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.

Nitrogen–Nitrogen Bond Formation Reactions Involved in Natural Product Biosynthesis

“…57 A comproportionation reaction between NO 2 − and a hydroxyamino group has been proposed to form the diazeniumdiolate moiety. 57,58 Referring to the two proposed transient intermediates involved in cremeomycin (7) biosynthesis, we suggest that the NO x -mediated nonenzymatic mechanism is also applicable to nitrogen−nitrogen bond formation in alanosine (6), fragin, and valdiazen biosynthesis.…”

Regulation of Plant Mineral Nutrition by Signal Molecules

“…Based on the homology observed among ham operons, it was suggested that members of the genera Pseudomonas , Pandorea, and Burkholderia may use the diffusible signal molecule—valdiazen (valinol diazeniumdiolate), which autoregulates its own biosynthesis and also of fragin. Fragin is a metallophore and its metal chelating ability is responsible for its antifungal property [ 184 , 185 , 186 ].…”

Discovery and biosynthesis of guanipiperazine from a NRPS-like pathway

“…[11] Mechanistically, the T domain delivers the acyl thioester to the R domain for reduction by NADPH to give an aldehyde product. For example, Lys2 can reduce α-aminoadipate at C-6 to the semialdehyde in the biosynthesis of lysine in Saccharomyces cerevisiae; [12] CmlP is involved in the biosynthesis of the antibiotic chloramphenicol (Scheme 1) in Streptomyces; [13] PvfC is responsible for the synthesis of a family of pyrazine N-oxides in Pseudomonas virulence; [14,15] and HamD is related to valdiazen biosynthesis in Burkholderia cenocepacia (Scheme 1). [16] CARs have also recently attracted interest as green biocatalysts due to their abilities to convert a broad range of carboxylic acid substrates to the corresponding aldehydes.…”