Kinetic and modeling studies of S3-S3' subsites of HIV proteinases.

Kinetic analysis and modeling studies of HIV-1 and HIV-2 proteinases were carried out using the oligopeptide substrate formula: see text] and its analogs containing single amino acid substitutions in P3-P3' positions. The two proteinases acted similarly on the substrates except those having certain hydrophobic amino acids at P2, P1, P2', and P3' positions (Ala, Leu, Met, Phe). Various amino acids seemed to be acceptable at P3 and P3' positions, while the P2 and P2' positions seemed to be more restrictive. Polar uncharged residues resulted in relatively good binding at P3 and P2 positions, while at P2' and P3' positions they gave very high Km values, indicating substantial differences in the respective S and S' subsites of the enzyme. Lys prevented substrate hydrolysis at any of the P2-P2' positions. The large differences for subsite preference at P2 and P2' positions seem to be at least partially due to the different internal interactions of P2 residue with P1', and P2' residue with P1. As expected on the basis of amino acid frequency in the naturally occurring cleavage sites, hydrophobic residues at P1 position resulted in cleavable peptides, while polar and beta-branched amino acids prevented hydrolysis. On the other hand, changing the P1' Pro to other amino acids prevented substrate hydrolysis, even if the substituted amino acid had produced a good substrate in other oligopeptides representing naturally occurring cleavage sites. The results suggest that the subsite specificity of the HIV proteinases may strongly depend on the sequence context of the substrate.