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Active Site<br />

The active site of COMT consists of the AdoMet binding domain and the catalytic site. The structural<br />

elements and individual interactions between COMT residues and the enzymatic-action-participating<br />

ligands are demonstrated in detail in Figures 5 to 8.<br />

Page 349<br />

AdoMet Binding <strong>by</strong> COMT. The active-site residues, which have significant interactions with the<br />

coenzyme, are shown in Figure 6. The loop region between strand β1 and helix α4 forms the AdoMetbinding<br />

consensus sequence (in COMT GAxxG) that is conserved in methyltransferases [23]. In this<br />

region the terminal amino and carboxyl groups of AdoMet are bound. The last residue of the strand β2,<br />

Glu90, forms a hydrogen bond to the ribose hydroxyls. The residue Met91 has face-to-face van der<br />

Waals contacts on one side, and His142 has edge-to-face contacts on the opposite side of the adenine<br />

ring. The residue Trp143 closes the adenine of AdoMet into the protein with face-to-edge contact.<br />

Furthermore, the N-6 atom of the adenine hydrogen binds to Ser119. The Met40 residue holds the<br />

sulphur of AdoMet with the methyl group in right position towards the hydroxyl group of the catechol<br />

substrate. As a result of the various hydrogen bonds and van der Waals contacts, AdoMet has a high<br />

affinity to COMT with a dissociation constant of 23 μM [19].<br />

Catalytic Site. The catalytic site of COMT is a rather simple environment formed <strong>by</strong> the metal ion and<br />

<strong>by</strong> the amino acids important for substrate binding and catalysis of the methylation reaction.<br />

The magnesium ion plays a crucial role for the catalytic activity of COMT. Figure 7 shows the binding<br />

of magnesium to COMT as derived from the<br />

Figure 7<br />

Magnesium binding in COMT. The magnesium ligands are Asp141, Asp169, Asn170,<br />

both hydroxyls of 3, 5-dinitrocatechol (DNC) and a water molecule (W).<br />

http://legacy.netlibrary.com/nlreader/nlReader.dll?bookid=12640&filename=Page_349.html (1 of 2) [4/5/2004 5:28:09 PM]

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