New challenges for artificial metalloenzymes based on the Biotin-(Strept)avidin technology

Artificial metalloenzymes are designed to gather advantages of both homogeneous catalysts and enzymes. The biotin-(strept)avidin technology insures the incorporation of an active catalyst in a proteinic scaffold which provides a chiral environment. The combination of these two parts provides efficient artificial metalloenzymes applied in various asymmetric reactions such as hydrogenation, transfer hydrogenation, allylic alkylation, etc…
In this work, the properties of (strept)avidin were used to explore new challenges for artificial metalloenzymes.
Artificial hydrogenases were used as model reaction to demonstrate that artificial metalloenzymes can be more than a concept. With this goal in mind, the loading of the artificial hydogenase was reduced to 0.1% and the concentration of substrate increased to 60 mM with very small erosion of activity and selectivity. The scale of the reaction was increased 15 times without any impact on conversion and ee. Recycling of the artificial hydrogenase was investigated by immobilizing streptavidin using biotin sepharose. The resulting artificial metalloenzymes could be used twice without any loss of activity. Lyophilisation of the artificial hydrogenase allows a convenient storage of the artificial hydrogenase.
The versatibilty of the biotin-streptavidin technology was exploited to create and optimize artificial allylic alkylases. The thermostability of streptavidin and its high tolerance towards organic solvents resulted good conversions and ee values up to 97%.
The thermostability of streptavidin was further investigated as a potential rapid purification strategy towards screening heat-treated cell-free extracts.
With the aim of selective modification of tyrosine-residues, the potential of the artificial allylic alkylases was extended towards the use of phenol as nucleophile. The reaction was optimized to operate under physiological conditions. Detailed investigations of the reaction suggested self-alyllation of the tyrosine residues of the artificial metalloenzyme.
Artificial ligases were developed by incorporation of a biotinylated Hoveyda-Grubbs 2nd generation catalyst in (strept)avidin. The optimization of the artificial metalloenzyme gave good conversions for ring closing metathesis of diallyltosylamine under acidic pH and high salt concentration.