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Plant Cell
2006 Dec 01;1812:3656-69. doi: 10.1105/tpc.106.041376.
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Structural basis for dual functionality of isoflavonoid O-methyltransferases in the evolution of plant defense responses.
Liu CJ
,
Deavours BE
,
Richard SB
,
Ferrer JL
,
Blount JW
,
Huhman D
,
Dixon RA
,
Noel JP
.
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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.
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