Abstract
An enzyme which is capable of catalyzing C-H amination reactions is considered to be a dream tool for chemists due to its pharmaceutical potential and greener approach. Recently, the Arnold group achieved this feat using an engineered CYP411 enzyme, which further undergoes a random directed evolution which increases its efficiency and selectivity. The present study provides mechanistic insight and the root cause of the success of these mutations to enhance the reactivity and selectivity of the mutant enzyme. This is achieved by means of comprehensive MD simulations and hybrid QM/MM calculations. The study shows that the efficient C-H amination by the engineered CYP411 is a combined outcome of electronic and steric effects. The mutation of the axial cysteine ligand to serine relays electron density to the Fe ion in the heme, and thereby enhances the bonding capability of the heme-iron to the nitrogen atom of the tosyl azide. In comparison, the native cysteine-ligated P450 cannot bind the tosyl azide. Additionally, the A78V and A82L mutations in P411 provide 'bulk' to the active site which increases the enantioselectivity via a steric effect. At the same time, the QM/MM calculations elucidate the C-H amination by the iron nitrenoid, revealing a mechanism analogous to Compound I in the native C-H hydroxylation by P450.