title: Evolution of the chalcone-isomerase fold from fatty-acid binding to stereospecific catalysis.
abstract: Specialized metabolic enzymes biosynthesize chemicals of ecological importance, often sharing a pedigree with primary metabolic enzymes. However, the lineage of the enzyme chalcone isomerase (CHI) remained unknown. In vascular plants, CHI-catalysed conversion of chalcones to chiral (S)-flavanones is a committed step in the production of plant flavonoids, compounds that contribute to attraction, defence and development. CHI operates near the diffusion limit with stereospecific control. Although associated primarily with plants, the CHI fold occurs in several other eukaryotic lineages and in some bacteria. Here we report crystal structures, ligand-binding properties and in vivo functional characterization of a non-catalytic CHI-fold family from plants. Arabidopsis thaliana contains five actively transcribed genes encoding CHI-fold proteins, three of which additionally encode amino-terminal chloroplast-transit sequences. These three CHI-fold proteins localize to plastids, the site of de novo fatty-acid biosynthesis in plant cells. Furthermore, their expression profiles correlate with those of core fatty-acid biosynthetic enzymes, with maximal expression occurring in seeds and coinciding with increased fatty-acid storage in the developing embryo. In vitro, these proteins are fatty-acid-binding proteins (FAPs). FAP knockout A. thaliana plants show elevated α-linolenic acid levels and marked reproductive defects, including aberrant seed formation. Notably, the FAP discovery defines the adaptive evolution of a stereospecific and catalytically 'perfected' enzyme from a non-enzymatic ancestor over a defined period of plant evolution.
authors.affiliation: Department of Genetics, Development, and Cell Biology, Iowa State University, Ames, Iowa 50011, USA.
authors.forename: Micheline N
authors.initials: MN
authors.lastname: Ngaki
authors.forename: Gordon V
authors.initials: GV
authors.lastname: Louie
authors.forename: Ryan N
authors.initials: RN
authors.lastname: Philippe
authors.forename: Gerard
authors.initials: G
authors.lastname: Manning
authors.forename: Florence
authors.initials: F
authors.lastname: Pojer
authors.forename: Marianne E
authors.initials: ME
authors.lastname: Bowman
authors.forename: Ling
authors.initials: L
authors.lastname: Li
authors.forename: Elise
authors.initials: E
authors.lastname: Larsen
authors.forename: Eve Syrkin
authors.initials: ES
authors.lastname: Wurtele
authors.forename: Joseph P
authors.initials: JP
authors.lastname: Noel
mesh_terms: D017360:Arabidopsis / Q000737:chemistry* / Q000201:enzymology / Q000235:genetics / Q000254:growth & development; D029681:Arabidopsis Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D055162:Biocatalysis; D018360:Crystallography, X-Ray; D019143:Evolution, Molecular; D050556:Fatty Acid-Binding Proteins / Q000737:chemistry / Q000172:deficiency / Q000235:genetics / Q000378:metabolism; D005227:Fatty Acids / Q000378:metabolism*; D019753:Intramolecular Lyases / Q000737:chemistry* / Q000172:deficiency / Q000235:genetics / Q000378:metabolism*; D008024:Ligands; D008958:Models, Molecular; D010641:Phenotype; D011485:Protein Binding; D017510:Protein Folding; D013237:Stereoisomerism; D017962:alpha-Linolenic Acid / Q000378:metabolism
chemical_list: D029681:Arabidopsis Proteins; D050556:Fatty Acid-Binding Proteins; D005227:Fatty Acids; D008024:Ligands; D017962:alpha-Linolenic Acid; D019753:Intramolecular Lyases; C019865:chalcone isomerase