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Page 203<br />

not form stable dimers was sequenced [48]. This ADH mutant has alanine-159 replaced with threonine.<br />

A 3D model of Drosophila ADH shows alanine-159 on the opposite surface of α helix F from tyrosine-<br />

153 and lysine-157 [48]. Alanine-159 along with alanine-158 form a hydrophobic anchor that stabilizes<br />

the dimer interface. These two residues of ADH and the homologous residues in other short-chain<br />

alcohol dehydrogenases have been overlooked in sequence analyses because they are not absolutely<br />

conserved. In fact, at least five amino acids are found in these positions among the different sec-alcohol<br />

dehydrogenases.<br />

G. Dimer Interface in 11β-and 17β-Hydroxysteroid Dehydrogenases<br />

Because α helix F at the dimer interface also contains the catalytic tyrosine and the near<strong>by</strong> lysine<br />

residue, any structural analysis of the catalytic site must also consider the structure of this part of the<br />

dimer interface. For this reason, we modeled α helix F on 11β-HSD-1 and -2 and 17β-HSD-1, -2, -3,<br />

and -4 to gain an insight into stabilizing interactions and how they may affect the catalytic site.<br />

H. Human 11β-HSD-1<br />

Figure 5 shows the modeled structure for the α helix F interface in human 11β-HSD-1, in which<br />

phenylalanine-188 and alanine-189 form an anchor. Alanine-189 is 3.5 Å and 4.7 Å from alanine-189<br />

and alanine-185, respectively, on the other subunit. The phenylalanine-188 side chain is 3.2 Å from<br />

glycine-192. There is a hydrogen bond between serine-185 and serine-196, which are 3.2 Å apart.<br />

Alanine-185 is 4.7 Å from phenylalanine-193. There also is a hydrophobic interaction between<br />

phenylalanine-193 and alanine-181, which are 3.9 Å apart.<br />

This web of interaction between side chains on the outer surface of α helix F on each subunit influences<br />

residues that have side chains oriented to the interior where catalysis occurs. Alanine-185, which is<br />

stabilized <strong>by</strong> interactions with phenylalanine-193, and serine-184, which interacts with serine-196, are<br />

between the conserved tyrosine-183 and lysine-187. Phenylalanine-193 is adjacent to phenylalanine-<br />

194, which is positioned into the catalytic site.<br />

I. Human 11β-HSD-2<br />

Figure 5 shows the α helix F interface of human 11β-HSD-2. Alanine-237 is about 3 Å from leucine-<br />

241; alanine-238 is about 3.7 Å from both alanine-238 and threonine-234. Threonine-234 has a<br />

stabilizing hydrophobic interaction<br />

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