Structural basis for dual functionality of isoflavonoid O-methyltransferases in the evolution of plant defense responses.
In leguminous plants such as pea (Pisum sativum), alfalfa (Medicago sativa), barrel medic (Medicago truncatula), and chickpea (Cicer arietinum), 4'-O-methylation of isoflavonoid natural products occurs early in the biosynthesis of defense chemicals known as phytoalexins. However, among these four species, only pea catalyzes 3-O-methylation that converts the pterocarpanoid isoflavonoid 6a-hydroxymaackiain to pisatin. In pea, pisatin is important for chemical resistance to the pathogenic fungus Nectria hematococca. While barrel medic does not biosynthesize 6a-hydroxymaackiain, when cell suspension cultures are fed 6a-hydroxymaackiain, they accumulate pisatin. In vitro, hydroxyisoflavanone 4'-O-methyltransferase (HI4'OMT) from barrel medic exhibits nearly identical steady state kinetic parameters for the 4'-O-methylation of the isoflavonoid intermediate 2,7,4'-trihydroxyisoflavanone and for the 3-O-methylation of the 6a-hydroxymaackiain isoflavonoid-derived pterocarpanoid intermediate found in pea. Protein x-ray crystal structures of HI4'OMT substrate complexes revealed identically bound conformations for the 2S,3R-stereoisomer of 2,7,4'-trihydroxyisoflavanone and the 6aR,11aR-stereoisomer of 6a-hydroxymaackiain. These results suggest how similar conformations intrinsic to seemingly distinct chemical substrates allowed leguminous plants to use homologous enzymes for two different biosynthetic reactions. The three-dimensional similarity of natural small molecules represents one explanation for how plants may rapidly recruit enzymes for new biosynthetic reactions in response to changing physiological and ecological pressures.
Liu
Chang-Jun
CJ
Howard Hughes Medical Institute, Jack H. Skirball Center for Chemical Biology and Proteomics, Salk Institute for Biological Studies, La Jolla, California, 92037, USA.
Deavours
Bettina E
BE
Richard
Stéphane B
SB
Ferrer
Jean-Luc
JL
Blount
Jack W
JW
Huhman
David
D
Dixon
Richard A
RA
Noel
Joseph P
JP
D000595:Amino Acid Sequence; D001665:Binding Sites; D005075:Biological Evolution; D001711:Biotransformation; D018360:Crystallography, X-Ray; D007113:Immunity, Innate; D046913:Medicago truncatula / Q000166:cytology / Q000201:enzymology; D008745:Methylation; D008780:Methyltransferases / Q000737:chemistry* / Q000378:metabolism*; D008969:Molecular Sequence Data; D010636:Phenols / Q000378:metabolism; D010935:Plant Diseases / Q000276:immunology*; D010940:Plant Proteins / Q000737:chemistry* / Q000378:metabolism*; D017433:Protein Structure, Secondary; D036343:Pterocarpans / Q000096:biosynthesis / Q000737:chemistry / Q000378:metabolism; D012435:S-Adenosylhomocysteine / Q000378:metabolism; D012436:S-Adenosylmethionine / Q000378:metabolism; D021241:Spectrometry, Mass, Electrospray Ionization; D013237:Stereoisomerism; D013329:Structure-Activity Relationship; D013379:Substrate Specificity; D000094162:Phytoalexins
D008780:Methyltransferases; D010636:Phenols; D010940:Plant Proteins; D036343:Pterocarpans; D012435:S-Adenosylhomocysteine; D012436:S-Adenosylmethionine; C047353:medicarpin; D000094162:Phytoalexins; C021452:isoflavone O4'-methyltransferase; C003161:pisatin