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position eight from forming the extra bond with Thr 242. In fact, it set up an unfavorable, repulsive<br />

clash between the threonine and the added amino group.<br />

Page 163<br />

In the absence of detailed structural information, it would have been extremely difficult to explain why<br />

affixing the amino group to the carbon in position eight proved unhelpful. But crystallography quickly<br />

provided the explanation. 9-Deazaguanine itself would be a better choice for the purine component of an<br />

inhibitor. This experience underscores the wonderful economy of the structure-<strong>based</strong> approach. Without<br />

crystallographic data, we might have pursued a logical but unproductive avenue of research much longer<br />

than we did.<br />

B. Ribose Site<br />

The next task was to fill the sugar binding site. The sugar in a nucleoside does not attach to PNP<br />

primarily <strong>by</strong> forming hydrogen bonds, but through hydrophobic attractions. The sugar binding pocket of<br />

PNP consists of three hydrophobic amino acids: Phe 200 and Tyr 88 from the same monomer that binds<br />

guanine and Phe 159 from the adjacent monomer. Several known inhibitors carried a benzene group<br />

attached to position 9 of the purine in place of the sugar in the nucleoside. An initial series of<br />

compounds was synthesized to exploit the hydrophobic region in the ribose binding site.<br />

A number of 9-substituted 9-deazapurine analogs were prepared with various aromatic, heteroaromatic,<br />

and cycloaliphatic substituents. The first 9-deazaguanine derivatives synthesized, such as 9-benzyl-9deazaguanine,<br />

were three to six times more potent than the most potent known inhibitor, 8-amino-9-(2thienylmethyl)guanine.<br />

The optimum spacer between the purine base and the aromatic substituent<br />

proved to be a single methylene group. Crystallographic data showed that generally the planes of the<br />

aromatic rings tend to orient in a reproducible conformation. The aromatic groups optimize their<br />

interaction with Phe 159 and Phe 200, which results in the classic “herringbone” arrangement reported<br />

in a variety of aromatic systems [26].<br />

Inhibitors with cycloaliphatic substituents at N9 of deazaguanine were also as potent as the aromatic<br />

analogs. The cycloaliphatic substituents occupied the same general volume as the aromatic groups. As<br />

with the aromatic series, the optimum spacer between the 9-deazaguanine and the hydrophobic<br />

substituent is one carbon atom. X-ray analysis of the PNP complexes of 9-cyclohexyl-9-deazaguanine, a<br />

relatively poor inhibitor, and the complex of 9-cyclohexylmethyl-9-deazaguanine, a potent inhibitor,<br />

showed the two cyclohexyl groups occupy approximately the same space in the active site with the<br />

purine base pulled out of its optimal position in the former.<br />

The chemistry is more straightforward with the aromatic series. From modeling studies, we saw that the<br />

sugar binding pocket could be filled more<br />

http://legacy.netlibrary.com/nlreader/nlReader.dll?bookid=12640&filename=Page_163.html [4/5/2004 5:02:22 PM]

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