Enzyme recruitment and its role in metabolic expansion

Although more than 10(9) years have passed since the existence of the last universal common ancestor, proteins have yet to reach the limits of divergence. As a result, metabolic complexity is ever expanding. Identifying and understanding the mechanisms that drive and limit the divergence of protein sequence space impact not only evolutionary biologists investigating molecular evolution but also synthetic biologists seeking to design useful catalysts and engineer novel metabolic pathways. Investigations over the past 50 years indicate that the recruitment of enzymes for new functions is a key event in the acquisition of new metabolic capacity. In this review, we outline the genetic mechanisms that enable recruitment and summarize the present state of knowledge regarding the functional characteristics of extant catalysts that facilitate recruitment. We also highlight recent examples of enzyme recruitment, both from the historical record provided by phylogenetics and from enzyme evolution experiments. We conclude with a look to the future, which promises fruitful consequences from the convergence of molecular evolutionary theory, laboratory-directed evolution, and synthetic biology.