A particular histidine residue is known to play an important role in the catalyt

Question

A particular histidine residue is known to play an important role in the catalytic function of an enzyme. However, it is not clear if this histidine needs to be in its protonated (charged) or unprotonated state in order to work. To answer this question, the enzyme’s activity was measured over a range of different pH conditions. Based on the results (shown at right), which form of histidine is required for enzyme activity. I cannot get the picture to attatch but it is a negative slope with pH on the x axis and activity on the y axis.

Explanation / Answer

Histidine contains the following three functional groups,

With increase in pH, there is a decrease in enzyme activity. Increase in pH, the deprotonated form of histidine dominates. Therefore, based on the enzyme activity graph, it is known that the protonated form of histidine is required for enzyme activity.

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