In Vitro Reconstitution Reveals a Central Role for the N‐Oxygenase PvfB in (Dihydro)pyrazine‐N‐oxide and Valdiazen Biosynthesis

Abstract

The Pseudomonas virulence factor (pvf) operon is essential for the biosynthesis of two very different natural product scaffolds: the (dihydro)pyrazine-N-oxides and the diazeniumdiolate, valdiazen. PvfB is a member of the nonheme diiron N-oxygenase enzyme family that commonly convert anilines to their nitroaromatic counterparts. In contrast, we show that PvfB catalyzes N-oxygenation of the aamine of valine, first to the hydroxylamine and then the nitroso, while linked to the carrier protein of PvfC. PvfB modification of PvfC-tethered valine was observed directly by protein NMR spectroscopy, establishing the intermediacy of the hydroxylamine. This work reveals a central role for PvfB in the biosynthesis of (dihydro)pyrazine-N-oxides and valdiazen. ThePseudomonas virulence factor (pvf, Figure 1 A), a widely conserved operon in proteobacteria, [1] plays important roles in virulence and bacterial cell-to-cell signaling. [2] This cluster, first reported from Pseudomonas entomophila L48, has now been identified in more than 500 strains (Supporting Information, Dataset S1). [1] We showed that pvf-encoded enzymes from P. entomophila L48 produce a novel family of molecules-(dihydro)pyrazine-N-oxides [(d)PNOs, Figure 1 B)]. Concurrently, the ham operon, a pvf-like cluster from Burkholderia cenocepacia H111, was implicated in the production of a very different molecule, valdiazen-a valinol diazeniumdiolate (Figure 1 B). [3] The (d)PNOs and valdiazen both contain unusual NO functionalities but differ in overall structure. We set out to understand the biosynthetic chemistries for these different compounds. Pvf commonly comprises four genes: a nonribosomal peptide synthetase (NRPS, pvfC or hamD), a non-heme diiron enzyme (pvfB or hamC), and two genes of unknown function, pvfA/hamA and pvfD/hamE (Figure 1 A). The ham operon contains a fifth gene, hamB, encoding a putative cupin protein. Both pvfC and pvfB from P. entomophila are necessary to produce the (d)PNOs. [4] NRPSs selectively activate and tether amino or carboxylic acids to thiolation domains where substrates undergo modifications by tailoring enzymes. [5] The NPRS PvfC activates and tethers l-valine to its thiolation domain (Figure 1 B), [1, 4] and similar chemistry could be predicted for HamD. The role of the putative diiron N-oxygenases PvfB or HamC, however, remained unclear.

Synthetic and biosynthetic routes to nitrogen–nitrogen bonds

“…160,161 Indeed, pvfB and pvfC were demonstrated to be necessary and sufficient for the production of 207-209 in vivo. Recently, the N-hydroxylation mechanism was shown by protein NMR analysis to operate in both fragin and diisopropylpyrazine pyrazine N-oxides biosynthesis as follows: the putative diiron N-oxygenase PvfB/HamC, catalyzes the 2-electron oxidation of valine tethered PCP domain of PvfC/HamD to form a hydroxylamine intermediate 162 (Fig. 32D).…”

Functional Biosynthetic Stereodivergence in a Gene Cluster via a DihydrosydnoneN-oxide

“…26 HamC, which shares homology with diiron N -oxygenase AurF, 27 was found to be responsible for the in vitro nitrogen oxidation of valine, while the substrate was bound to the NRPS complex. 11,23,28 The cupin domain of HamB was suggested nonessential for the production of valdiazen, 11 and that HamA (PvfA) and HamE (PvfD) were nonessential for the production of hydroxylamine intermediates towards valdiazen and pyrazine N -oxide derivatives. 22 From the second operon, HamG was predicted to be an aminotransferase, and HamF a starter condensation domain, 11 and both of them are indispensable for the production of fragin.…”

“…fragin and (±)-valdiazen, based on previous findings from our group and others. 11,[13][14][15][16][19][20][21][22][23][24][25][26] The ham cluster consists of seven genes distributed in two oppositely oriented operons. 11,26 As demonstrated by isotopically labelled feeding experiments, the biosynthesis starts with loading L-valine onto the HamD nonribosomal peptide synthetase (NRPS) complex.…”

Signal Antagonists SuppressPseudomonas syringaePathogenicity

“…However, general efficient synthetic routes to access diazeniumdiolate analogs were missing in the literature. While the biosynthesis of some natural diazeniumdiolates has been investigated, 19,20,47,50–52,54–60 only a limited number of synthetic derivatives have been described. 18–20,61 In the case of leudiazen, the previous synthesis in 4 steps from DL- leucinol yielded only a 15% overall yield (Scheme 1A), 12 being far from efficient.…”