Structural and Functional Adaptation of Vancomycin Resistance VanT Serine Racemases

Abstract

Vancomycin is one of the drugs of last resort against Gram-positive antibiotic-resistant pathogens. However, bacteria have evolved a sophisticated mechanism which remodels the drug target, the D-alanine ending precursors in cell wall synthesis, into precursors terminating with D-lactate or D-serine, to which vancomycin has less affinity. D-Ser is synthesized by VanT serine racemase, which has two unusual characteristics: (i) it is one of the few serine racemases identified in bacteria and (ii) it contains a membrane-bound domain involved in L-Ser uptake. The structure of the catalytic domain of VanTG showed high similarity to alanine racemases, and we identified three specific active site substitutions responsible for L-Ser specificity. The data provide the molecular basis for VanT evolution to a bifunctional enzyme coordinating both transport and racemization. Our findings also illustrate the evolution of the essential alanine racemase into a vancomycin resistance enzyme in response to antibiotic pressure.