Engineering a dirhodium artificial metalloenzyme for selective olefin cyclopropanation

Abstract

Artificial metalloenzymes (ArMs) formed by incorporating synthetic metal catalysts into protein scaffolds have the potential to impart to chemical reactions selectivity that would be difficult to achieve using metal catalysts alone. In this work, we covalently link an alkyne-substituted dirhodium catalyst to a prolyl oligopeptidase containing a genetically encoded L-4-azidophenylalanine residue to create an ArM that catalyses olefin cyclopropanation. Scaffold mutagenesis is then used to improve the enantioselectivity of this reaction, and cyclopropanation of a range of styrenes and donor-acceptor carbene precursors were accepted. The ArM reduces the formation of byproducts, including those resulting from the reaction of dirhodium-carbene intermediates with water. This shows that an ArM can improve the substrate specificity of a catalyst and, for the first time, the water tolerance of a metal-catalysed reaction. Given the diversity of reactions catalysed by dirhodium complexes, we anticipate that dirhodium ArMs will provide many unique opportunities for selective catalysis.

Figure 1. ArM formation and reactivity.

(a) ArM formation using the SPAAC reaction. (b) Structure of cofactor 1. (c) Representative reactions catalysed by dirhodium complexes.

Figure 2. Homology model of Pfu POP.

The hydrolase domain is shown in green, the propeller domain is shown in grey and cofactor 1 linked at Z477 is shown in red. Sites of different mutations introduced into Pfu POP are shown as coloured spheres.

Figure 3. Kinetic analysis of cyclopanation reactions.

(a) Comparison of product yield versus time for cyclopropantion of styrene using 2 catalysed by various ArMs or 5 (0.5 mol%). (see Supplementary Fig. 3) (b) Structure of 5.

Figure 4. CD spectra for POP variants and ArMs.

(a) Comparing different constructs (10 μM). (b) CD spectra of POP-ZA₄-HFF acquired at 10 °C intervals from 50 to 100 °C (see also Supplementary Fig. 7).