Taxol binding site

Fig. 5. 3D EM shows how kinesin and tau bind to microtubules. (A) Reconstruction of a microtubule decorated with kinesin heads (ochre). One kinesin head binds per afi-tubulin heterodimer (grey) and, due to its asymmetric form, can be used to distinguish between the subunits. (B) Inside view of a microtubule that was coassembled with gold-labeled tau and decorated with kinesin heads. The kinesin heads can be seen on the outside through the holes between the protofilaments. The labeled repeat motif of tau binds to the inside face of microtubule. The averaged nanogold density (yellow), which is attached to a repeat motif of tau through a linker, can only be seen near the Taxol binding site of -tubulin, but not on the a subunit (Kar et al, 2003a). The ribbon diagram of the refined zinc-sheet structure is also shown for reference (see Figure 3).

Discodermolide (DDM, Fig. 16) is a marine natural product that promotes MT assembly more potently than PTX and is active against some PTX-resistant cell lines [118-120], The photoaffmity analogue C19-BPC-DDM labels (3-tubulin in close proximity to the taxol binding site [121] and DDM itself is a competitive inhibitor of PTX binding to MT [120], suggesting that DDM also binds to the PTX binding site on P-tubulin. 

Fig. 30 Taxoid site on (3-tubulin and predicted peloruside site on a-tubulin. a Surface representation (view from the inner side of the microtubule) of a tubulin dimer with FIX (red) bound to P-tubulin (green) and peloruside A (orange) bound to the predicted site in a-tubulin (blue), b View of the peloruside binding site. Hydrogen bonds are represented as yellow dashed lines, and the residues involved in these bonds are labeled. Some secondary structure elements are also labeled, c View of the taxol binding site. Some secondary structure elements are labeled. In panels b and c, H7 is colored in orange, and the N-terminal and intermediate domains are colored in green and blue, respectively. (Reprinted with permission from [17]. Copyright 2006 American Chemical Society)...

Andreu, J. M. Barasoain, I. The interaction of baccatin III with Taxol binding site of microtubules determined by a homogeneous assay with fluorescent taxoid. Biochemistry, 2001, 40 11975-11984. 

Rao, S., He, L., Chakravarty, S., et al. (1999) Characterization of the taxol binding site on the microtubule. Identification of Arg282 in P-Tubulin as the site of photoincorporation of a 7-benzophenone analogue of taxol. Journal of Biological Chemistry, 274, 37990-37994. 

Known antimitotic agents (see Fig. 2) bind to a number of characterized sites on the tubulin protein. The most well characterized sites include the taxol binding site, the vinca alkaloid domain and the colchicine site. 

Rao S, Orr GA, Chaudhary AG et al. Characterization of the taxol binding site on the microtubule. 2-(m-Azido benzoyljtaxol photolabels a pepdde (amino acids 217-231) of tubulin. J Biol Chem 1995 270 20235-20238. 

A recent paper describes the use of Flutax-1 and -2 (11.3.12 and 11.3.13) as probes for the study of the binding of taxol to microtubules. A binding mechanism consisting of a fast bimolecular reaction followed by at least two slow monomolecular rearrangements was developed. The results also suggested that the taxol binding site is directly accessible, in opposition to some other models of microtubules (470). 

These include rhazinilam (15.4.6) 494), which inhibits the disassembly of microtubules but has a different mechanism of action than taxol, laulimalide (15.4.7) and isolaulimalide (15.4.8) 496), WS9885B (15.4.9) 497), and polyisoprenylated benzophenones such as guttiferone E (15.4.10) 497). The naturally occurring 3(2H)-furanone derivative geiparvin has been found to counteract the microtubule-assembly effects of taxol, suggesting that it is a competitive inhibitor at the taxol-binding site of tubulin 498). 

Andreu JM, Barasoain I (2001) The Interaction of Baccatin III with the Taxol Binding Site of Microtubules Determined by a Homogeneous Assay with Fluorescent Taxoid. Biochemistry 40 11975... 

As indicated above, the antitumor activity of Epo B is based on its ability to bind to microtubules and to alter their intrinsic stability and dynamic properties. (For excellent reviews on microtubule structure and function see ref. 6 and 47-49.) Epothilones prevent the Ca or cold-induced depolymerization of pre-existing microtubule polymers in cell-free systems at the same time, they promote the polymerization of soluble tubulin into microtubule-like polymers under conditions that would normally destabilize microtubules.As demonstrated by kinetic experiments, epothilones inhibit the binding of taxol to microtubules in a competitive manner and they bind to the taxol binding site on p-tubulin with affinities that exceed (Epo B) or are comparable (Epo A) to taxol affinity likewise Epo B is a more potent tubulin-polymerizing agent than taxol or Epo Structural studies on... 

Computational studies have been of paramount importance in order to integrate the results of the experimental studies indicated above with the electron crystallographic data known. According to Snyder and co-workers, a satisfactory and experimentally verifiable model of the tubulin-binding site and of the conformation of paclitaxel has been obtained by computational methods on the first electron crystallographic model. In this context, a new paclitaxel conformer, T-Taxol (Fig. 10c), was proposed [59, 81, 82],... 

This type of tubulin activity has so far been exclusively found in the four above-mentioned natural products and some derivatives, although far more then 140000 synthetic compounds and extracts have been tested. Of these four compounds, epothilones appear to be the best candidates. They are equally or even more active, e.g. up to 35 000 times better then Taxol in resistant cell lines [2]. They also have better cytotoxic potential connected to the tubulin activity, as not all microtubule stabilizers lead to sufficient cell death, and they allow extensive derivatization much faster then Taxol or discodermolide [3, 4]. Also, improvements in the applicability to patients compared to the sparingly soluble Taxol arc expected, eliminating some of the severe side effects connected to the latter drug. Since the binding sites of Taxol and epothilones overlap, epitope comparisons and models of binding... 

Based on analysis of electronic crystallography and NMR data for the bindings of Taxol and epothilone A to tubulin subunits, it was proposed that they did not share a common pharmacophore (similar binding mode and sites) as hypothesized for a long time, because they bind to their receptors uniquely and independently. Also, the T-shape conformation of Taxol binding to tubulin was supported from this study. 

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