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 (k(cat)/K-m) at pH 7.5 for cleavage is 5.2 M(-1) s(-1). This value increases to 38 Rr(-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 (k(cat)/K-m = 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 (k(cat)/K-m = 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.