PubMed · 2113372
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W Wiesner, M K Otto, K D Kulbe. 1990. Bacterial enzymes in halogenation processes.. https://doi.org/10.1111/j.1749-6632.1990.tb24283.x
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[structure: see text] Rate constants for two-electron oxidation reactions of Compound I from chloroperoxidase (CPO) with a variety of substrates were measured by stopped-flow kinetic techniques. The thiolate ligand of CPO Compound I activates the iron-oxo species with the result that oxidation reactions are 2 to 3 orders of magnitude faster than oxidations by model iron(IV)-oxo porphyrin radical cations containing weaker binding counterions.
Using a combination of density functional calculations and Mössbauer spectroscopy, we have examined chloroperoxidase compound II (CPO-II). The Mössbauer spectrum of CPO-II suggests the presence of two distinct ferryl species in an approximately 70:30 ratio. Density functional calculations and cryogenic reduction and annealing experiments allow us to assign the major species as an Fe(IV)OH intermediate. The Mössbauer parameters of the minor component are indicative of an authentic iron(IV)oxo species, but we have found the 70:30 ratio to be pH invariant. The unchanging ratio of component concentrations is in agreement with CPO-II's visible absorption spectrum, which shows no change over the enzyme's range of pH stability.
The classical heme-monooxygenase active intermediate, compound I (Cpd-I), incorporates a heme which is oxidized by two equivalents above the resting ferric state, one equivalent associated with a ferryl center, [Fe=O]2+ (FeS = 1), and the other with an active-site radical (RS = 1/2). Theoretical calculations on models of a Cpd-I with a thiolato axial ligand have presented divergent views about its electronic structure. In one picture, the radical is on the porphyrin; in the other, it is on the sulfur. In this report, ENDOR spectroscopy answers the question, does Cpd-I of the enzyme chloroperoxidase contain a porphyrin pi-cation radical or an iron-bound cysteinyl radical: the radical is predominantly on the porphyrin, with spin density on sulfur having an upper bound, rhoS </= rhoSmax approximately 0.23. We further suggest that the same answer applies to Cpd-I of cytochromes P450.