Identification of amino acid residues important in the cyclization reactions of chalcone and stilbene synthases.
Chalcone synthase (CHS) and stilbene synthase (STS) catalyse condensation reactions of p-coumaroyl-CoA and three C(2) units from malonyl-CoA up to a common tetraketide intermediate but then catalyse different cyclization reactions to produce naringenin chalcone and resveratrol respectively. On the basis of sequence alignment with other condensing enzymes including 3-ketoacyl-(acyl carrier protein) synthases of polyketide and fatty-acid synthases, site-directed mutagenesis was performed on the active-site G(372)FGPG loops in CHS and STS. The CHS-P375G mutant showed a 6-fold decrease in overall condensing activity with selectively increased production of p-coumaroyltriacetic acid lactone (CTAL, the derailment product of the tetraketide intermediate). Meanwhile, resveratrol production by STS-P(375)G strongly decreased to give various products in the order CTAL> resveratrol approximately bisnoryangonin>naringenin. As a result, naringenin production (cross-reaction) by STS-P(375)G was close to 30% of resveratrol production. Both G(374)L mutants of CHS and STS showed no condensing activity with residual malonyl-CoA decarboxylase activity. These results suggested that the G(372)FGPG loop in CHS and STS contribute to a determination of the outcome during cyclization reactions by serving as a part of the active-site scaffold on which the stereochemistry of cyclization is performed. These observations provide the first biochemical indication that cyclization reactions are modulated by active-site geometry. The implications for the evolutionary relationship of these enzymes are also discussed.
Faculty of Pharmaceutical Sciences, Toyama Medical and Pharmaceutical University, 2630 Sugitani, Toyama 930-0194, Japan.
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Suh
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Fukuma
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Kagami
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Yamazaki
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Shibuya
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Ebizuka
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Sankawa
D000217:Acyltransferases / Q000737:chemistry / Q000235:genetics / Q000302:isolation & purification / Q000378:metabolism*; D000595:Amino Acid Sequence; D007700:Kinetics; D008969:Molecular Sequence Data; D016297:Mutagenesis, Site-Directed; D011993:Recombinant Fusion Proteins / Q000737:chemistry / Q000235:genetics / Q000302:isolation & purification / Q000378:metabolism; D017386:Sequence Homology, Amino Acid
D011993:Recombinant Fusion Proteins; D000217:Acyltransferases; C022674:stilbene synthase; C020141:flavanone synthetase