Design and evolution of chimeric streptavidin for protein-enabled dual gold catalysis

Artificial metalloenzymes result from anchoring an organometallic catalyst within an evolvable protein scaffold. Thanks to its dimer of dimers quaternary structure, streptavidin allows the precise positioning of two metal cofactors to activate a single substrate, thus expanding the reaction scope accessible to artificial metalloenzymes. To validate this concept, we report herein on our efforts to engineer and evolve an artificial hydroaminase based on dual gold activation of alkynes. Guided by modelling, we designed a chimeric streptavidin equipped with a hydrophobic lid shielding its active site, which enforces the advantageous positioning of two synergistic biotinylated gold cofactors. Three rounds of directed evolution using Escherichia coli cell-free extracts led to the identification of mutants favouring either the anti-Markovnikov product (an indole carboxamide with 96% regioselectivity, 51 turnover numbers), resulting from a dual gold σ,π-activation of an ethynylphenylurea substrate, or the Markovnikov product (a phenyl-dihydroquinazolinone with 99% regioselectivity, 333 turnover numbers), resulting from the π-activation of the alkyne by gold. Dual catalysis is widely employed by natural metalloenzymes to functionalize challenging substrates. Now, this concept is applied to artificial metalloenzymes by designing a hydroaminase with two biotinylated gold cofactors enabling an unnatural σ,π-activation mechanism of terminal alkynes.