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Figure 4<br />

Amino acid sequence comparison of known COMT sequences (rat [11], human<br />

[14], pig [13]). Secondary structure elements in the sequence of rat soluble COMT<br />

are indicated as well as important active-site residues involved in binding of ligands<br />

(a, AdoMet; m, magnesium; s, substrate/inhibitor). The numbering of the residues<br />

corresponds to the soluble enzyme. The extension of the MB-COMT consists of the first<br />

50 amino terminal residues. In the human sequence of COMT determined <strong>by</strong> Bertocci<br />

[13], Val108 is replaced <strong>by</strong> Met108.<br />

Under physiological concentrations of catecholamines in the brain, MB-COMT may play a more<br />

important role than S-COMT [19,20].<br />

D. Three-Dimensional <strong>Structure</strong> of COMT<br />

Backbone<br />

Page 347<br />

The crystal structure of rat soluble COMT has been solved at 2.0 Å resolution [21]. The COMT enzyme<br />

has a single domain α/β-folded structure, in which eight α-helices are arranged around the the central<br />

mixed β sheet. The sheet contains five parallel β strands and one antiparallel β hairpin. An overview of<br />

COMT is illustrated in Figure 5.<br />

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