title: Dissecting paclitaxel-microtubule association: quantitative assessment of the 2'-OH group.
abstract: Paclitaxel (PTX) is a microtubule-stabilizing agent that is widely used in cancer chemotherapy. This structurally complex natural product acts by binding to β-tubulin in assembled microtubules. The 2'-hydroxyl group in the flexible side chain of PTX is an absolute requirement for activity, but its precise role in the drug-receptor interaction has not been specifically investigated. The contribution of the 2'-OH group to the affinity and tubulin-assembly efficacy of PTX has been evaluated through quantitative analysis of PTX derivatives possessing side chain deletions: 2'-deoxy-PTX, N-debenzoyl-2'-deoxy-PTX, and baccatin III. The affinity of 2'-deoxy-PTX for stabilized microtubules was more than 100-fold lower than that of PTX and only ~3-fold greater than the microtubule affinity of baccatin III. No microtubule binding activity was detected for the analogue N-debenzoyl-2'-deoxy-PTX. The tubulin-assembly efficacy of each ligand was consistent with the microtubule binding affinity, as was the trend in cytotoxicities. Molecular dynamics simulations revealed that the 2'-OH group of PTX can form a persistent hydrogen bond with D26 within the microtubule binding site. The absence of this interaction between 2'-deoxy-PTX and the receptor can account for the difference in binding free energy. Computational analyses also provide a possible explanation for why N-debenzoyl-2'-deoxy-PTX is inactive, in spite of the fact that it is essentially a substituted baccatin III. We propose that the hydrogen bonding interaction between the 2'-OH group and D26 is the most important stabilizing interaction that PTX forms with tubulin in the region of the C-13 side chain. We further hypothesize that the substituents at the 3'-position function to orient the 2'-OH group for a productive hydrogen bonding interaction with the protein.
authors.affiliation: Department of Chemistry, Binghamton University, State University of New York, Binghamton, NY 13902, USA.
authors.forename: Shubhada
authors.initials: S
authors.lastname: Sharma
authors.forename: Chandraiah
authors.initials: C
authors.lastname: Lagisetti
authors.forename: Barbara
authors.initials: B
authors.lastname: Poliks
authors.forename: Robert M
authors.initials: RM
authors.lastname: Coates
authors.forename: David G I
authors.initials: DG
authors.lastname: Kingston
authors.forename: Susan
authors.initials: S
authors.lastname: Bane
mesh_terms: D000818:Animals; D000972:Antineoplastic Agents, Phytogenic / Q000737:chemistry / Q000494:pharmacology; D001665:Binding Sites; D002417:Cattle; D002460:Cell Line; D006801:Humans; D008870:Microtubules / Q000378:metabolism*; D008958:Models, Molecular; D017239:Paclitaxel / Q000031:analogs & derivatives / Q000737:chemistry* / Q000494:pharmacology*; D011485:Protein Binding; D043823:Taxoids / Q000494:pharmacology; D014404:Tubulin / Q000378:metabolism*; D050257:Tubulin Modulators / Q000737:chemistry* / Q000494:pharmacology*
chemical_list: D000972:Antineoplastic Agents, Phytogenic; D043823:Taxoids; D014404:Tubulin; D050257:Tubulin Modulators; D017239:Paclitaxel