Activation of the Herpes Simplex Virus Type 1 Protease

The catalytic efficiency of the mature HSV-1 protease has been examined as a function of solvent composition. With the peptide substrate HTYLQASEKFKMWG-amide, the specificity constant (kcat/Km) at pH 7.5 for cleavage is 5.2 M-1 s-1. This value increases to 38 M-1 s-1 when 25% glycerol is present in the reaction mixture. It was found that glycerol activation is but one case of the general phenomenon of HSV-1 protease activation by kosmotropes, or water structure-forming cosolvents. For example, an 860-fold increase in the protease activity (kcat/Km = 4500 M-1 s-1) occurs in the presence of 0.8 M sodium citrate. Similarly, the presence of 0.8 M sodium phosphate activates the catalytic efficiency by 420-fold (kcat/Km = 2200 M-1 s-1). The extent of HSV-1 protease activation by various anions correlates with the Hofmeister series. Both the susceptibility to proteolysis by trypsin and the protein fluorescence spectra of the HSV-1 protease change in the presence of activating solvents, suggesting a conformational change accompanying activation.