X-ray Structural Study on Polyketide Synthase Involved in Olivetolic Acid Biosynthesis in Cannabis sativa

Cannabis sativa L. produces unique secondary metabolites known as cannabinoids which have psychotropic and pharmacological activities. Major cannabinoids including DELTA9-tetrahydrocannabinol are biosynthesized from a common precursor olivetolic acid. A type III polyketide synthetase (PKS) from C. sativa named PKS-1 is considered to synthesize the tetraketide intermediate of olivetolic acid by condensation of hexanoyl-CoA with three malonyl-CoA molecules. Whereas most plant PKSs can react with various CoA esters, PKS-1 exhibits a relatively strict substrate specificity for hexanoyl-CoA as the starter substrate. The unique enzymatic property of PKS-1 is important in the cannabinoid pathway. To clarify the structure-function relationship of PKS-1, we have determined X-ray crystal structures of recombinant apo PKS-1 and complexes with hexanoyl-CoA or malonyl-CoA, at resolutions of 1.6 Å, 1.6 Å and 1.8 Å, respectively. The overall structure of PKS-1 is similar to those of other plant PKSs. As for the loop regions around the active site of PKS-1, which are thought to characterize the enzymatic properties, their structures are most similar to those of pyrone synthase from Gerbera hybrida. The binding pattern of the CoA moiety to PKS-1 is similar between hexanoyl-CoA and malonyl-CoA. Compared to other plant PKSs, several amino acid side chains of PKS-1 that constitute the active site cavity are positioned to effectively reduce the cavity volume and bind the aliphatic moiety of hexanoyl-CoA tightly. These structural features of PKS-1 are considered to enable selective binding of hexanoyl-CoA leading to production of cannabinoids unique to C. sativa.第20回日本蛋白質科学会年