In vitro reconstitution of the first steps of anatoxin-a biosynthesis in Oscillatoria PCC 6506: from free L-proline to acyl carrier protein bound dehydroproline.

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 in cyanobacteria and proposed a biosynthetic scheme starting from L-proline and involving three polyketide synthases for which the starter would be (S)-1-pyrroline-5-carboxylate bound to an acyl carrier protein, AnaD. We now report the in vitro reconstitution of the first steps of this biosynthesis in Oscillatoria PCC 6506. We identified in PCC 6506 the gene coding for an Sfp-like phosphopantetheinyl transferase and purified the gene product, OsPPT, that catalyzed the transfer of the phosphopantetheinyl arm to the serine 41 of AnaD. The pure adenylation protein AnaC loaded L-proline on holo-AnaD and was specific for L-proline (K(m) = 0.97 mM, k(cat) = 68 min(-1)) among the 20 natural amino acids. Among six close structural analogues of L-proline, including (S)-1-pyrroline-5-carboxylate, we only found 3,4-dehydro-L-proline to be an alternate substrate for AnaC (K(m) = 1.5 mM, k(cat) = 29 min(-1)). The putative prolyl-AnaD dehydrogenase, AnaB, purified to homogeneity as a histidine-tagged protein, showed an absorption spectrum characteristic of FAD-containing proteins. It oxidized prolyl-AnaD to dehydroprolyl-AnaD as shown by tryptic digestion of the protein followed by liquid chromatography coupled to tandem mass spectrometry. Alignment of the amino acid sequence of this dehydrogenase with related enzymes showed that AnaB belongs to the acyl-CoA dehydrogenase superfamily and thus probably catalyzes an alpha-beta-dehydrogenation of the thioester-bound proline followed by an aza-allylic isomerization to yield (S)-pyrroline-5-carboxyl-AnaD, the proposed starter for the subsequent polyketide synthase, AnaE.