Structural Elucidation of Chalcone Reductase and Implications for Deoxychalcone Biosynthesis

Erin K Bomati,Michael B Austin,Marianne E Bowman,Richard A Dixon,Joseph P Noel

doi:10.1074/jbc.m502239200

Erin K Bomati, Michael B Austin + Show 3 more

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https://doi.org/10.1074/jbc.m502239200

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Abstract

4,2',4',6'-Tetrahydroxychalcone (chalcone) and 4,2',4'-trihydroxychalcone (deoxychalcone) serve as precursors of ecologically important flavonoids and isoflavonoids. Deoxychalcone formation depends on chalcone synthase and chalcone reductase; however, the identity of the chalcone reductase substrate out of the possible substrates formed during the multistep reaction catalyzed by chalcone synthase remains experimentally elusive. We report here the three-dimensional structure of alfalfa chalcone reductase bound to the NADP+ cofactor and propose the identity and binding mode of its substrate, namely the non-aromatized coumaryl-trione intermediate of the chalcone synthase-catalyzed cyclization of the fully extended coumaryl-tetraketide thioester intermediate. In the absence of a ternary complex, the quality of the refined NADP+-bound chalcone reductase structure serves as a template for computer-assisted docking to evaluate the likelihood of possible substrates. Interestingly, chalcone reductase adopts the three-dimensional structure of the aldo/keto reductase superfamily. The aldo/keto reductase fold is structurally distinct from all known ketoreductases of fatty acid biosynthesis, which instead belong to the short-chain dehydrogenase/reductase superfamily. The results presented here provide structural support for convergent functional evolution of these two ketoreductases that share similar roles in the biosynthesis of fatty acids/polyketides. In addition, the chalcone reductase structure represents the first protein structure of a member of the aldo/ketoreductase 4 family. Therefore, the chalcone reductase structure serves as a template for the homology modeling of other aldo/keto-reductase 4 family members, including the reductase involved in morphine biosynthesis, namely codeinone reductase.

Highlights

Introduction4,2؅,4؅,6؅-tetrahydroxychalcone (chalcone) and 4,2؅,4؅trihydroxychalcone (deoxychalcone) serve as precursors of ecologically important flavonoids and isoflavonoids
4,2؅,4؅,6؅-tetrahydroxychalcone and 4,2؅,4؅trihydroxychalcone serve as precursors of ecologically important flavonoids and isoflavonoids
The chalcone reductase structure serves as a template for the homology modeling of other aldo/keto-reductase 4 family members, including the reductase involved in morphine biosynthesis, namely codeinone reductase

Summary

Introduction

4,2؅,4؅,6؅-tetrahydroxychalcone (chalcone) and 4,2؅,4؅trihydroxychalcone (deoxychalcone) serve as precursors of ecologically important flavonoids and isoflavonoids. Chalcone Synthase (CHS) and Chalcone Reductase (CHR) play key roles in the formation of an important set of phytoalexin precursors (recently reviewed in Ref. 3) In both legumes and non-legumes, CHS catalyzes the first committed step of flavonoid biosynthesis, forming 4,2Ј,4Ј6Јtetrahydroxychalcone (chalcone) from coumaroyl-CoA and three molecules of malonyl-CoA. CHR sits at a critical biosynthetic branch point that has afforded legumes with the additional ability to synthesize a set of related deoxychalcone-derived phytoalexins in response to herbivore or pathogen attack including isoflavonoids, coumestans, pterocarpans, and isoflavans [2]. In addition to their activity as phytoalexins, CHR products function in the induction of symbiotic root nodulation by Rhizobium bacteria leading to nitrogen fixation [4, 5]. CHR bears sequence similarity to aldo-keto reductase (AKR) superfamily enzymes from primary metabolism, such as aldose reductase [21] and 3␣-hydroxy steroid dehydrogenase [22]

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