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Effects of substituents on the acidity of phenols: Existence of electron attracting group, (for example-NO2, -X, - NR, -CN, -CHO, -COOH) on the benzene ring increases the acidity of phenol since it enables the ring to draw much more electrons from the phenoxy oxygen and so releasing easily the proton.
pH affects carbonic anhydrase in a sigmoidal fashion. The higher the pH, the more active the enzyme is (since it is in the optimal conditions for deprotonation). 1) The binding of zinc lowers the pKa of water from 15.7 to 7, generating a hydroxide ion (OH) to attack carbon dioxide.