title: The folding free-energy surface of HIV-1 protease: insights into the thermodynamic basis for resistance to inhibitors.
abstract: Spontaneous mutations at numerous sites distant from the active site of human immunodeficiency virus type 1 protease enable resistance to inhibitors while retaining enzymatic activity. As a benchmark for probing the effects of these mutations on the conformational adaptability of this dimeric beta-barrel protein, the folding free-energy surface of a pseudo-wild-type variant, HIV-PR(*), was determined by a combination of equilibrium and kinetic experiments on the urea-induced unfolding/refolding reactions. The equilibrium unfolding reaction was well described by a two-state model involving only the native dimeric form and the unfolded monomer. The global analysis of the kinetic folding mechanism reveals the presence of a fully folded monomeric intermediate that associates to form the native dimeric structure. Independent analysis of a stable monomeric version of the protease demonstrated that a small-amplitude fluorescence phase in refolding and unfolding, not included in the global analysis of the dimeric protein, reflects the presence of a transient intermediate in the monomer folding reaction. The partially folded and fully folded monomers are only marginally stable with respect to the unfolded state, and the dimerization reaction provides a modest driving force at micromolar concentrations of protein. The thermodynamic properties of this system are such that mutations can readily shift the equilibrium from the dimeric native state towards weakly folded states that have a lower affinity for inhibitors but that could be induced to bind to their target proteolytic sites. Presumably, subsequent secondary mutations increase the stability of the native dimeric state in these variants and, thereby, optimize the catalytic properties of the resistant human immunodeficiency virus type 1 protease.
authors.affiliation: Department of Biochemistry and Molecular Pharmacology, University of Massachusetts Medical School, Worcester, MA 01605, USA.
authors.forename: Amanda F
authors.initials: AF
authors.lastname: Noel
authors.forename: Osman
authors.initials: O
authors.lastname: Bilsel
authors.forename: Agnita
authors.initials: A
authors.lastname: Kundu
authors.forename: Ying
authors.initials: Y
authors.lastname: Wu
authors.forename: Jill A
authors.initials: JA
authors.lastname: Zitzewitz
authors.forename: C Robert
authors.initials: CR
authors.lastname: Matthews
mesh_terms: D000595:Amino Acid Sequence; D019943:Amino Acid Substitution; D001665:Binding Sites / Q000235:genetics; D002942:Circular Dichroism; D019281:Dimerization; D024882:Drug Resistance, Viral / Q000235:genetics; D004795:Enzyme Stability / Q000235:genetics; D016333:HIV Protease / Q000737:chemistry* / Q000235:genetics; D017320:HIV Protease Inhibitors / Q000494:pharmacology; D015497:HIV-1 / Q000187:drug effects / Q000201:enzymology* / Q000235:genetics; D007700:Kinetics; D008958:Models, Molecular; D008969:Molecular Sequence Data; D016297:Mutagenesis, Site-Directed; D009154:Mutation; D017510:Protein Folding; D020836:Protein Structure, Quaternary; D011994:Recombinant Proteins / Q000737:chemistry / Q000235:genetics; D013050:Spectrometry, Fluorescence; D013816:Thermodynamics
chemical_list: D017320:HIV Protease Inhibitors; D011994:Recombinant Proteins; D016333:HIV Protease; C514803:p16 protease, Human immunodeficiency virus 1