Enzymes are proteins possessing catalytic activity. Proteases are a class of enzymes that catalyze the hydrolysis (i.e., cleavage by the addition of water) of the peptide bonds linking amino acids in proteins according to the following reaction:
Protein + H2O ? protein fragments
As proteins themselves, enzymes are comprised of amino acids. The protease from the human immunodeficiency virus 1 (HIV-1) contains two aspartate (Asp) amino acid residues, referred to here as Asp-1 and Asp-2, that are essential for catalysis. The functional group of aspartate side chains is a carboxyl group that can exist in either the acid (-COOH) or base (-COO-; carboxylate) forms. The pKa of the carboxyl group of Asp-1 is 3.3 while that of Asp-2 is 5.3.
Because changes in pH can change the nature of the acid/base groups on a protein, the catalytic activity of enzymes may increase or decrease with changes in pH and shows maximum activity within a narrow pH range. In order for the HIV-1 protease to be active, one of the Asp residues must be in the base form and one of the Asp residues must be in the acid form.
(a) Which Asp residue, Asp-1 or Asp-2, is the one that will be in the base form in the active enzyme?
(b) Which Asp residues, Asp-1 or Asp-2, is the one that will be in the acid form in the active enzyme?
(c) At what pH will enzyme activity be maximal?
(d) Plot below the % maximal enzyme activity (i.e., maximal activity = 100%) on the Y-axis vs the pH on the X-axis over the pH range 2-7. Your Y-values need only be ballpark (i.e., no calculations are necessary). However, be sure to label clearly on your plots the activity present at pH 3.3 and 5.3 and the pH at which maximum activity occurs.