Mixed Regioselectivity in the Arg-177 Mutants of Corynebacterium diphtheriae Heme Oxygenase as a Consequence of in-Plane Heme Disorder

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It has been reported that the R183E and R183D mutants of rat heme oxygenase-1 (r-HO-1) produce approximately 30% δ-biliverdin [Zhou, H., et al. (2000) J. Am. Chem. Soc. 122, 8311-8312]. Two plausible mechanisms were proposed to explain the observations. (a) Electrostatic repulsion between E183 (D183) and one of the heme propionates forces the heme to rotate, thereby placing the δ-meso carbon in a position that is susceptible to oxidation. (b) Rearrangement of the distal pocket structure is triggered by the formation of a hydrogen bond between E183 (D183) and K179. A change in the pKa for the FeIII-H2O to FeIII-OH transition of the mutants was interpreted to be consistent with rearrangement of the hydrogen bond network in the distal pocket. The large similarities between the high-frequency portion of the 1H NMR spectra corresponding to the wild type and R183E and R183D mutants were interpreted to indicate that the heme in the mutants is not rotated to a significant extent. We have reexamined this issue by studying the corresponding R177 mutants in heme oxygenase from Corynebacterium diphtheriae (cd-HO). Replacing R177 with E or D results in the formation of approximately 55% α- and 45% δ-biliverdin, whereas the R177A mutant retains α-regioselectivity. In addition, the K13N/Y130F/ R177A triple mutant catalyzed the formation of 60% δ- and 40% α-biliverdin, while single mutants K13N and Y130F did not appreciably change the regioselectivity of the reaction. The pKa of the Fe III-H2O to FeIII-OH transition in wild-type cd-HO is 9.1, and those of the R177E, R177D, R177A, and K13N/ Y130F/R177A mutants are 9.4, 9.5, 9.2, and 8.0, respectively. Thus, no obvious correlation exists between the changes in pKa and the altered regioselectivity. NMR spectroscopic studies conducted with the R177D and R177E mutants of cd-HO revealed the presence of three heme isomers: a major (M) and a minor (m) heme orientational isomer related by a 180° rotation about the α-γ meso axis and an alternative seating (m′) which is related to m by an 85° in-plane rotation of the macrocycle. The in-plane rotation of m to acquire conformation m′ is triggered by electrostatic repulsion between the side chains of D or E at position 177 and heme propionate-6. As a consequence, the δ-meso carbon in m′ is placed in the position occupied by the α-meso carbon in m, where it is susceptible to hydroxylation and subsequent formation of δ-biliverdin.

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