Elaboration of the corticosteroid synthesis pathway in primates through a multi-step enzyme

Metabolic networks are complex cellular systems dependent on the interactions among, and regulation of, the enzymes in the network. However, the mechanisms that lead to the expansion of networks are not well understood. While gene duplication is a major driver of the expansion and functional evolution of metabolic networks, the effect and fate of retained duplicates in a network is poorly understood. Here, I study the evolution of an enzyme family that performs multiple subsequent enzymatic reactions in the corticosteroid pathway in primates to illuminate the mechanisms that shape network components following duplication. The products of the pathway (aldosterone, corticosterone, and cortisol) are steroid hormones that regulate metabolism and stress in tetrapods. These steroids are synthesized by a multi-step enzyme Cytochrome P450 11B (CYP11B) that performs subsequent steps on different carbon atoms of the steroid derivatives. Through ancestral state reconstruction and in vitro characterization, I find the ancestor of the CYP11B1 and CYP11B2 paralogs (in primates) had moderate ability to synthesize cortisol and aldosterone. Following duplication in the primate lineage the CYP11B1 homolog specialized on the production of cortisol while its paralog, CYP11B2, maintained its ability to perform multiple subsequent steps as in the ancestral pathway. Unlike CYP11B1, CYP11B2 could not specialize on the production of aldosterone because it is constrained to perform earlier steps in the corticosteroid synthesis pathway to achieve the final product aldosterone. These results suggest that pathway context, along with tissue-specific regulation, both play a role in shaping potential outcomes of metabolic network elaboration.