Enzymology of Pyran Ring A Formation in Salinomycin Biosynthesis

Tetrahydropyran rings are a common feature of complex polyketide natural products, but much remains to be learned about the enzymology of their formation. The enzyme SalBIII from the salinomycin biosynthetic pathway resembles other polyether epoxide hydrolases/cyclases of the MonB family, but SalBIII plays no role in the conventional cascade of ring opening/closing. Mutation in the salBIII gene gave a metabolite in which ring A is not formed. Using this metabolite in vitro as a substrate analogue, SalBIII has been shown to form pyran ring A. We have determined the X‐ray crystal structure of SalBIII, and structure‐guided mutagenesis of putative active‐site residues has identified Asp38 and Asp104 as an essential catalytic dyad. The demonstrated pyran synthase activity of SalBIII further extends the impressive catalytic versatility of α+β barrel fold proteins.     

Insights into the Dual Activity of a Bifunctional Dehydratase‐Cyclase Domain

Oxygen‐containing heterocycles are a common structural motif in polyketide natural products and contribute significantly to their biological activity. Here, we report structural and mechanistic investigations on AmbDH3, a polyketide synthase domain with dual activity as dehydratase (DH) and pyran‐forming cyclase in ambruticin biosynthesis. AmbDH3 is similar to monofunctional DH domains, using H51 and D215 for dehydration. V173 was confirmed as a diagnostic residue for cyclization activity by a mutational study and enzymatic in vitro experiments. Similar motifs were observed in the seemingly monofunctional AmbDH2, which also shows an unexpected cyclase activity. Our results pave the way for mining of hidden cyclases in biosynthetic pathways. They also open interesting prospects for the generation of novel biocatalysts for chemoenzymatic synthesis and pyran‐polyketides by combinatorial biosynthesis.     

Nitrile Synthesis with Aldoxime Dehydratases: A Biocatalytic Platform with Applications in Asymmetric Synthesis,Bulk Chemicals,and Biorefineries

Nitriles comprise a broad group of chemicals that are currently being industrially produced and used in fine chemicals and pharmaceuticals, as well as in bulk applications, polymer chemistry, solvents, etc. Aldoxime dehydratases catalyze the cyanide-free synthesis of nitriles starting from aldoximes under mild conditions, holding potential to become sustainable alternatives for industrial processes. Different aldoxime dehydratases accept a broad range of aldoximes with impressive high substrate loadings of up to >1 Kg L⁻¹ and can efficiently catalyze the reaction in aqueous media as well as in non-aqueous systems, such as organic solvents and solvent-free (neat substrates). This paper provides an overview of the recent developments in this field with emphasis on strategies that may be of relevance for industry and sustainability. When possible, potential links to biorefineries and to the use of biogenic raw materials are discussed.     

A Dehydratase Domain in Ambruticin Biosynthesis Displays Additional Activity as a Pyran‐Forming Cyclase

Hydropyran rings are a common structural motif in reduced polyketides. Information on their biosynthetic formation and particularly the biochemical characterization of the responsible enzymes has only been reported in few cases. The dehydratase domain AmbDH3 from the ambruticin polyketide synthase was investigated. Through in vitro assay of the recombinant domain with synthetically‐derived substrate surrogates, it was shown that it has a second catalytic activity as a cyclase that performs oxa‐conjugate addition. Probing AmbDH3 with synthetic substrate analogues revealed stereoselectivity and substrate tolerance in both substeps. This is the first characterization of a pyran‐forming cyclase from a cis‐AT PKS system and the first report of a polyketide synthase domain with this kind of dual activity. Finally, it was revealed that this domain shows potential for application in chemoenzymatic synthesis.     

Pac13 is a Small,Monomeric Dehydratase that Mediates the Formation of the 3′‐Deoxy Nucleoside of Pacidamycins

The uridyl peptide antibiotics (UPAs), of which pacidamycin is a member, have a clinically unexploited mode of action and an unusual assembly. Perhaps the most striking feature of these molecules is the biosynthetically unique 3′‐deoxyuridine that they share. This moiety is generated by an unusual, small and monomeric dehydratase, Pac13, which catalyses the dehydration of uridine‐5′‐aldehyde. Here we report the structural characterisation of Pac13 with a series of ligands, and gain insight into the enzyme's mechanism demonstrating that H42 is critical to the enzyme's activity and that the reaction is likely to proceed via an E1cB mechanism. The resemblance of the 3′‐deoxy pacidamycin moiety with the synthetic anti‐retrovirals, presents a potential opportunity for the utilisation of Pac13 in the biocatalytic generation of antiviral compounds.     

The Interplay between a Multifunctional Dehydratase Domain and a C‐Methyltransferase Effects Olefin Shift in Ambruticin Biosynthesis

The olefin shift is an important modification during polyketide biosynthesis. Particularly for type I cis‐AT PKS, little information has been gained on the enzymatic mechanisms involved. We present our in vitro investigations on the olefin shift occurring during ambruticin biosynthesis. The unique, multifunctional domain AmbDH4 catalyzes consecutive dehydration, epimerization, and enoyl isomerization. The resulting 3‐enethioate is removed from the equilibrium by α‐methylation catalyzed by the highly specific C‐methyltransferase AmbM. This thermodynamically unfavorable overall process is enabled by the high, concerted substrate specificity of the involved enzymes. AmbDH4 shows close relationship to DH domains and initial mechanistic studies suggest that the olefin shift occurs via a similar proton‐shuttling mechanism as previously described for EI domains from trans‐AT‐PKS.