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Page 156<br />

The stumbling block did not lie with obtaining pure PNP or converting the protein into crystals. Robert<br />

E. Parks, Jr. and Johanna D. Stoeckler of Brown University had already isolated the enzyme from<br />

human cells. They supplied quantities of protein to William J. Cook, who succeeded in preparing the<br />

well-ordered crystals required for x-ray studies [16]. We established that PNP crystals function normally<br />

as a catalyst. Thus crystalline PNP is essentially identical to PNP in the body. If it were profoundly<br />

different, one would have no justification for basing drug design on the crystal structure.<br />

In the early years we had to depend on a relatively low-intensity x-ray source. High-resolution data was<br />

obtained through collaboration with John R. Helliwell and his group at the Daresbury Laboratory<br />

Synchrotron Radiation Source in England. Today greatly improved equipment and more synchrotron<br />

facilities are available for protein crystallography.<br />

The three-dimensional structure was determined <strong>by</strong> multiple isomorphous replacement techniques using<br />

synchrotron radiation [17]. The native and guanine-PNP complex structures have been refined to 2.8 Å<br />

resolution [18,19].<br />

A. <strong>Structure</strong> of the Enzyme<br />

Crystals of human PNP are grown from ammonium sulfate solution and stored in artificial mother liquor<br />

solution made of 60% ammonium sulfate in 0.05 M citrate buffer at pH 5.4. The space group is R32 with<br />

hexagonal cell parameters a=142.9(1) Å and c=165.2(1) Å. The PNP crystals contain about 76% solvent<br />

and diffract to around 2.8 Å resolution.<br />

The x-ray data established that PNP crystals contain a high percentage of water. This feature proved<br />

very useful; proposed drugs could easily be soaked into the active site without disrupting the crystal<br />

packing. Figure 4A shows the<br />

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