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netLibrary - eBook Summary Structure-based Drug Design by ...

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Figure 8<br />

The catalytic machinery of COMT. Shown are the COMT residues<br />

important for catalysis, Mg 2+ binding, the catechol as substrate, and the<br />

methyl-donating coenzyme AdoMet. The hydrophobic walls are defined<br />

<strong>by</strong> two tryptophane residues and a proline residue.<br />

Page 350<br />

crystallographic studies. Magnesium has an octahedral coordination to two aspartic acid residues<br />

(Asp141 and Asp169), to an asparagine residue (Asn170), to both catechol hydroxyls of the substrate,<br />

and to a water molecule. In addition to the Mg 2+ ion, Lys144 and Glu199 participate directly in the<br />

methylation reaction as shown in Figure 8. The “gate keeper” residues Trp38 and Pro174 form the<br />

hydrophobic “walls” and define the selectivity of the enzyme to different side chains of the substrate.<br />

They play a significant role in the binding of the substrates and inhibitors of COMT [19, 21].<br />

III. Mechanism of the Catalytic Action of COMT<br />

The catalytic site is a shallow groove with the catalytic machinery at the bottom as illustrated in Figure<br />

8. The two hydroxyl oxygens of a catechol substrate bind directly to the Mg 2+ ion. The active methyl<br />

group of AdoMet is near one of the hydroxyl groups, on one side of the catechol ring. The amino group<br />

of Lys144 is also located near this hydroxyl group, on the other side of the catechol ring from AdoMet.<br />

The Glu199 residue is near the other hydroxyl group.<br />

http://legacy.netlibrary.com/nlreader/nlReader.dll?bookid=12640&filename=Page_350.html [4/5/2004 5:28:14 PM]

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