation enzyme in Corynebacterium diphtheriae, is coordinated axially to a neutral imidazole of a proximal histidine residue in Hmu O. To identify which of the eight histidines in Hmu O is the proximal heme ligand, we have constructed and expressed the plasmids for eight His → Ala Hmu O mutants. Reconstituted with hemin, the active site structures and enzymatic activity of these mutants have been examined by EPR, resonance Raman, and optical absorption spectroscopy. EPR of the NO-bound ferrous heme-Hmu O mutant complexes reveals His20 as the proximal heme ligand in Hmu O, and this is confirmed by resonance Raman results from the ligand-free ferrous heme-H20A. All eight His → Ala mutants bind hemin stoichiometrically, proving that none of the histidines is essential for hemin-Hmu O formation. However, His20 is crucial to Hmu O catalysis. Its absence by point mutation has inhibited the conversion of hemin to biliverdin. The ferric heme-H20A complex is pentacoordinate. Resonance Raman of the CO-bound ferrous heme-H20A corroborates this and reveals an Fe-C-O bending mode, δ(Fe-C-O), the first reported for a pentacoordinate CO-bound hemeprotein. The appearance of δ(Fe- C-O) in C. diphtheriae Hmu O H20A but not mammalian HO-1 mutant H25A indicates that the heme environment between the two heme oxygenases is different.

Original languageEnglish
Pages (from-to)17494-17500
Number of pages7
JournalJournal of Biological Chemistry
Issue number23
Publication statusPublished - 2000 Jun 9

ASJC Scopus subject areas

  • Biochemistry
  • Molecular Biology
  • Cell Biology


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