Structural and Biochemical Characterization of AidC, a Quorum-Quenching Lactonase with Atypical Selectivity

Quorum-quenching catalysts are of interest for potential application as biuchemical tools for interrogating interbacterial cominunication pathways, as arttibiofonling agents, and as anti-infective agents in plants and animals. Herein, the structure and function of AidC, an N-acyl-Lhornoserine lactone (AHL) lactonase froth. Chryseobaoterium, is characterized. Steady-state kinetics show that zinc-supplemented AidC is the most efficient wild-type quorum-quenthing enzymes characterized to date,. with a k(cat)/K-M value cif approximately 2 X 10(6) M-1 s(-1) tor N-heptanoyl-L-homoserine lactone. The enzyme has stricter substrate selectivity and significantly lower K-M values (ca. 50 mu M for'preferred substrates) compared to those of typical AHL,lactonases (ca. >1 mM). X-ray crystal structures of AidC alone and with the product N-hexanoyl-L-homoSeritie, were determined at resolutions of 1.09 and 1.67 angstrom, respectively. Each structure displays as a dimer, and-climerie oligibmerization was also observed in solution by size-exclusion chromatography coupled with multiangle light scattering. The structures reveal two atypical features as, compared to previously characteriZed AHL lactonaseS: a kinked alpha-helix that forms part of a closed binding pocket, that provides affinity and enforces selectivity for Al-IL substrates and an active-site His substitution that is usually found, in a homologous family of phosphodiesterases. Implications for the catalytic mechanism of AHL lactonaseS are discussed.