Figure ), and its Fe II -N1 bond making orbital has a small weight contribution of Fe II (0.15) in the respective MO. It is apparent therefore that the corresponding iron ion, in the cysteine ligated heme, will coordinate the TAZ very feebly. On the other hand, the high orbital density for the serine-ligated iron, creates a stronger binding site for TAZ. In a nutshell, our theoretical investigation beautifully explains the enhanced activity of the C-H amination in cysteine→ serine mutation and complements the experimentally observed results. 17