= Aspartic protease
An <aspartic protease> is a <protease> whose catalytic chemistry depends on <aspartic acid> side chains. In common examples, their carboxyl groups position and activate a <water> molecule for <peptide bond> <hydrolysis>. Water supplies the attacking <nucleophile>, unlike a mechanism in which an enzyme side chain itself makes the initial covalent attack.<MEROPS glossary>[Catalytic Type, activated water and aspartic residues]{ref}
The name concerns catalytic groups, rather than requiring an aspartic-acid residue at every substrate cleavage site. Substrate recognition and catalytic mechanism are distinct properties.
<HIV-1 protease> provides an experimentally characterized example. In 1989, <T. D. Meek> and colleagues measured apparent masses of approximately 11,000 under denaturing conditions and 22,000 under nondenaturing conditions; <protein cross-linking> supplied additional evidence for an active <protein dimer>. The ratio $22{,}000/11{,}000=2$ explains the subunit interpretation.<Human immunodeficiency virus 1 protease expressed in Escherichia coli behaves as a dimeric aspartic protease by T. D. Meek and others>[Abstract, molecular masses and cross-linking]{ref}
The same study found pH-dependent inactivation consistent with a required unprotonated <carboxyl group> in the <active site>. Combining this chemical perturbation with cleavage activity and subunit measurements supported a catalytic classification more specifically than the enzyme's molecular mass alone.<Human immunodeficiency virus 1 protease expressed in Escherichia coli behaves as a dimeric aspartic protease by T. D. Meek and others>[Abstract, inactivation kinetics]{ref}
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