You discover a new cysteine protease similar to papain Cyste

You discover a new cysteine protease similar to papain. Cysteine proteases are proteolytic enzymes that utilize a cysteine residue in the active site for the nucleophilic attack of a peptide bond, while a second residue acts as a base for proton abstraction in the reaction (acid-base catalysis). In this novel enzyme the pKa of the cysteine residue is significantly lowered by the active site environment to pKa=4.

Show all steps to the solutions of the following problems.

a. What is the fraction of deprotonated Cys and protonated B at the calculated optimum pH?

b. How are these fractions changing at 1 pH unit below the optimal pH? Which fraction of the enzyme will be active at this pH?

Solution

a. pH = pKa - log (HA)/ (A-)

    7 = 4 - log (HA)/ (A-)

(HA)/ (A-) = 1/ 1000

b. cysteine there is 1000 times as much deprotonated cysteine as there is protonated cysteine. In this case a much greater protion of the cysteine are deprotonated and active part in chemical reaction.