title: Elucidating the role of the proximal cysteine hydrogen-bonding network in ferric cytochrome P450cam and corresponding mutants using magnetic circular dichroism spectroscopy.
abstract: Although extensive research has been performed on various cytochrome P450s, especially Cyt P450cam, there is much to be learned about the mechanism of how its functional unit, a heme b ligated by an axial cysteine, is finely tuned for catalysis by its second coordination sphere. Here we study how the hydrogen-bonding network affects the proximal cysteine and the Fe-S(Cys) bond in ferric Cyt P450cam. This is accomplished using low-temperature magnetic circular dichroism (MCD) spectroscopy on wild-type (wt) Cyt P450cam and on the mutants Q360P (pure ferric high-spin at low temperature) and L358P where the "Cys pocket" has been altered (by removing amino acids involved in the hydrogen-bonding network), and Y96W (pure ferric low-spin). The MCD spectrum of Q360P reveals fourteen electronic transitions between 15200 and 31050 cm(-1). Variable-temperature variable-field (VTVH) saturation curves were used to determine the polarizations of these electronic transitions with respect to in-plane (xy) and out-of-plane (z) polarization relative to the heme. The polarizations, oscillator strengths, and TD-DFT calculations were then used to assign the observed electronic transitions. In the lower energy region, prominent bands at 15909 and 16919 cm(-1) correspond to porphyrin (P) → Fe charge transfer (CT) transitions. The band at 17881 cm(-1) has distinct sulfur S(π) → Fe CT contributions. The Q band is observed as a pseudo A-term (derivative shape) at 18604 and 19539 cm(-1). In the case of the Soret band, the negative component of the expected pseudo A-term is split into two features due to mixing with another π → π* and potentially a P → Fe CT excited state. The resulting three features are observed at 23731, 24859, and 25618 cm(-1). Most importantly, the broad, prominent band at 28570 cm(-1) is assigned to the S(σ) → Fe CT transition, whose intensity is generated through a multitude of CT transitions with strong iron character. For wt, Q360P, and L358P, this band occurs at 28724, 28570, and 28620 cm(-1), respectively. The small shift of this feature upon altering the hydrogen bonds to the proximal cysteine indicates that the role of the Cys pocket is not primarily for electronic fine-tuning of the sulfur donor strength but is more for stabilizing the proximal thiolate against external reactants (NO, O(2), H(3)O(+)), and for properly positioning cysteine to coordinate to the iron center. This aspect is discussed in detail.
authors.affiliation: Department of Chemistry, University of Michigan, Ann Arbor, Michigan 48109, United States.
authors.forename: Mary Grace I
authors.initials: MG
authors.lastname: Galinato
authors.forename: Tatyana
authors.initials: T
authors.lastname: Spolitak
authors.forename: David P
authors.initials: DP
authors.lastname: Ballou
authors.forename: Nicolai
authors.initials: N
authors.lastname: Lehnert
mesh_terms: D001665:Binding Sites; D019475:Camphor 5-Monooxygenase / Q000737:chemistry* / Q000235:genetics* / Q000378:metabolism; D002384:Catalysis; D002942:Circular Dichroism; D003545:Cysteine / Q000737:chemistry* / Q000378:metabolism; D006860:Hydrogen Bonding; D008958:Models, Molecular; D009154:Mutation; D013059:Spectrum Analysis, Raman
chemical_list: D019475:Camphor 5-Monooxygenase; D003545:Cysteine