Insights into the Reaction Mechanism of the Prolyl–Acyl Carrier Protein Oxidase Involved in Anatoxin-a and Homoanatoxin-a Biosynthesis

Anatoxin-a and homoanatoxin-a are two potent cyanobacterial neurotoxins. We recently reported the identification of the gene cluster responsible for the biosynthesis of these toxins as well as the in-vitro reconstitution of the first steps of this biosynthesis. We now report experimental evidence supporting the proposed reaction mechanism of AnaB, a flavoprotein homologous to acyl-CoA dehydrogenase. AnaB catalyzes the two-electron oxidation of prolyl-AnaD, which is proline linked to the acyl carrier protein holo-AnaD, to dehydroprolyl-AnaD using oxygen as the second substrate. AnaB is thus an oxidase. By using liquid chromatography coupled to tandem mass spectrometry (LC-MS/MS), we have identified and characterized dehydroprolyl-AnaD, the AnaB product. We estimated an apparent catalytic constant of 1 min^–1 for AnaB catalysis. We synthesized several deuterium-labeled prolines and enzymatically transformed them into their corresponding prolyl-AnaD. These deuterium-labeled prolyl-AnaDs were oxidized in the presence of AnaB, and the deuterium labeling in the remaining substrate and in the product was determined by LC-MS/MS. The data supported a reaction mechanism starting with a rapid enolization followed by a slow oxidation to give the conjugated imine, which in turn was isomerized to pyrroline-5-carboxyl-AnaD. We also showed that cis- and trans-4-fluoro-l-prolyl-AnaD and 3,4-dehydro-l-prolyl-AnaD were transformed into pyrrole-2-carboxyl-AnaD by AnaB. Thus, the 4-fluoro-analogues experienced a β-elimination supporting the AnaB-catalyzed aza–allylic isomerization. We identified by sequence alignment the AnaB active site base, Glu244. We produced, purified, and characterized the E244A AnaB mutant, which is inactive, supporting the catalytic role of E244 as a base.