4‐Hydroxy‐3‐methyl‐6‐(1‐methyl‐2‐oxoalkyl)pyran‐2‐one Synthesis by a Type III Polyketide Synthase from Rhodospirillum centenum

Abstract

The purple photosynthetic bacterium Rhodospirillum centenum has a putative type III polyketide synthase gene (rpsA). Although rpsA was known to be transcribed during the formation of dormant cells, the reaction catalyzed by RpsA was unknown. Thus we examined the RpsA reaction in vitro, using various fatty acyl‐CoAs with even numbers of carbons as starter substrates. RpsA produced tetraketide pyranones as major compounds from one C10–14 fatty acyl‐CoA unit, one malonyl‐CoA unit and two methylmalonyl‐CoA units. We identified these products as 4‐hydroxy‐3‐methyl‐6‐(1‐methyl‐2‐oxoalkyl)pyran‐2‐ones by NMR analysis. RpsA is the first bacterial type III PKS that prefers to incorporate two molecules of methylmalonyl‐CoA as the extender substrate. In addition, in vitro reactions with 13C‐labeled malonyl‐CoA revealed that RpsA produced tetraketide 6‐alkyl‐4‐hydroxy‐1,5‐dimethyl‐2‐oxocyclohexa‐3,5‐diene‐1‐carboxylic acids from C14–20 fatty acyl‐CoAs. This class of compounds is likely synthesized through aldol condensation induced by methine proton abstraction. No type III polyketide synthase that catalyzes this reaction has been reported so far. These two unusual features of RpsA extend the catalytic functions of the type III polyketide synthase family.

Novel Type III Polyketide Synthases Biosynthesize Methylated Polyketides in Mycobacterium marinum

“…3c , Supplementary Table S2 ). Mass spectrometric identification of similar molecules has been previously reported for reaction products of PKS10 and RpsA proteins from Msmeg and R. centenum , respectively 36 , 46 . Both these products could be identified in the three chromatographic peaks observed for MMAR_2474 reactions.…”

Type III Polyketide Synthases: Functional Classification and Phylogenomics

“…BpsA mainly catalyzes the formation of triketide pyrones (Lb‐3‐L) from (branched) long‐chain (C 15 , C 16 , and C 17 ) acyl‐CoA starters in vivo, whereas in vitro it also catalyzes the formation of tetraketide pyrones (L‐4‐L) and alkylresorcinols (L‐4‐A) from starter substrates of various chain lengths (C 4 –C 22 ) . RpsA, like SrsA and FtpA, uses two kinds of extender substrates, but it incorporates the extenders in a different order from SrsA/FtpA to produce tetraketide dimethylpyrones (L‐4m‐L) and tetraketide dimethylresorcylic acids (dimethylresorcinols; L‐4m‐A) from long‐chain (C 6 –C 20 and C 14 –C 20 , respectively) acyl‐CoA starters . The aldol cyclization between methine C2 and C7 is a unique feature of RpsA, because the reaction is disfavored in relation to the usual aldol cyclization between methylene C2 and C7.…”

“…[121] RpsA, like SrsA andF tpA, uses two kinds of extender substrates, but it incorporates the extenders in ad ifferent order from SrsA/FtpA to produce tetraketide dimethylpyrones (L-4m-L) and tetraketide dimethylresorcylica cids (di-methylresorcinols;L -4m-A) from long-chain (C 6 -C 20 and C 14 -C 20 ,r espectively)a cyl-CoA starters. [122] The aldol cyclization between methine C2 and C7 is au nique feature of RpsA, because the reactioni sd isfavored in relation to the usual aldol cyclization between methylene C2 and C7. MtbPKS18 preferentially uses long-chain (C 12 to > C 20 )a cyl-CoA starters to produce triand tetraketide alkylpyrones (L-3-L and L-4-L).…”

Exploiting the Biosynthetic Potential of Type III Polyketide Synthases

“…The presence of the tryptophan residue at position 284 resulted in a modulation of the position of Met293 into the active site, greatly decreasing the space for polyketide synthesis ( Figure 13). A type III PKS from Rhodospirillum centenum, Rhodospirillum polyketide synthase (RpsA), was characterized and found to utilize a variety of acyl-CoAs (C6-C20) as starters, followed by the decarboxylative condensations of one methylmalonyl-CoA, one malonyl-CoA, and another methylmalonyl-CoA to yield alkyldimethylpyrones 45 and/or alkyldimethylresorcylic acids 46 (Scheme 18) [81]. RpsA is the first bacterial type III PKS capable of accepting two units of methylmalonyl-CoA as extenders efficiently.…”

“…2 1 -oxoalkylresorcylic acid synthase (ORAS) from N. crassa was found to catalyze the decarboxylative condensation of four units of malonyl-CoA with a unit of long-chain acyl-CoA (C18) as the preferred starter, followed by a C2 to C7 aldol condensation to generate stearoyl pentaketide resorcylic acid (47) (Scheme 19) [20]. A type III PKS from Rhodospirillum centenum, Rhodospirillum polyketide synthase (RpsA), was characterized and found to utilize a variety of acyl-CoAs (C6-C20) as starters, followed by the decarboxylative condensations of one methylmalonyl-CoA, one malonyl-CoA, and another methylmalonylCoA to yield alkyldimethylpyrones 45 and/or alkyldimethylresorcylic acids 46 (Scheme 18) [81]. RpsA is the first bacterial type III PKS capable of accepting two units of methylmalonyl-CoA as extenders efficiently.…”