General Acid Catalysis is a catalytic mechanism commonly found in enzymes. It is not limited soley to enzymes, for example it can be used to speed up a keto-enol tautomerization.
In general acid catalysis a weak acid partially transfers a proton, thereby stabilizing the transition state and lowering the activation energy.
With this type of reaction the rate can be increased at a given pH by increasing the weak acid concentration. Since the weak acid must be protonated for catalysis to occur the pH must be below the pKa.
Four Types of Enzyme Catalysis:
1) Acid-Base Catalysis
2) Covalent Catalysis
3) Metal Ion Catalysis
4) Electrostatic Catalysis
General acid catalyis falls into the first category along with General Base Catalysis and Concerted Acid-Base Catalysis.
Examples of enzymes that use this mechanism are Serine Proteases, RNase and lysozyme. There are many others that also use this type of catalysis. Often enzymes will use multiple types of catalysis.