Proposed mechanism for post-translational self-modification of Co-NHase based on Co²⁺ diffusion limitation

Nitrile hydratase (NHase), was an excellent biocatalyst for the synthesis of amide compounds. NHase was typical heterodimeric metalloprotein, required of the assistance of activator for active expressions. In this work, we found a special Co-NHase HBA from Caldalkalibacillus thermarum, which had the ability of post-translational self-modification and could incorporate Co²⁺ into the catalytic center in the absence of activator.

We simulated the movement of Co²⁺ in silico and established a hypothetical model to predict the Co²⁺ incorporation efficiency (X_Co ) of NHases. According to the simulation results, NHase mutants with different positive charge distribution were constructed. Compared with wild-type, the Co²⁺ incorporation efficiency of K1 (M10K) was increased by 2.1-fold from 0.36 to 0.76, and the specific activity was increased by 3.2-fold from 136.3 to 432.0 U mg^-1 , while mutant K1H1 (M10K, D11H) and K2H2 (M10K, D11H, E20K, N21H) lost the ability of post-translation self-modification.

The interactions of positively charged residues near the catalytic center, such as lysine with strong electrostatic repulsive interaction, arginine with weak electrostatic repulsive interaction and histidine with metal affinity, could limit the free diffusion of Co²⁺ in NHase and affect the efficiency of post-translational self-modification. This work also provided an effective strategy for protein engineering of NHases and other metalloenzymes.