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Figure 13<br />

The inhibitor 2-((3,4-dihydroxy-2-nitrophenyl)vinyl)phenylketone modeled<br />

into the active site of rat (a) and pig (b) COMT.<br />

Page 356<br />

ously this one amino acid differences in the active site of the isoenzymes can explain the significant<br />

differences in the inhibitory potency of vinylphenylketone against these enzymes. From the structural<br />

point of view it seems that the position of the side-chain substitution (for example at C5 in entacapone<br />

and C4 in vinylphenylketone) is not critical for the inhibitor binding. In both cases the substituent has<br />

sufficient space to adapt to the protein structure, and in fact, large substituents reach from the active site<br />

cavity to the solvent (Figure 13).<br />

The tenfold higher inhibitory activity of entacapone compared with vinylphenylketone against human<br />

COMT can be accounted for <strong>by</strong> the electron- withdrawing effect of the side-chain substitution. In the<br />

case of entacapone, the side chain at position C5 has a more beneficial electronic influence on the 2-<br />

hydroxyl of the inhibitor producing a better inhibition (see Section VI).<br />

VI. Mechanism of the COMT Inhibition By Nitrocatechols<br />

As described above, catechols with strong electronegative groups are potent inhibitors of COMT. These<br />

compounds seem to bind well to the active site, but in spite of that they are very poor substrates. It has<br />

been shown, with nitecapone<br />

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