Although enzymes can be really specific on the reactions they catalyze, they often have different strategies. Some examples of such strategies are: covalent, acid-base, metal ion, and catalysis by approximation. During the process of catalysis in covalent catalysis, the active site has a reactive group (usually a nucleophile) that becomes temporarily covalently attached to a section of the substrate. In acid-base catalysis, a molecule (except for water) plays the role of either a proton donor or a proton acceptor. In metal ion catalysis, metal ions can act in different ways. It can ease the formation of nucleophiles, serve as an electrophile (therefore stabilizing a negative charge on a reaction intermediate), or serve as a bridge between enzyme and substrate. Finally in catalysis by approximation, the reaction (generally having two different substrates) is favored by bringing together the two substrates to a single binding surface on the enzyme.
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