title: Structural insights into CPT-11 activation by mammalian carboxylesterases.
abstract: Mammalian carboxylesterases cleave the anticancer prodrug CPT-11 (Irinotecan) into SN-38, a potent topoisomerase I poison, and 4-piperidino-piperidine (4PP). We present the 2.5 A crystal structure of rabbit liver carboxylesterase (rCE), the most efficient enzyme known to activate CPT-11 in this manner, in complex with the leaving group 4PP. 4PP is observed bound adjacent to a high-mannose Asn-linked glycosylation site on the surface of rCE. This product-binding site is separated from the catalytic gorge by a thin wall of amino acid side chains, suggesting that 4PP may be released through this secondary product exit pore. The crystallographic observation of a leaving group bound on the surface of rCE supports the 'back door' product exit site proposed for the acetylcholinesterases. These results may facilitate the design of improved anticancer drugs or enzymes for use in viral-directed cancer cotherapies.
authors.affiliation: Department of Chemistry, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599, USA.
authors.forename: Sompop
authors.initials: S
authors.lastname: Bencharit
authors.forename: Christopher L
authors.initials: CL
authors.lastname: Morton
authors.forename: Escher L
authors.initials: EL
authors.lastname: Howard-Williams
authors.forename: Mary K
authors.initials: MK
authors.lastname: Danks
authors.forename: Philip M
authors.initials: PM
authors.lastname: Potter
authors.forename: Matthew R
authors.initials: MR
authors.lastname: Redinbo
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D000972:Antineoplastic Agents, Phytogenic / Q000737:chemistry / Q000378:metabolism*; D001216:Asparagine / Q000378:metabolism; D001665:Binding Sites; D002166:Camptothecin / Q000031:analogs & derivatives / Q000737:chemistry / Q000378:metabolism*; D002265:Carboxylic Ester Hydrolases / Q000737:chemistry* / Q000378:metabolism*; D018360:Crystallography, X-Ray; D006031:Glycosylation; D000077146:Irinotecan; D008099:Liver / Q000201:enzymology; D008958:Models, Molecular; D008969:Molecular Sequence Data; D018583:Pliability; D011355:Prodrugs / Q000737:chemistry / Q000378:metabolism*; D011817:Rabbits; D013329:Structure-Activity Relationship
chemical_list: D000972:Antineoplastic Agents, Phytogenic; D011355:Prodrugs; D001216:Asparagine; D000077146:Irinotecan; D002265:Carboxylic Ester Hydrolases; D002166:Camptothecin