Evolution of the catalytic mechanism at the dawn of the Baeyer-Villiger monooxygenases

Enzymes are crucial for the emergence and sustenance of life on earth. How they became catalytically active during their evolution is still an open question. Two opposite explanations are plausible: acquiring a mechanism in a series of discrete steps or all at once in a single evolutionary event. Here, we use molecular phylogeny, ancestral sequence reconstruction, and biochemical characterization to follow the evolution of a specialized group of flavoprotein monooxygenases, the bacterial Baeyer-Villiger monooxygenases (BVMOs). These enzymes catalyze an intricate chemical reaction relying on three different elements: a reduced nicotinamide cofactor, dioxygen, and a substrate. Characterization of ancestral BVMOs shows that the catalytic mechanism evolved in a series of steps starting from a FAD-binding protein and further acquiring reactivity and specificity toward each of the elements participating in the reaction. Together, the results of our work portray how an intrinsically complex catalytic mechanism emerged during evolution.