title: Role of the colchicine ring A and its methoxy groups in the binding to tubulin and microtubule inhibition.
abstract: The roles of the methoxy substituents on ring A of two ring colchicine (COL) analogues were probed by the synthesis of a number of drugs and the examination of their effect on binding to tubulin, inhibition of microtubule assembly, and induction of GTPase activity. Selective elimination of ring A methoxy groups at positions 2, 3, and 4 weakened all three processes. The effects on binding and inhibition were independent of the nature of ring C (or C'). Specifically, excision of the 2- or 3-methoxy groups weakened binding by ca. 0.4 kcal mol-1, while that of the 4-methoxy group of ring A was weakened by 1.36 +/- 0.15 kcal mol-1. The effect on the inhibition of microtubule assembly, expressed as the equilibrium constant for the binding of the tubulin-drug complex to the end of a microtubule, was more complex and strongly dependent on the nature of ring C (or C'). This was attributed to the abilities of various groups on ring C' to overcome the wobbling in the tubulin-drug complex introduced by the weakening of the anchoring provided by ring A. It is concluded that ring A of COL is not germane to the mechanism of the inhibition of tubulin self-assembly. It serves only as a complex-stabilizing anchor. The control of this process resides in the interactions that key oxygen atoms of ring C of COL or C' of structural analogues establish with the protein. It is proposed that the 4-methoxy group of ring A serves as a key attachment point for immobilization of the drugs on the protein.
authors.affiliation: Department of Biochemistry, Brandeis University, Waltham, Massachusetts 02254-9110, USA.
authors.forename: J M
authors.initials: JM
authors.lastname: Andreu
authors.forename: B
authors.initials: B
authors.lastname: Perez-Ramirez
authors.forename: M J
authors.initials: MJ
authors.lastname: Gorbunoff
authors.forename: D
authors.initials: D
authors.lastname: Ayala
authors.forename: S N
authors.initials: SN
authors.lastname: Timasheff
mesh_terms: D000818:Animals; D001665:Binding Sites / Q000187:drug effects; D002417:Cattle; D003078:Colchicine / Q000031:analogs & derivatives / Q000737:chemistry* / Q000378:metabolism* / Q000494:pharmacology; D004790:Enzyme Induction / Q000187:drug effects; D020558:GTP Phosphohydrolases / Q000096:biosynthesis; D008024:Ligands; D008870:Microtubules / Q000187:drug effects* / Q000201:enzymology / Q000378:metabolism*; D014404:Tubulin / Q000378:metabolism*; D050257:Tubulin Modulators
chemical_list: D008024:Ligands; D014404:Tubulin; D050257:Tubulin Modulators; D020558:GTP Phosphohydrolases; D003078:Colchicine