Kinetic and structural analysis of redox-reversible artificial imine reductases

Three artificial imine reductases, constructed via supramolecular anchoring utilising Fe^III-azotochelin, a natural siderophore, to bind an iridium-containing catalyst to periplasmic siderophore-binding protein (PBP) scaffolds, have previously been synthesised and subjected to catalytic testing. Despite exhibiting high homology and possessing conserved siderophore anchor coordinating residues, the three artificial metalloenzymes (ArMs) displayed significant variability in turnover frequencies (TOFs). To further understand the catalytic properties of these ArMs, their kinetic behaviour was evaluated with respect to the reduction of three cyclic imines: dihydroisoquinoline, harmaline, and papaverine. Kinetic analyses revealed that all examined ArMs adhere to Michaelis-Menten kinetics, with the most pronounced saturation profile observed for the substrate harmaline. Additionally, molecular docking studies suggested varied hydrogen-bonding interactions between substrates and residues within the artificial binding pocket. Pi-stacking and pi-cation interactions were identified for harmaline and papaverine, corroborating the higher affinity of these substrates for the ArMs in comparison to dihydroisoquinoline. Furthermore, it was demonstrated that multiple cavities are capable of accommodating substrates in close proximity to the catalytic centre, thereby rationalising the moderate enantioselectivity conferred by the unmodified scaffolds.