The use of small molecule mimics to provide insight into the reactive mechanism of lipoxygenases

Lipoxygenases (LOs) are mononuclear non-heme metalloenzymes that regio- and stereospecifcally convert unsaturated fatty acids into alkyl hydroperoxides. The rate-determining step is generally accepted to be hydrogen to yield a metal(II)-water species and an organic radical. Reported here are the syntheses and characterizations of [FeIII(PY5)(OMe)](CF ₃SO₃)₂, [FeIII(PY5)(OH)](CF ₃SO₃)₂, and [MnIII(PY5)(OH)](CF ₃SO₃)₂, that are structural and functional mimics of the proposed active species in the LO mechanisms. The ligand PY5 (2,6-bis(bis(2-pyridyl)methoxymethane)pyridine) was developed to simulate the histidine-dominated coordination sphere of LOs. Reactivity of the metal(III) complexes with a number of hydrocarbons possessing weak C-H bonds scales best with the substrates' bond dissociation energies (BDEs), rather than pK _{step in the oxidation. Thermodynamic analyses of the three oxidants and their metal(II) end-products estimate the strengths of the O-H bonds in [Fe II(PY5)(MeOH)]2+, [MnII( PY5)(H2O)]2+, and [FeII( PY5)(H2O)]2+ to be 84 (±2.0), 82 (±2.0), and 80 (±2.0) kcal mol−1, respectively. These high BDEs correlate to the high reduction potentials of [FeIII( PY5)(OMe)]2+, [MnIII(PY5)(OH)] 2+, and [FeIII(PY5)(OH)]2+ and provide a large enthalpic driving force for the oxidation of weak C-H bonds (<90 kcal mol−1). These results suggest that the analogously high reduction potentials of the metal(III) sites in LOs allow the enzymes to perform their unusual oxidation chemistry.; Comparison of the three oxidants to non-PY5 oxidants shows that although a strong correlation exists between the thermodynamic driving force and the rate of reaction, other factors modulate the reactivity. [Mn III(PY5)(OH)]2+ is hindered by the greater amount of structural reorganization that occurs upon reduction, as the Mn(II) product lacks the Jahn-Teller distortion of the Mn(III) complex. A greater influence appears to be substrate accessibility to the hydrogen atom accepting atom in the oxidant. Calculations suggest that the methyl group of the exogenous methoxide in [FeIII(PY5)(OMe)]2+ slows its reactivity by a factor of 15 relative to [FeIII( PY5)(OH)]2+, negating the thermodynamic benefit of the methoxide for hydroxide substitution. Thermodynamically weaker oxidants are thus viable in an enzymatic system if the surrounding protein can attract and properly orient the substrate more quickly than its competitors.}