http://en.wikibooks.org/wiki/Structural_Biochemistry/Enzyme/Acid-Base_Catalysis
 
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General acid-base catalysis involves a molecule besides water that acts as a proton donor or acceptor during the enzymatic reaction. Acid-base catalysis facilitates a reaction by stabilizing the charges in the transition state through the use of an acid or base, which donates protons or accepts them, respectively. Nucleophilic and electrophilic groups are activated as a result of the proton addition or removal and causes the reaction to proceed. Many acid-base catalysis reactions involve histidine because it has a pHclose to 7, allowing it to act as both an acid and a base. When a functional group accepts a proton, it will release or donate a proton by the end of the catalytic cycle, and vice versa. Functional groups that participate in reaction are His imidazole, alpha-amino group, alpha-carboxyl group, thiol of Cys, R group of carboxyls of Glu and Asp, aromatic OH of Tyr, and guanidino group of ARG. The protonation of these functional groups are dependent on the pH, therefore the enzymatic catalytic activity is sensitive to the pH level.

General acid-base catalysis is involved in a majority of enzymatic reactions, wherein the side chains of various amino acids act as general acids or general basis. General acid–base catalysis needs to be distinguished from specific acid–base catalysis.

Specific acid–base catalysis means specifically, –OH or H+ accelerates the reaction. The reaction rate is dependent on pH only (which of course is a function of –OH and H+ concentrations), and not on buffer concentration.
In General acid–base catalysis, the buffer aids in stabilizing the transition state via donation or removal of a proton. Therefore, the rate of the reaction is dependent on the buffer concentration, as well as the appropriate protonation state. -General base - partial abstraction of a proton by a Brønsted base lowers the free energy of the transition state rate of reaction increases with increase in pH and increase in Brønsted base.