Rate-determining steps in HIV-1 protease catalysis. The hydrolysis of the most specific substrate.

The human immunodeficiency virus type-1 (HIV-1) encodes a protease which is essential for the production of infectious virus. The protease prefers substrates that contain glutamic acid or glutamine at the P2' position. The catalytic role of these residues has been studied by using a highly specific fluorogen substrate, 2-aminobenzoyl-Thr-Ile-Nle-Phe(NO2)-Gln-Arg (substrate QR), and its counterpart (substrate ER) containing Glu in place of Gln. The newly designed substrate ER that contains a pair of charged residues at P2' and P3' sites is the most specific substrate described so far for HIV-1 protease. The specificity rate constant (kcat/Km = 2.1 x 10(7) M-1 s-1) approaches, but does not reach, the diffusion limit. This follows from the appreciable solvent kinetic deuterium isotope effects on the rate constants, indicating that, independent of the salt concentration, the rate-limiting step of the catalysis is a chemical process rather than a physical one. The reaction also has positive entropy of activation. On the other hand, the rate-limiting step for substrate QR changes with increasing salt concentration from a physical to chemical step, while the negative activation entropy becomes positive. The rate increase with substrate ER is 50-fold with respect to substrate QR in the presence of 0.1 M NaCl and diminishes to 3.5-fold at 2.0 M NaCl concentration, as a consequence of a considerable rate increase at high salt concentration with substrate QR but not with substrate ER. The Km value is much lower for the substrate ER (0.8 microM) than for substrate QR (15 microM), indicating a more effective binding for substrate ER at 0.1 M NaCl. Unexpectedly, the strong binding appears to be achieved by the unionized form of Glu in P2', as follows from the remarkably different pH-rate profiles for substrates QR and ER. The effective binding elicited by the glutamic acid may be utilized in designing inhibitors for therapeutic purposes.