The crystal structure of an LLL-configured depsipeptide substrate analogue bound to isopenicillin N synthase

Isopenicillin N synthase (IPNS) is a non-heme iron(II) oxidase, which catalyses the biosynthesis of isopenicillin N (IPN) from the tripeptide δ-L-α-aminoadipoyl-L-cysteinyl-D-valine (LLD-ACV) in a remarkable oxidative bicyclisation reaction. The natural substrate for IPNS is the LLD-configured tripeptide. LLL-ACV is not turned over by the enzyme, but inhibits turnover of the LLD-tripeptide. The mechanism by which this inhibition takes place is not fully understood. Recent studies have employed a range of LLD-configured depsipeptide substrate analogues in crystallographic studies to probe events preceding β-lactam closure in the IPNS reaction cycle. Herein, we report the first crystal structure of IPNS in complex with an LLL-configured depsipeptide analogue, δ-L-α-aminoadipoyl-L-cysteine (1-(R)-carboxy-2-thiomethyl)ethyl ester (LLL-ACOmC). This report describes the crystal structure of the IPNS:Fe(II):LLL-ACOmC complex to 2.0 Å resolution, and discusses attempts to oxygenate this complex at high pressure in order to probe the mechanism by which LLL-configured substrates inhibit IPNS catalysis.