Quantum mechanical/molecular mechanical study of mechanisms of heme degradation by the enzyme heme oxygenase: The strategic function of the water cluster

Hui Chen, Yohann Moreau, Etienne Derat, Sason Shaik*

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

67 Scopus citations

Abstract

Heme degradation by heme oxygenase (HO) enzymes is important in maintaining iron homeostasis and prevention of oxidative stress, etc. In response to mechanistic uncertainties, we performed quantum mechanical/molecular mechanical investigations of the heme hydroxylation by HO, in the native route and with the oxygen surrogate donor H2O2. It is demonstrated that H2O2 cannot be deprotonated to yield FeIIIOOH, and hence the surrogate reaction starts from the FeHOOH complex. The calculations show that, when starting from either FeIIIOOH or Fe IIIHOOH, the fully concerted mechanism involving O-O bond breakage and O-Cmeso bond formation is highly disfavored. The low-energy mechanism involves a nonsynchronous, effectively concerted pathway, in which the active species undergoes first O-O bond homolysis followed by a barrier-free (small with FeIIIHOOH) hydroxyl radical attack on the meso position of the porphyrin. During the reaction of FeIIIHOOH, formation of the Por+•FeIV=O species, compound I, competes with heme hydroxylation, thereby reducing the efficiency of the surrogate route. All these conclusions are in accord with experimental findings (Chu, G. C.; Katakura, K.; Zhang, X.; Yoshida, T.; Ikeda-Saito, M. J. Biol. Chem. 1999, 274, 21319). The study highlights the role of the water cluster in the distal pocket in creating "function" for the enzyme; this cluster affects the O-O cleavage and the O-Cmeso formation, but more so it is responsible for the orientation of the hydroxyl radical and for the observed α-meso regioselectivity of hydroxylation (Ortiz de Montellano, P. R. Acc. Chem. Res. 1998, 31, 543). Differences/similarities with P450 and HRP are discussed.

Original languageEnglish
Pages (from-to)1953-1965
Number of pages13
JournalJournal of the American Chemical Society
Volume130
Issue number6
DOIs
StatePublished - 13 Feb 2008

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