Plant polyketide synthases: a chalcone synthase-type enzyme which performs a condensation reaction with methylmalonyl-CoA in the biosynthesis of C-methylated chalcones.
Heterologous screening of a cDNA library from Pinusstrobus seedlings identified clones for two chalcone synthase (CHS) related proteins (PStrCHS1 and PStrCHS2, 87.6% identity). Heterologous expression in Escherichia coli showed that PStrCHS1 performed the typical CHS reaction, that it used starter CoA-esters from the phenylpropanoid pathway, and that it performed three condensation reactions with malonyl-CoA, followed by the ring closure to the chalcone. PstrCHS2 was completely inactive with these starters and also with linear CoA-esters. Activity was detected only with a diketide derivative (N-acetylcysteamine thioester of 3-oxo-5-phenylpent-4-enoic acid) that corresponded to the CHS reaction intermediate postulated after the first condensation reaction. PstrCHS2 performed only one condensation, with 6-styryl-4-hydroxy-2-pyrone derivatives as release products. The enzyme preferred methylmalonyl-CoA against malonyl-CoA, if only methylmalonyl-CoA was available. These properties and a comparison with the CHS from Pinus sylvestris suggested for PstrCHS2 a special function in the biosynthesis of secondary products. In contrast to P. sylvestris, P. strobus contains C-methylated chalcone derivatives, and the methyl group is at the position predicted from a chain extension with methylmalonyl-CoA in the second condensation of the biosynthetic reaction sequence. We propose that PstrCHS2 specifically contributes the condensing reaction with methylmalonyl-CoA to yield a methylated triketide intermediate. We discuss a model that the biosynthesis of C-methylated chalcones represents the simplest example of a modular polyketide synthase.
Schröder
J
J
Institut für Biologie II, Universität Freiburg, Germany. jschroe@ruf.uni-freiburg.de
Raiber
S
S
Berger
T
T
Schmidt
A
A
Schmidt
J
J
Soares-Sello
A M
AM
Bardshiri
E
E
Strack
D
D
Simpson
T J
TJ
Veit
M
M
Schröder
G
G
D000214:Acyl Coenzyme A / Q000737:chemistry / Q000378:metabolism*; D000595:Amino Acid Sequence; D001483:Base Sequence; D002599:Chalcone / Q000737:chemistry / Q000378:metabolism*; D003001:Cloning, Molecular; D004789:Enzyme Activation; D005419:Flavonoids / Q000378:metabolism; D008745:Methylation; D008969:Molecular Sequence Data; D009097:Multienzyme Complexes / Q000737:chemistry / Q000378:metabolism*; D013379:Substrate Specificity; D014197:Trees / Q000201:enzymology* / Q000235:genetics
D000214:Acyl Coenzyme A; D005419:Flavonoids; D009097:Multienzyme Complexes; C015357:methylmalonyl-coenzyme A; D002599:Chalcone