3. (4 points) Every enzyme works at its highest efficiency at a particular pH. F

Question

3. (4 points) Every enzyme works at its highest efficiency at a particular pH. For example, the enzymes in the lysosome in the cell (most all of which are involved in protein degradation) work best when the pH is low, between 4 and 5. One such enzyme, hexosaminidase A, has a Glu at its active site. a. The glutamate side chain has a pKa of 4.4. Calculate the percentage of Glu that is unprotonated at a pH of 4.7. The pH of the cytosol is 7.2. How will this affect the activity of hexosaminidase A if it is released from the lysosome into the cell? Explain your rationale for this outcome b.

Explanation / Answer

Answer a) We shall use the formula given below

pH = pKa + log (conjugate base)/(acid)

The given pH = 4.7, so

4.7 = 4.4 + log (conjugate base)/(acid)

Therefore, log (conjugate base)/(acid) = 4.7-4.4 = 0.3

(conjugate base)/(acid) = antilog of 0.3 = 1.995

thus conjugate base: acid ratio is 1.995:1 meaning for every 1 protonated molecule (the acid form), there will be 1.995 conjugate baes
so % protonated form = 1*100/1.995 = 50.12%

and % of unprotonated form = 100-50.12 = 49.88%

Answer b) In the lysosomes, pH is maintained between 3.0 to 6.0. All the lysosomal enzymes work at a low pH optima. With increasing pH.there will be decrease in the activity of hexaminidase in cytosol.

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