Protease viral aspartic

As an example, we can consider the discovery and development of an antiviral drug. Inhibitors of the HIV protease, an enzyme essential for the maturation of the virus, can potentially cure HIV-infected people. The discovery process may consist of finding samples able to inhibit the viral aspartic protease over a certain threshold, while having little or no effect on another protease of the same class, such as pepsin. The hits will be submitted to further biological tests to identify leads, patentable compounds that are capable, for example, of inhibiting viral replication in cellular models. 

Pis act by impeding the action of the HTV-1 protease, an aspartic protease that cleaves the gag and gag-pol precursor molecules into smaller structural proteins and enzymes. By doing this, Pis prevent viral maturation and produce defective vkal particles without an electrodense core, and these are unable to infect new cells. Several pharmacological properties of Pis are presented in Table 41.5. 

Inhibit viral aspartate protease Indinavir, ritonavir, saquinavir, nelfinavir... 

The HIV-l protease is a remarkable viral imitation of mammalian aspartic proteases It is a dimer of identical subunits that mimics the two-lobed monomeric structure of pepsin and other aspartic proteases. The HIV-l protease subunits are 99-residue polypeptides that are homologous with the individual domains of the monomeric proteases. Structures determined by X-ray diffraction studies reveal that the active site of HIV-l protease is formed at the interface of the homodimer and consists of two aspartate residues, designated Asp and Asp one contributed by each subunit (Figure 16.29). In the homodimer, the active site is covered by two identical flaps, one from each subunit, in contrast to the monomeric aspartic proteases, which possess only a single active-site flap. 

Because they must often cleave large polyproteins, many viruses encode processing proteases.464-4661 For example, the entire RNA genome of the poliovirus encodes a large polyprotein which is cut by two virally encoded chymotrypsin-like cysteine proteases within Tyr-Gly and Gln-Gly sequences 341465 Asn-Ser sequences are also cut, apparently autocatalytically. As we have already seen, retroviruses encode their own aspartic protease. Most cellular and secreted proteins of bacteria or eukaryotes also undergo processing. 

In the life cycle of HIV, its RNA is translated into a polypeptide chain that is composed of several individual proteins including protease, integrase and reverse transcriptase, but in this form these enzymes are not functional. They must be cleaved by viral proteases from the assembled sequence in order for them to become functional. These posttranslational modifications allow the enzymes to facilitate the production of new viruses. The protease itself is made up of two 99-amino-acid monomers, and an aspartic acid residue in the monomer is required for the cleavage. The protease inhibitors inhibit the enzyme protease and consequently interfere with viral replication and maturation by preventing proteases from cleaving proteins into peptides. In humans, these drugs inhibit cleavage of HIV gag and pol polyproteins, which are part of the essential viral structural components, P7, P9, P17 and P24, and... 

All the HIV protease inhibitors have in common a specific effect against the aspartic HIV protease that cleaves viral proteins to yield structural proteins. Competitive inhibition of this process by the protease inhibitors results in the production of immature, non-infections virus particles. These drugs are also characterized by their high specificity, being more than a thousand-fold more active against viral than human aspartic proteases. 

E. The HIV proteins synthesized by the host cell are produced as a long polyprotein that must be cleaved to the active HIV enzymes and structural proteins. HIV protease inhibitors bind to and inhibit the aspartic protease that hydrolyzes the polyprotein, thus preventing the assembly of infective viral particles. 

Aspartate protease (pol gene encoded) is a viral enzyme that cleaves precursor polypeptides in HIV buds to form the proteins of the mature virus core. The enzyme contains a dipeptide structure not seen in mammalian proteins. Pis bind to this dipeptide, inhibiting the enzyme. 

Viral replication in the presence of nelfinavir selects for drug resistance. The primary nelfinavir resistance mutation is unique to this drug and occurs at HIV protease codon 30 (aspartic acid-to-asparagine substitution) this mutation results in a sevenfold decrease in susceptibility. Isolates with only this mutation retain full sensitivity to other HIV protease inhibitors. Less conunonly, a primary resistance mutation occurs at position 90, which can confer crossresistance. In addition, secondary resistance mutations can accumulate at codons 35, 36, 46, 71, 77, 88, and 90, and these are associated with further resistance to nelfinavir, as well as cross-resistance to other HIV protease inhibitors. 

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