Light teaches (co)enzymes new tricks Light is widely used in organic synthesis to excite electrons in a substrate or catalyst, opening up reactive pathways to a desired product. Biology uses light sparingly in this way, but coenzymes such as flavin can be driven to excited states by light. Biegasiewicz et al. investigated this reactivity and found a suite of flavoenzymes that catalyze asymmetric radical cyclization when exposed to light. “Ene”-reductases, when reduced and illuminated, converted starting materials containing an α-chloroamide and an alkene into five-, six-, seven-, or eight-membered lactams. Different enzymes furnished different stereochemistry in the products, likely because of changes in active-site pocket geometry. Science , this issue p. 1166