Reinterpreting the Mechanism of Inhibition of <i>Mycobacterium tuberculosis</i> d‑Alanine:d‑Alanine Ligase by d‑Cycloserine

Abstract

d-Cycloserine is a second-line drug approved for use in the treatment of patients infected with <i>Mycobacterium tuberculosis</i>, the etiologic agent of tuberculosis. The unique mechanism of action of d-cycloserine, compared with those of other clinically employed antimycobacterial agents, represents an untapped and exploitable resource for future rational drug design programs. Here, we show that d-cycloserine is a slow-onset inhibitor of MtDdl and that this behavior is specific to the <i>M. tuberculosis</i> enzyme orthologue. Furthermore, evidence is presented that indicates d-cycloserine binds exclusively to the C-terminal d-alanine binding site, even in the absence of bound d-alanine at the N-terminal binding site. Together, these results led us to propose a new model of d-alanine:d-alanine ligase inhibition by d-cycloserine and suggest new opportunities for rational drug design against an essential, clinically validated mycobacterial target