Decarboxylation in P450OleT: To study the mechanism of decarboxylation reaction in the WT P450OleT enzyme, we performed QMMM calculations for H atom transfer (HAT) from the prochiral CH2 group in the DCR substrate, employing a representative snapshot from the most populated structure of the MD simulations. The optimized structure of the reactants' complex (RC) is shown in Figure . The Pro-R hydrogen of the β position is positioned at 2.6 Å and is closer to the oxo moiety of Cpd I than the Pro-S hydrogen at 3.3Å. Moreover, in the optimized geometry of the reactant cluster (RC) the O---H bond length of water W1 is 1.08Å, namely, longer than the pristine O-H bond length (0.98 Å) in the water molecules (cf. Figure ). The potential energy profile for the doublet-spin state for Pro-R H abstraction (cf. Figure ) exhibits an energy barrier of 9.6 kcal/mol, thus revealing a very facile H-abstraction from the β position. To assure the correct energy profile all the way to Cpd II, we also performed the calculations, using the ChemShell routine Nudged Elastic Band, for the H-abstraction process and the energy profile was practically identical (9.9 kcal/mol).