Aspartate and lysine are in the active site of an enzyme. They are both known to

Question

Aspartate and lysine are in the active site of an enzyme. They are both known to participate directly in catalysis. The pKa's of the residues are found to be 3.2 and 9.6, respectively for aspartate and lysine. The optimum pH for the enzyme is 6.4. Which forms of these two residues will predominate when the enzyme is most active?

aspartate is protonated; lysine is deprotonated

both residues are protonated

aspartate is deprotonated; lysine is protonated

both residues are deprotonated

I know the answer is C, but i need to know why. Thank you!

aspartate is protonated; lysine is deprotonated

both residues are protonated

aspartate is deprotonated; lysine is protonated

both residues are deprotonated

I know the answer is C, but i need to know why. Thank you!

Explanation / Answer

pH = 6.4 > pka of aspartate which is 3.2  

which means we had higher basic medium compared to pka of aspartate , hence aspartate is deprotonated

pH = 6.4 < pka of lysine 9.6   which means we had higher acidic medium compared to pka of lysine. Hence lysine is protonated form .

Hence option C is answer

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