On the Origins of Enzymes: Phosphate-Binding Polypeptides Mediate Phosphoryl Transfer to Synthesize Adenosine Triphosphate

[Image: see text] Reactions involving the transfer of a phosphoryl (−PO(3)(2–)) group are fundamental to cellular metabolism. These reactions are catalyzed by enzymes, often large and complex, belonging to the phosphate-binding loop (P-loop) nucleoside triphosphatase (NTPase) superfamily. Due to their critical importance in life, it is reasonable to assume that phosphoryl-transfer reactions were also crucial in the pre-LUCA (last universal common ancestor) world and mediated by precursors that were simpler, in terms of their sequence and structure, relative to their modern-day enzyme counterparts. Here, we demonstrate that short phosphate-binding polypeptides (∼50 residues) comprising a single, ancestrally inferred, P-loop or Walker A motif mediate the reversible transfer of a phosphoryl group between two adenosine diphosphate molecules to synthesize adenosine triphosphate and adenosine monophosphate. This activity, although rudimentary, bears resemblance to that of adenylate kinase (a P-loop NTPase enzyme). The polypeptides, dubbed as “P-loop prototypes”, thus relate to contemporary P-loop NTPases in terms of their sequence and function, and yet, given their simplicity, serve as plausible representatives of the early “founder enzymes” involved in proto-metabolic pathways.