INTRODUCTION
Flavonoids are a class of specialized plant metabolites that play
pivotal roles in the survival of land plants in terrestrial
environments. Currently, more than 6900 flavonoids with different
structures have been identified and are known to have indispensable
roles in land plants, not only owing to their antioxidant or
UV-screening functions but also for their reproductions or defense
mechanisms against various stresses.1 Isoflavonoids
are one of the flavonoid groups produced by almost all legumes such asGlycine max (L.) Merr (soybean). Chalcone synthase (CHS), the
first committed enzyme of the flavonoid biosynthetic pathway, catalyzes
the consecutive condensations of three malonyl-CoA molecules into
one p -coumaroyl-CoA molecule to produce
2’,4,4’,6’-tetrahydroxychalcone, which serves as a common flavonoid
precursor (Fig. S1). We have previously reported that an isozyme of
soybean CHS (termed GmCHS1) is a component of the soybean isoflavonoid
metabolon and interacts with other components such as
2-hydroxyisoflavanone synthase isozymes, providing mechanistic evidence
regarding the importance of the protein-protein interactions for the
efficient production of 5-deoxyisoflavonoids in the
soybean.2,3 More recently, we reported the
interactions of CHS, including GmCHS1, with its nonenzymatic interacting
partner, chalcone isomerase-like protein, in a wide variety of land
plants.4 Interactions of this protein with CHS are
widely conserved among land plants and reduce the intrinsic catalytic
promiscuity of CHS to enhance the efficiency of flavonoid productionin planta .4 Although these observations
indicate the functional significance of protein-protein interactions
among CHS and other enzymes and proteins in the metabolons, there is no
atomic-level structural information for protein-protein interactions
that mediate flavonoid metabolon formation. Thus, in the first step of
addressing this issue, we determined the crystal structure of
recombinantly expressed GmCHS1 as one of the key components of a
well-characterized flavonoid metabolon. We also determined the complex
structure of GmCHS1 with bound naringenin, a product isomer of the
CHS-catalyzed reaction, to better understand the catalytic mechanism of
GmCHS1.