Pseudoglycosyltransferase Catalyzes Nonglycosidic C–N Coupling in Validamycin A Biosynthesis

Abstract

Glycosyltransferases are ubiquitous in nature. They catalyze a glycosidic bond formation between sugar donors and sugar or non-sugar acceptors to produce oligo/polysaccharides, glycoproteins, glycolipids, glycosylated natural products, and other sugar-containing entities. However, a trehalose 6-phosphate synthase-like protein has been found to catalyze an unprecedented non-glycosidic C-N bond formation in the biosynthesis of the aminocyclitol antibiotic validamycin A. This dedicated ‘pseudoglycosyltransferase’ catalyzes a condensation between GDP-valienol and validamine 7-phosphate to give validoxylamine A 7′-phosphate with net retention of the ‘anomeric’ configuration of the donor cyclitol in the product. The enzyme operates in sequence with a phosphatase, which dephosphorylates validoxylamine A 7′-phosphate to validoxylamine A.

Distinct Substrate Specificity and Catalytic Activity of the Pseudoglycosyltransferase VldE

“…As described above, this hybrid compound is relatively unstable, requiring early termination of incubation and immediate analysis of the sample for clear observation of the product. Its low stability may explain why this product was not observed in our previous experiments (Asamizu et al, 2011). In light of the present results, we conclude that the C-terminal domain of VldE can recognize both GDPvalienol ( K m 0.06 ± 0.012 mM) (Asamizu et al, 2011) and GDP-glucose ( K m >2.2 mM) as substrate, but with >37-fold greater affinity toward GDP-valienol.…”

“…Both GDP-valienol and validamycin 7-phosphate were prepared according to procedures reported previously (Asamizu et al, 2011). As expected, chimera-1 was able to catalyze the coupling between GDP-glucose and validamine 7-phosphate to give a hybrid pseudo-aminodisaccharide with an apparent K m value of 1.1 ± 0.07 mM and k cat / K m 0.15 s −1 •mM −1 for GDP-glucose and K m 0.975 ± 0.1 mM and k cat / K m 0.11 s −1 •mM −1 for validamine 7-phosphate (Figures 4K and S5E–F).…”

“…Recently, we discovered a number of putative GTs that can catalyze nonglycosidic C-N couplings in the biosynthesis of pseudosugar-containing natural products, such as the α-glucosidase inhibitors acarbose and amylostatins, the antibiotic pyralomicin 1a, and the antifungal agent validamycin A (Figure 1) (Asamizu et al, 2013; Asamizu et al, 2011; Flatt et al, 2013). The term “pseudosugars” refers to analogs of monosaccharides in which the ring oxygen has been replaced by a methylene group (Barbaud et al, 1995; Barton et al, 1988; Brunkhorst et al, 1999; Callam and Lowary, 2000; Suami and Ogawa, 1990; Yoshikawa et al, 1994).…”

“…The enzymes that catalyze the transfer of pseudosugars, or “pseudoglycosyltransferases” (PsGTs), are believed to have evolved from ancestral GTs. Our recent studies on VldE, a trehalose 6-phosphate synthase (TPS)-like protein from the validamycin biosynthetic pathway, revealed that the enzyme does not catalyze the coupling between UDP-glucose and glucose 6-phosphate, but catalyzes an unprecedented nonglycosidic C-N coupling of GDP-valienol and validamine 7-phosphate to give validoxylamine A 7´-phosphate with an α,α-amino linkage (Figure 2) (Asamizu et al, 2011). This finding raises fundamental questions as to how a GT-like enzyme catalyzes a condensation of two non-sugar molecules with retention of configuration.…”

Synthesis, Biological Activity and Molecular Docking of New Tricyclic Series as α-glucosidase Inhibitors

“…Among them, is the Ncontaining carbasugars such as validamine and valienamine, which have proven to be lead compounds for the development of clinically important agents such as acarbose ( Fig. 1) 2 . It has been shown that the valienamine moiety in acarbose mimics the oxocarbenium ion-like transition state that binds to and inhibits α-glucosidase 3 .…”