The 'push' effect of the thiolate ligand in cytochrome P450: A theoretical gauging

The 'push' effect of the thiolate ligand in cytochrome P450 is investigated using density functional calculations. Theory supports Dawson's postulate that the 'push' effect is crucial for the heterolytic O-O bond cleavage of ferric-peroxide, as well as for controlling the Fe(III)/Fe(II) redox process and gating the catalytic cycle. Two energetic factors that contribute to the 'push' effect are revealed. The dominant one is the field factor (ΔE_field=54-103 kcal/mol) that accounts for the classical electrostatic repulsion with the negative charge of thiolate. The smaller factor is a quantum mechanical effect (ΔE_QM(σ)=39 kcal/mol, ΔE_QM(π)=4 kcal/mol), which is associated with the σ- and π-donor capabilities of thiolate. The effects of ligand replacement, changes in hydrogen bonding and dielectric screening are discussed in term of these quantities. In an environment with a dielectric constant of 5.7, the total 'push' effect is reduced to 29-33 kcal/mol. Manifestations of the 'push' effect on other properties of thiolate enzymes are discussed.