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with the tryptophane residues 38 and 143. The hydroxyl groups of the inhibitor are coordinated to the
Mg 2+ ion. The hydroxyl group in position 1 has an important hydrogen bond to the carboxyl group of
Glu199. The other hydroxyl is near the methyl group donated by AdoMet. The pK a of this hydroxyl
group is low (about 3.4) [26]. The 3-nitro group of the inhibitor has favourable van der Waals
interactions with Trp143. The Trp38 residue is located edge-to-face with the catechol plane, which
allows an ideal aromatic hydrophobic contact. Such aromatic hydrophobic interactions have been
described to be important in proteins and for the binding of ligands [44,45]. The binding mode of the
catechol ring of other crystallographically determined potential nitrocatecholtype inhibitors complexed
with COMT is essentially the same as with 3,5- dinitrocatechol (J.Vidgren, unpublished results).
C. Differences in the Active Site of Human, Rat, and Pig COMT
Models for human and pig COMT are easy to build using the experimental structure of the rat COMT,
due to the high degree of homology between the rat, human, and pig COMT enzymes (Figure 4). The
active sites are especially well conserved—the few differences in the active-site residues are collected in
Table 2. The kinetic data show that the K m values of common substrates for rat and human COMT are
very similar. Pig COMT shows, however, a considerably higher K m value for catechol [46]. The same
difference is apparent for inhibitors represented by the K i values in Table 1.
The model for the binding of vinylphenylketone to pig and rat COMT is shown in Figure 13. Assuming
that the catechol part of the inhibitor adopts the same position as found in the crystal structure with
dinitrocatechol, the vinylphenylketone substituent has enough room to bind to both enzymes. The most
significant difference between these enzymes lies in residue 38, the hydrophobic tryptophan in rat (and
human) COMT and the polar arginine in pig COMT. If Arg38 is directed towards the hydrophobic core
of the enzyme in a similar conformation as Trp38 (shown in Figure 13), it causes repulsion with the
catechol ring of the inhibitor. However, it is probable that the polar Arg38 is directed towards the
solvent. In this case the substrates and inhibitors will lack the favorable contacts that exist with Trp38 in
human and rat enzymes. Obvi-
Table 2 Differences in the Active Sites Between Rat, Human, and Pig COMT
Position Rat Human Pig
38 Trp Trp Arg
173 Val Cys Cys
201 Met Arg Ser
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