Activity of Tethered Human Immunodeficiency Virus 1 Protease Containing Mutations in the Flap Region of One Subunit

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Abstract
The tethered‐dimer protease of human immunodeficiency virus 1 (HIV‐1) [Cheng Y.‐S. E., Yin, F. H., Foundling, S., Blomstrom, D. & Kettner, C. A. (1990) Proc. Natl Acad. Sci. USA 87, 9660–9664] and its mutants containing amino acid substitutions or deletions or both in only one flap region were expressed in Escherichia coll. These mutant enzymes showed various degrees of self‐processing and significantly reduced catalytic activity toward oligopeptide substrates compared with the wild type. Kinetic parameters determined for one of the oligopeptide substrates showed a dramatic increase in Km and decrease in kcat values. Unexpectedly, the substrate cleavage was more efficient in low salt concentration for a mutant containing a shortened hydrophilic flap. Assays with oligopeptides representing naturally occurring cleavage sites or oligopeptides containing single amino acid substitutions at the P2 and P2′ substrate positions showed only moderate changes in the substrate specificity of the mutant proteases. Predicted structures for the mutants were constructed by molecular modeling and used to interpret the results of kinetic measurements. In general, the data suggest that the mutated part of the flaps does not have a major role in determining substrate specificity; rather, it provides the hydrophobic environment and hydrogen‐bond interactions with the conserved water that are necessary for efficient substrate binding and catalysis.