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Abstract How does enzymatic activity emerge? To shed light on this fundamental question, we study type B dihydrofolate reductases (DfrB), which were discovered for their role in antibiotic resistance. These rudimentary enzymes are evolutionarily distinct from the ubiquitous, monomeric FolA dihydrofolate reductases targeted by the antibiotic trimethoprim. DfrB is unique: it homotetramerizes to form a highly symmetrical central tunnel that accommodates its substrates in close proximity and the right orientation, thus promoting the metabolically essential production of tetrahydrofolate. It is the only known enzyme built from the ancient Src Homology 3 fold, typically a binding module. Strikingly, by studying the evolution of this enzyme family, we observe that no active-site residues are conserved across catalytically active homologs. Integrating experimental and computational analyses, we identify an intricate relationship between homotetramerization and catalytic activity, where formation of a tunnel featuring positive electrostatic potential proves to be a powerful predictor of activity. We demonstrate that the DfrB enzymes have not evolved in response to the synthetic antibiotic to which they confer strong resistance, and propose that DfrB domains evolved the capacity for rudimentary catalysis from a binding capacity. That (rudimentary) catalysis can emerge from the homotetramerization of a binding domain, and that it has been recently recruited by pathogenic bacteria, manifests the opportunistic nature of evolution.