title: Crystal structure of the polyketide cyclase AknH with bound substrate and product analogue: implications for catalytic mechanism and product stereoselectivity.
abstract: AknH is a small polyketide cyclase that catalyses the closure of the fourth carbon ring in aclacinomycin biosynthesis in Streptomyces galilaeus, converting aklanonic acid methyl ester to aklaviketone. The crystal structure analysis of this enzyme, in complex with substrate and product analogue, showed that it is closely related in fold and mechanism to the polyketide cyclase SnoaL that catalyses the corresponding reaction in the biosynthesis of nogalamycin. Similarity is also apparent at a functional level as AknH can convert nogalonic acid methyl ester, the natural substrate of SnoaL, to auraviketone in vitro and in constructs in vivo. Despite the conserved structural and mechanistic features between these enzymes, the reaction products of AknH and SnoaL are stereochemically distinct. Supplied with the same substrate, AknH yields a C9-R product, like most members of this family of polyketide cyclases, whereas the product of SnoaL has the opposite C9-S stereochemistry. A comparison of high-resolution crystal structures of the two enzymes combined with in vitro mutagenesis studies revealed two critical amino acid substitutions in the active sites, which contribute to product stereoselectivity in AknH. Replacement of residues Tyr15 and Asn51 of AknH, located in the vicinity of the main catalytic residue Asp121, by their SnoaL counter-parts phenylalanine and leucine, respectively, results in a complete loss of product stereoselectivity.
authors.affiliation: Department of Biochemistry and Food Chemistry, University of Turku, FIN-20014 Turku, Finland.
authors.forename: Pauli
authors.initials: P
authors.lastname: Kallio
authors.forename: Azmiri
authors.initials: A
authors.lastname: Sultana
authors.forename: Jarmo
authors.initials: J
authors.lastname: Niemi
authors.forename: Pekka
authors.initials: P
authors.lastname: Mäntsälä
authors.forename: Gunter
authors.initials: G
authors.lastname: Schneider
mesh_terms: D015250:Aclarubicin / Q000031:analogs & derivatives / Q000096:biosynthesis / Q000737:chemistry; D000595:Amino Acid Sequence; D001426:Bacterial Proteins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D001665:Binding Sites; D002384:Catalysis; D018360:Crystallography, X-Ray; D007535:Isomerases / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D008958:Models, Molecular; D008969:Molecular Sequence Data; D015394:Molecular Structure; D016297:Mutagenesis, Site-Directed; D009621:Nogalamycin / Q000096:biosynthesis / Q000737:chemistry; D011485:Protein Binding; D020836:Protein Structure, Quaternary; D017434:Protein Structure, Tertiary; D021122:Protein Subunits / Q000737:chemistry / Q000378:metabolism; D011993:Recombinant Fusion Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D013302:Streptomyces / Q000201:enzymology*; D013379:Substrate Specificity
chemical_list: D001426:Bacterial Proteins; D021122:Protein Subunits; D011993:Recombinant Fusion Proteins; C011157:aclacinomycins; D015250:Aclarubicin; D007535:Isomerases; C091661:aklanonic acid methyl ester cyclase; D009621:Nogalamycin