Since more than 75% of all drugs involve an N-containing heterocyclic ring, this has started a race among biochemists to develop an effective biocatalyst for C-N bond formation using inert C-H bonds. Such bioengineering was demonstrated by Gellman in 1985 using a porphyrin mimetic, and then by the Fasan group in 2014 using intra-molecular C-H amination catalyzed by CYP450, albeit with a low yield. Ultimately, the Arnold group bioengineered an efficient enzyme, P411, which is a variant of CYP450BM3, by mutating the most conserved axial-ligand cysteine to serine. This newly engineered CYP450 variant was sufficiently powerful to accomplish C-H amination reaction, however, the regioselectivity remained uncontrolled. In a subsequent feat of engineering, the Arnold group used P411 as a scaffold, and reported the first-ever intermolecular C-H amination with significant enantioselectivity. The C-H amination reaction in Scheme 1a is supposed to be mediated by an active ironnitrenoid oxidant (complex 3 in Scheme 2), in a catalytic cycle shown in Scheme 2 (note that 3 is a Compound I (Cpd I) analog). As can be seen, the scheme involves three main catalytic steps that begin with a single electron reduction of the resting ferric complex, 1. The so-formed reduced ferrous complex, 2, readily reacts with the nitrene source (tosyl azide) and forms a short-lived active oxidant 'iron nitrenoid', 3, that directly facilitates the C-H activation. The third step may bifurcate into either an unproductive nitrene reduction or the productive nitrene transfer, which affects the efficacy of the so engineered enzyme. The root cause of this bifurcation remains an enigma, which is the focus of this work. Guided by the above mechanistic questions, we carried out several MD simulations, Density Functional Theory (DFT) calculations, and hybrid QM/MM calculations. We have performed a comprehensive and sequential study starting with the characterization of electronic states of different catalytic steps in Scheme 2, studied the topology of key protein residues by several MD simulations, verified the mechanism of C-H amination by hybrid QM/MM calculation, and revealed the root cause that triggers the unorthodox C-H amination due to serine mutation. We will see how theoretical calculations coherently explain the elegant choreography of protein matrix engineered by directed evolution and ultimately leads to an efficient and selective C-H amination.