Designing Enzymatic Reactivity with an Expanded Palette.

The expanding applications of biocatalysis in the chemical and pharmaceutical sectors herald a greener future for these industries. Yet, the range of chemical reactions known to enzymes only covers a small fraction of what is required for modern synthetic routes. To continue the increases in sustainability afforded by converting chemical processes into enzymatic ones, fundamentally new kinds of biocatalytic reactivity are required. Perhaps the very components from which enzymes are constructed, a palette of canonical amino acids and cofactors, inherently limit their catalytic possibilities, even if all the available natural sequence space can be explored. In recent years, there has been an explosion of strategies to produce new biocatalytic function through the incorporation of noncanonical amino acids and synthetic cofactors, new colors which are added to the enzyme design palette. This has enabled new enzymatic reactions that proceed via organocatalytic, organometallic, and photocatalytic mechanisms. Aside from designing new enzymatic activities from scratch, exogenous photocatalysts have recently also been used in synergy with natural enzyme active sites to diverge their reactivity towards radical pathways. This review will highlight recent developments in enriching enzymatic chemistry with new unnatural components, providing an outlook for future directions and needed developments for practicality and sustainability.