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netLibrary - eBook Summary Structure-based Drug Design by ...

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the active site of HIV PR-16 that could form hydrogen bonds directly and two in which the interaction is<br />

mediated <strong>by</strong> the conserved Wat301. The total solvent accessible surface area of the eight subsites of the<br />

HIV PR active site is approximately 1150 Å 2. Because of the large number of groups with hydrogen-bondforming<br />

potential, 450 Å 2 of the surface has a polar character, and the nonpolar area of the subsites is<br />

slightly larger, approximately 700 Å 2.<br />

B. Structural Flexibility of HIV PR<br />

In the process of viral assembly, HIV PR specifically cleaves nine cleavage sites on GAG and GAG-POL<br />

polypeptides [21]. Examination of the amino acid composition of the recognized substrate sites (Table 1)<br />

indicates their hydrophobic character and significant sequence variability. The loose specificity of HIV PR<br />

most likely reflects its functions in a world of reduced complexity within the confines of the budding<br />

virion. The length of the viral protein precursors (approximately 1500 amino acids) reduces the number of<br />

potential sequences the protease must discriminate from in selecting its nine cleavage sites. Therefore,<br />

HIV PR and other retroviral proteases are not enzymes that have evolved to carry out a single reaction at a<br />

rapid rate, but rather enzymes with minimum specificity required to cleave the viral precursors in a<br />

specific and orderly manner.<br />

The loose specificity requirements demonstrated <strong>by</strong> effective binding and catalytic processing of all nine<br />

sequences, albeit at different rates [22], was the<br />

Table 1 The Sequences of the Proteolytic Processing Sites of HIV-1<br />

HIV-1 PR<br />

Cleavage sites<br />

Scissile bond<br />

P17/P24 V S Q N Y P I V Q N<br />

P24/P2 K A R V L A E A M S<br />

P2/P7 S A T I M M Q R G N<br />

P7/P1 E R Q A N F L G K I<br />

P1/P6 R P G N F L Q S R P<br />

TF/PR V S F S F P Q I T L<br />

PR/RT C T L N F P I S P I<br />

RT/RN G A E T F Y V D G A<br />

RN/IN I R K V L F L D G I<br />

Schechter-Berger<br />

notation<br />

P5 P4 P3 P2 P1 P1' P2' P3' P4' P5'<br />

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