title: 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 C(10-14) 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 (13)C-labeled malonyl-CoA revealed that RpsA produced tetraketide 6-alkyl-4-hydroxy-1,5-dimethyl-2-oxocyclohexa-3,5-diene-1-carboxylic acids from C(14-20) 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.
authors.affiliation: Department of Biotechnology, Graduate School of Agricultural and Life Sciences, The University of Tokyo, Bunkyo-ku, Tokyo 113-8657, Japan.
authors.forename: Takayoshi
authors.initials: T
authors.lastname: Awakawa
authors.forename: Yoshinori
authors.initials: Y
authors.lastname: Sugai
authors.forename: Kanae
authors.initials: K
authors.lastname: Otsutomo
authors.forename: Shukun
authors.initials: S
authors.lastname: Ren
authors.forename: Shinji
authors.initials: S
authors.lastname: Masuda
authors.forename: Yohei
authors.initials: Y
authors.lastname: Katsuyama
authors.forename: Sueharu
authors.initials: S
authors.lastname: Horinouchi
authors.forename: Yasuo
authors.initials: Y
authors.lastname: Ohnishi
mesh_terms: D000214:Acyl Coenzyme A / Q000378:metabolism*; D000217:Acyltransferases / Q000235:genetics / Q000378:metabolism*; D056426:Genetic Loci; D008316:Malonyl Coenzyme A / Q000378:metabolism; D011714:Pyrans / Q000737:chemistry* / Q000378:metabolism*; D042182:Rhodospirillum centenum / Q000737:chemistry / Q000201:enzymology* / Q000235:genetics / Q000378:metabolism; D013379:Substrate Specificity
chemical_list: D000214:Acyl Coenzyme A; D011714:Pyrans; C015357:methylmalonyl-coenzyme A; D008316:Malonyl Coenzyme A; D000217:Acyltransferases; C020141:flavanone synthetase