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J. Biol. Chem., Vol. 275, Issue 34, 26265-26276, August 25, 2000
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From the We have determined the kinetic scheme and the
reaction rates of binding to microtubules of two fluorescent taxoids,
7-O-[N-(4'-fluoresceincarbonyl)-L-alanyl]Taxol (Flutax-1) and
7-O-[N-(2,7-difluoro-4'-fluoresceincarbonyl)-L-alanyl]Taxol (Flutax-2). Flutax-1 and Flutax-2 bind to microtubules with high affinity (Ka
Molecular Recognition of Taxol by Microtubules
KINETICS AND THERMODYNAMICS OF BINDING OF FLUORESCENT TAXOL
DERIVATIVES TO AN EXPOSED SITE*
§,
**, and
Centro de Investigaciones Biológicas,
Consejo Superior de Investigaciones Científicas,
C/Velázquez, 144, 28006 Madrid, Spain, the ¶ Laboratory
of Biomolecular Dynamics, Katholieke Universiteit Leuven,
Celestijnenlaan 200D, B-3001 Heverlee, Belgium, and
Instituto de
Química Orgánica, Consejo Superior de Investigaciones
Científicas, C/Juan de la Cierva 3, 28006 Madrid, Spain
107
M
1, 37 °C). The binding
mechanism consists of a fast bimolecular reaction followed by at least
two monomolecular rearrangements, which were characterized with
stopped-flow techniques. The kinetic constants of the bimolecular
reaction were 6.10 ± 0.22 × 105
M
1 s
1
and 13.8 ± 1.8 × 105
M
1 s
1
at 37 °C, respectively. A second slow binding step has been measured employing the change of fluorescence anisotropy of the probe. The
reversal of this reaction is the rate-limiting step of
dissociation. A third step has been detected using small angle x-ray
scattering and involves a 2-nm increase in the diameter of
microtubules. It is suggested that the first step entails the binding
of the Taxol moiety and the second a relative immobilization of the
fluorescent probe. The equilibrium and some kinetic measurements
required the use of stabilized cross-linked microtubules, which
preserved taxoid binding. The results indicate that the Taxol binding
site is directly accessible, in contrast with its location at lumen in
the current model of microtubules. An alternative structural model is
considered in which the binding site is located between protofilaments,
accessible from the microtubule surface.
*
This work was supported by Comunidad Autonoma de
Madrid Grant 07B/0025/99, Dirección General de Enseñanza
Superior e Investigación Científica (DGESIC) Grants PB-95-0116
and APC 96-0071, and Fundación Científica de la
Asociación Española contra el Cancer (to J. M. A.) and
DGESIC Grant PB-96-0852 (to U. Acuna).The costs of publication of this
article were defrayed in part by the
payment of page charges. The article
must therefore be hereby marked
"advertisement" in
accordance with 18 U.S.C. Section
1734 solely to indicate this fact.
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