Formation of the Δ^18,19 Double Bond and Bis(spiroacetal) in Salinomycin Is Atypically Catalyzed by SlnM, a Methyltransferase‐like Enzyme

Abstract

Salinomycin is aw idely used polyether coccidiostat and was recently found to have antitumor activities.However, the mechanism of its biosynthesis remained largely speculative until now.R eported herein is the identification of an unprecedented function of SlnM, homologous to O-methyltransferases,b yc orrelating its activity with the formation of the D 18,19 double bond and bis(spiroacetal). Detailed in vivo and in vitro investigations revealed that SlnM, using positively charged S-adenosylmethionine (SAM) or sinefungin as the cofactor, catalyzedt he spirocyclization-coupled dehydration of C19 in ahighly atypical fashion to yield salinomycin.

Reconstitution of a mini‐gene cluster combined with ribosome engineering led to effective enhancement of salinomycin production in Streptomyces albus

“…Based on the proposed salinomycin biosynthetic pathway (Fig. 2B) (Jiang et al, 2012;Yurkovich et al, 2012;Jiang et al, 2015;Luhavaya et al, 2015), the epoxidase gene salC, epoxide hydrolase genes salBI and salBII, dehydratase gene salE, furan synthase gene salBIII, P450 monooxygenase gene salD and the cognate ferredoxin gene salF as well as resistance genes salH and salI were selected for evaluation. The transcriptional units (or gene modules) containing tailoring or resistance genes salBIII-E, salD-F, salH-I, salBII-BI and salC were overexpressed under the control of P kasO* or P neo promoter respectively.…”

“…In order to know whether the resulting Str-99 can be used as a better chassis host than the wild-type strain for further enhancing salinomycin production, more assessments were performed. It has been reported that the expression of SAM synthetase MetK was increased in RsmG mutants of several Streptomyces strains (Nishimura et al, 2007;Tanaka et al, 2009), whereas SAM is a co-factor of SalE that catalyses the last step of the salinomycin synthetic pathway (Jiang et al, 2015). It is intriguing to know whether the transcription of metK is upregulated in Str-99, and how it correlates with salinomycin production.…”

SAM-dependent enzyme-catalysed pericyclic reactions in natural product biosynthesis

“…Given that the SAM-binding motif (GXGXG) of LepI is strictly conserved with other methyltransferases, we next probed the involvement of SAM in the LepI-catalysed reactions 7 , 23 – 24 . Since no exogenous SAM was added in the assays, we investigated if SAM copurified with LepI and remained bound.…”

Diversity of the reaction mechanisms of SAM-dependent enzymes

“…The unique structure endows it with the ability to chelate metal ions and outstanding potency in killing cancer stem cells 98 , 99 , 100 , indicating the significance of understanding its biosynthetic mechanism. The formation of its Δ 18 , 19 double bond and bis(spiroacetal) is catalyzed in an unprecedented manner by the SAM-dependent enzyme SlnM 29 . SlnM is moderately homologous to TcmP, an O -MTase 101 , and contains a glycine-rich motif for SAM binding 13 , 102 , 103 .…”

“…Intriguingly, there is a group of SAM-dependent enzymes that share highly similar core domains with MTases but catalyze diverse types of reactions such as decarboxylation, oxidation, cyclization and hydroxylation ( Table 1 20 , 21 , 22 , 23 , 24 , 25 , 26 , 27 , 28 , 29 , 30 , 31 , 32 , 33 , 34 ). In addition, radical SAM-dependent (RS) enzymes, which have been widely studied in radical chemistry, also exhibit the ability to catalyze multiple reactions 35 , 36 , 37 , 38 , 39 , 40 .…”