title: Crystal structure and functional analysis of tetracenomycin ARO/CYC: implications for cyclization specificity of aromatic polyketides.
abstract: Polyketides are a class of natural products with highly diverse chemical structures and pharmaceutical activities. Polyketide cyclization, promoted by the aromatase/cyclase (ARO/CYC), helps diversify aromatic polyketides. How the ARO/CYC promotes highly specific cyclization is not well understood because of the lack of a first-ring ARO/CYC structure. The 1.9 A crystal structure of Tcm ARO/CYC reveals that the enzyme belongs to the Bet v1-like superfamily (or STAR domain family) with a helix-grip fold, and contains a highly conserved interior pocket. Docking, mutagenesis, and an in vivo assay show that the size, shape, and composition of the pocket are important to orient and specifically fold the polyketide chain for C9-C14 first-ring and C7-C16 second-ring cyclizations. Two pocket residues, R69 and Y35, were found to be essential for promoting first- and second-ring cyclization specificity. Different pocket residue mutations affected the polyketide product distribution. A mechanism is proposed based on the structure-mutation-docking results. These results strongly suggest that the regiospecific cyclizations of the first two rings and subsequent aromatizations take place in the interior pocket. The chemical insights gleaned from this work pave the foundation toward defining the molecular rules for the ARO/CYC cyclization specificity, whose rational control will be important for future endeavors in the engineered biosynthesis of novel anticancer and antibiotic aromatic polyketides.
authors.affiliation: Department of Molecular Biology and Biochemistry, Department of Chemistry, University of California, Irvine, CA 92697, USA.
authors.forename: Brian Douglas
authors.initials: BD
authors.lastname: Ames
authors.forename: Tyler Paz
authors.initials: TP
authors.lastname: Korman
authors.forename: Wenjun
authors.initials: W
authors.lastname: Zhang
authors.forename: Peter
authors.initials: P
authors.lastname: Smith
authors.forename: Thanh
authors.initials: T
authors.lastname: Vu
authors.forename: Yi
authors.initials: Y
authors.lastname: Tang
authors.forename: Shiou-Chuan
authors.initials: SC
authors.lastname: Tsai
mesh_terms: D000596:Amino Acids; D001141:Aromatase / Q000737:chemistry* / Q000378:metabolism; D001426:Bacterial Proteins / Q000737:chemistry; D001665:Binding Sites; D003198:Computer Simulation; D018360:Crystallography, X-Ray; D003500:Cyclization; D018942:Macrolides; D009279:Naphthacenes; D013302:Streptomyces; D013379:Substrate Specificity
chemical_list: D000596:Amino Acids; D001426:Bacterial Proteins; D018942:Macrolides; D009279:Naphthacenes; C080889:5838 DNI; D001141:Aromatase