An enzyme contains an active site aspartic acid with a p K a = 5.0, which acts a
ID: 1059708 • Letter: A
Question
An enzyme contains an active site aspartic acid with a pKa = 5.0, which acts as a general acid catalyst. The curve of enzyme activity (reaction rate) versus pHfor the enzyme (assume the protein is stably folded between pH 2-12 and that the active site Asp is the only ionizable residue involved in catalysis).
Part B
Briefly explain the shape of your curve.
Check all that apply.
The Asp must be protonated to act as a general acid catalyst; thus, activity will be higher when pH < pKa and lower when pH > pKa. At pH = pKa expect 50% of maximal activity because the Asp will be 50% protonated. At pH = pKa expect 100% of maximal activity because the Asp will be 100% protonated. The Asp must be fully deprotonated to act as a general acid catalyst; thus, activity will be higher when pH < pKa and lower when pH > pKa. At pH = pKa expect 100% of maximal activity because the Asp will be 100% deprotonated. NH enzyme activityExplanation / Answer
The graph shown is non-linear curve, yoo can observe that the % enzyme activity is more at lower PH and
% is less at higher PH. From this kind of relation we can infer that PH is the inverse function of % specific activity.
Most of the times a catalytic reaction is initiated by the transfer of a proton from a protein residue which is the proton donor to the substrate and this happens when the steps in an enzymatic reaction mechanism is normally a nucleophilic attack on a substrate. So, for catalysis to take place the proton donor must be protonated.
Enzymes which have Asp residues that act as proton donors and catalytic nucleophiles are expected to
show an upward shift in the PKa value of the proton donor. Below two choices will be your answer
The Asp must be protonated to act as a general acid catalyst. The activity will be higher when pH<pKa and
lower when pH>pKa
At pH=pKa there is 50% of the maximal activity because the Asp will be 50% protonated