HIV1 Protease

PR is encoded within the polregion of the HIV-1 genome (Fig. 2A). The enzyme is an aspartic protease, which functions as a molecular scissor to cleave the Gag and Gag/Pol polyprotein precursors into structural proteins (matrix, capsid, nucleocapsid and p6) and enzymes (PR, RT, and integrase) (Fig. 2B). At least eight different target sequences (Fig. 2B, A-G) in the polyprotein serve as substrates for PR and are cleaved in an ordered process during maturation of virions (Fig. 2C).5

The functional form of HIV-1 PR is a dimeric molecule comprised of two identical 99-amino acid monomers. Three functional domains are involved in PR activity: the active-site cleft with two catalytic aspartic acid

FIGURE 1. HIV-1 Life Cycle. Themain stepsof the HIV-1 life cycle are designated by numbers and include (1) viral attachment and entry, (2) reverse transcription, (3) integration,

(4) viral RNA transcription and processing,

(5) translation ofviral mRNAs into polyprotein precursors of structural and enzymatic proteins, (6) assembly, (1) budding. Retroviral enzymes reverse transcriptase (RT), integrase (INT), andprotease (PR) are active during specified steps in the virus life cycle.

FIGURE 2. HIV-1 Genome. HIV-1 is an RNA virus comprised of two identical, 10-kb RNA molecules. (A) Genome organization of HIV-1. The three main coding regions of HIV-1, gag, pol, and env, and the accessory genes are represented with respect to their reading frames. Two identical long terminal repeats (LTR) flank the viral genome. (B) Processing of precursor polyproteins. The Pr55 gag and Pr160 gag-pol polyproteins are depicted with respect to the reading frame. Dotted lines represent protease cleavage sites for the generation of mature viral proteins. The structural proteins encoded by gag and the enzymatic proteins encoded by pol are shown in their relative locations within the virion. The ribosomal frameshift is located at D'. Frameshift removes the D' and D cleavage sites. The Pr55 and Pr160 polyproteins are translated at a ratio of 20:1. (C) Virion structure. MA, matrix or p17; CA, capsid or p24; NC, nucleocapsid or p7.

FIGURE 2. HIV-1 Genome. HIV-1 is an RNA virus comprised of two identical, 10-kb RNA molecules. (A) Genome organization of HIV-1. The three main coding regions of HIV-1, gag, pol, and env, and the accessory genes are represented with respect to their reading frames. Two identical long terminal repeats (LTR) flank the viral genome. (B) Processing of precursor polyproteins. The Pr55 gag and Pr160 gag-pol polyproteins are depicted with respect to the reading frame. Dotted lines represent protease cleavage sites for the generation of mature viral proteins. The structural proteins encoded by gag and the enzymatic proteins encoded by pol are shown in their relative locations within the virion. The ribosomal frameshift is located at D'. Frameshift removes the D' and D cleavage sites. The Pr55 and Pr160 polyproteins are translated at a ratio of 20:1. (C) Virion structure. MA, matrix or p17; CA, capsid or p24; NC, nucleocapsid or p7.

residues, the flaps, and the psi loop. Binding of substrate in the active site induces conformational changes which close the flaps and bring the catalytic aspartate residues into position to mediate substrate cleavage. PR inhibitors are small molecules that compete with substrates for active-site binding. Once bound, the inhibitors cannot be cleaved and consequently inactivate the enzyme by remaining in the active site.

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