On the emergence of P-Loop NTPase and Rossmann enzymes from a Beta-Alpha-Beta ancestral fragment

This article is dedicated to the memory of Michael G. Rossmann. Dating back to the last universal common ancestor, P-loop NTPases and Rossmanns comprise the most ubiquitous and diverse enzyme lineages. Despite similarities in their overall architecture and phosphate binding motif, a lack of sequence identity and some fundamental structural differences currently designates them as independent emergences. We systematically searched for structure and sequence elements shared by both lineages. We detected homologous segments that span the first beta alpha beta motif of both lineages, including the phosphate binding loop and a conserved aspartate at the tip of beta 2. The latter ligates the catalytic metal in P-loop NTPases, while in Rossmanns it binds the nucleotide's ribose moiety. Tubulin, a Rossmann GTPase, demonstrates the potential of the beta 2-Asp to take either one of these two roles. While convergence cannot be completely ruled out, we show that both lineages likely emerged from a common beta alpha beta segment that comprises the core of these enzyme families to this very day.