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Enzyme activity is at a maximum at pH 4, that is, acidic conditions. In acidic conditions, the imidazole ring in the histidine molecule has two extra protons (i.e. it is protonated). As the pH increases above 6, one of the protons is lost to the solution. In alkaline conditions, both the protons are lost, deeming the histidine molecule unprotonated (and ineffective as a catalysis). Therefore, the protonated form of histidine is required for effective catalysis.