Role of the Invariant Peptide Fragment Forming NH···S Hydrogen Bonds in the Active Site of Cytochrome P-450 and Chloroperoxidase: Synthesis and Properties of Cys-Containing Peptide Fe(III) and Ga(III) (Octaethylporphinato) Complexes as Models

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Abstract
The primary sequence of Cys-X-Gly-Y− (X, hydrophobic residue; Y, hydrophilic residue) is highly conserved in cytochrome P-450s. The amide NHs of Leu, Gly, and X are assumed to form NH···S hydrogen bonds which are also found in the active site fragment, Cys-Pro-Ala-Leu, of chloroperoxidase (CPO). [FeIII(OEP)(Z-cys-Leu-Gly-Leu-OMe)] (OEP, octaethylporphinato; Z, benzyloxycarbonyl) and [FeIII(OEP)(Z-cys-Pro-Ala-Leu-OMe)] were synthesized as P-450 and CPO model complexes containing the invariant amino acid fragment of the active site. The corresponding gallium(III) complexes were also synthesized to investigate the solution structures using two-dimensional (2D) NMR experiments because the Ga(III) ion is similar to the Fe(III) ion in the ionic radii and in the coordination geometry. The solution structures of the peptide part of the gallium complexes indicate that the invariant fragments maintain a β I-turn−like conformation and then form NH···S hydrogen bonds between SγCys and NH of the third and fourth amino acid residues. The hydrogen bonds have also been confirmed by the 2H NMR spectra of N2H-substituted Fe(III) peptide complexes. The FeIII/FeII redox potentials of the Fe(III) complexes indicate that the NH···S hydrogen bonds in the fragments causes a slight positive shift of the redox potential. The tri- and tetrapeptide Fe(III) complexes containing the invariant fragments of P-450 are kinetically stable at 30 °C in CH2Cl2. In contrast, [FeIII(OEP)(Z-cys-Leu-OMe)] decomposed to give [FeII(OEP)] (22%) and the corresponding disulfide immediately in CD2Cl2 at 30 °C for 1 h. These results indicate that the invariant fragments involving the hydrogen bonds cause the stabilization of the high-spin Fe(III) resting state rather than the positive shift of FeIII/FeII redox potential.