Refer to the Figure Below

question 73

Multiple Choice

Refer to the figure below. The figure shows how a histidine group at the active site of an enzyme could function either as a base (left) or as an acid (right) .The pK_a of histidine is 6.0, which means that at pH 6.0, there are equal numbers of ionized and nonionized histidine groups since a pK_a is an acid dissociation constant.With this in mind, what would you expect about the pH sensitivity of enzyme X that uses histidine as a base catalyst compared with enzyme Y that uses histidine as an acid catalyst?

• A) Enzyme X and enzyme Y would both have the highest activities at pH 6.0, with lower activities at pH values above and below pH 6.0.
• B) Enzyme X and enzyme Y would both have lower activities at pH values below 6.0 and higher activities at pH values above 6.0.
• C) Enzyme X and enzyme Y would both have higher activities at pH values below 6.0 and lower activities at pH values above 6.0.
• D) Enzyme X would have higher activities below pH 6.0 and lower activities above pH 6.0, while enzyme Y would have lower activities below pH 6.0 and higher activities above pH 6.0.
• E) Enzyme X would have lower activities below pH 6.0 and higher activities above pH 6.0, while enzyme Y would have higher activities below pH 6.0 and lower activities above pH 6.0.

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