The ability to predict the reactivity and selectivity of cytochrome P450 en- zymes using computational methods has the potential to reduce the number of late- stage failures in the drug discovery process. The antiinfiammatory drug diclofenac and anticancer therapy tamoxifen provide interesting test-cases for in silica predic- tion of selectivity in P450-mediated drug metabolism because the CYP2C9 enzyme selectively aromatically hydroxylates diclofenac at the 4'-position and tamoxifen at the 4-position, whereas other P450 isozymes produce 5-hydroxydiclofenac and 4' - hydroxytamoxifen. Molecular Dynamics (MD) simulations have been performed to identify the key residues involved in the orientation of diclofenac and tamoxifen in the CYP2C9 active site. The MD simulations were also used to assess the flexibility of different regions of the enzyme. Combined quantum mechanical/molecular mechanical (QM/MM) methods have been applied to calculating the energy barriers to formation of 4'- and 5- hydroxy- diclofenac in CYP2C9. The calculated energy barrier to 4'-hydroxylation was con- sistent with experimental kcat values but the energy barrier to 5-hydroxylation was found to be lower than that to 4'-hydroxylation due to formation of stabilising internal hydrogen bond in the 5-hydroxy transition state. It was found that the orienting effect of a hydrogen bonding interaction between the Argl08 residue of CYP2C9 and diclofenac causes the selectivity for 4'-hydroxylation, and that this in- teraction is also responsible for preventing formation of an internal hydrogen bond in the 4'-hydroxylation transition state. QM/MM methods were used to model the 4- and 4'-hydroxylation of tamoxifen by CYP2C9. The energy barriers to each reaction were found to be similar and so it is likely that the relative binding-free energies of 4- and 4'- reactive orientations is responsible for the observed selectivity for 4-hydroxylation of tamoxifen by CYP2C9.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:555631 |
| Date | January 2011 |
| Creators | Houghton, Kerensa |
| Publisher | University of Bristol |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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