ABSTRACT: As a major component of plant specialized metabolism, phenylpropanoid biosynthetic pathways provide anthocyanins for pigmentation, flavonoids such as flavones for protection against UV photodamage, various flavonoid and isoflavonoid inducers of Rhizobium nodulation genes, polymeric lignin for structural support and assorted antimicrobial phytoalexins. As constituents of plant-rich diets and an assortment of herbal medicinal agents, the phenylpropanoids exhibit measurable cancer chemopreventive, antimitotic, estrogenic, antimalarial, antioxidant and antiasthmatic activities. The health benefits of consuming red wine, which contains significant amounts of 3,4',5-trihydroxystilbene (resveratrol) and other phenylpropanoids, highlight the increasing awareness in the medical community and the public at large as to the potential dietary importance of these plant derived compounds. As recently as a decade ago, little was known about the three-dimensional structure of the enzymes involved in these highly branched biosynthetic pathways. Ten years ago, we initiated X-ray crystallographic analyses of key enzymes of this pathway, complemented by biochemical and enzyme engineering studies. We first investigated chalcone synthase (CHS), the entry point of the flavonoid pathway, and its close relative stilbene synthase (STS). Work soon followed on the O-methyl transferases (OMTs) involved in modifications of chalcone, isoflavonoids and metabolic precursors of lignin. More recently, our groups and others have extended the range of phenylpropanoid pathway structural investigations to include the upstream enzymes responsible for the initial recruitment of phenylalanine and tyrosine, as well as a number of reductases, acyltransferases and ancillary tailoring enzymes of phenylpropanoid-derived metabolites. These structure-function studies collectively provide a comprehensive view of an important aspect of phenylpropanoid metabolism. More specifically, these atomic resolution insights into the architecture and mechanistic underpinnings of phenylpropanoid metabolizing enzymes contribute to our understanding of the emergence and on-going evolution of specialized phenylpropanoid products, and underscore the molecular basis of metabolic biodiversity at the chemical level. Finally, the detailed knowledge of the structure, function and evolution of these enzymes of specialized metabolism provide a set of experimental templates for the enzyme and metabolic engineering of production platforms for diverse novel compounds with desirable dietary and medicinal properties.
