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96 Chapter 5
CASE STUDY: HIV-1 protease
The discovery in 1983 that human immunodeWciency virus (HIV) is the causative
agent of acquired immunodeWciency syndrome (AIDS) prompted a huge
research eVort into this virus. The virus was found to contain an essential
aspartyl protease (known as ‘HIV protease’) which is required for the cleavage
of two 55-kDa and 160-kDa precursor polypeptides produced from the gag and
pol genes of HIV-1. The cleavage products of these precursor polypeptides are
the structural proteins and retroviral replication enzymes required for the
assembly of new HIV-1 virions and completion of the viral life cycle. This
enzyme, therefore, represented an immediate target for anti-HIV therapy: if
inhibitors could be devised for the HIV-1 protease, the virus would be unable to
synthesise its essential proteins and the life cycle would be blocked.
The HIV-1 protease has been overexpressed, puriWed and crystallised, and
several research groups have solved its X-ray crystal structure to high resolution.
The enzyme is a 99-kDa homodimer that is similar in structure to other
members of the family such as pepsin. Its active site lies at the interface of the
two sub-units, and the two active site carboxylate residues are Asp-25 from one
sub-unit and the complementary Asp-25 0 from the other sub-unit, as shown in
Figure 5.17. Preferred cleavage sites for the HIV-1 protease are at Aro–proline
sites, where Aro is an aromatic amino acid (tyrosine, phenylalanine, tryptophan).
Using synthetic substrates containing a tyrosine–proline cleavage site,
enzyme-catalysed exchange of 18 O from H 18
2 O into the amide carbonyl has been
Figure 5.17 Structure of HIV-1 protease (PDB Wle 1HVR), showing the catalytic aspartic acid
residues Asp-25 and Asp-25 0 in red, and a bound inhibitor in black.