This is a type of enzyme catalysis. The enzyme binds a positively charged metal ion, as a cofactor
Possible Functions of Metal Ions:
1) Electrophilic catalysis - In this type of catalysis the postitively charged metal ion stabilizes negative charges that may develop in the transition state.
2) Metal ions can exist in multiple oxidation states. Thus they can serve as catalysts in redox reactions.
3) Metal ions can act to coordinate substrate molecules and orient them for reaction.
4) Metal ions also have the ability to create reactive hydroxyl groups by coordinating a water molecule and thus increase its acidity. Carbonic anhydrase is an enzyme that uses this method to carry out catalysis.