Cu, Zn, Mo, Fe, and Co cations. Transition metal ions bind to enzymes much more strongly than the metal ions and form metalloenzymes. Mo and Fe are found in nitricoxide reductase; Fe is a component of Hb, the  carrying haemoprotein of erythrocytes. Co is found in vitamin B12. Superoxide dismutase is a copper-metalloenzyme, which catalyses the removal of highly reactive O₂-produced.

Bovine erythrocyte superoxide dismutase is a dimeric protein containing two Cu²⁺ ions and two Zn²⁺ ions. The Zn²⁺ ions appear to have a structural rather than a catalytic role, while Cu²⁺ ions are involved in reaction sequence.

In contrast to above, Zn²⁺ has a catalytic role in the reaction catalysed by carboxypeptidase A, where the C-terminal amino acid of a polypeptides is removed, provided it has a non-polar side chain.

Carboxypeptidase A – Zn²⁺ C-terminal acid of polypeptide is removed. Bovine pancreas carboxypeptidase A is a monomeric enzyme which contains one atom of Zn. The active site contains the coordinated Zn²⁺ ions bound to histidine-69, glumate-72, histidine-196, and H₂O as well as a groove for the polypeptide substrate and hydrophobic pocket for binding the side chain of the C-terminal amino acid. The terminal carboxyl group of the substrate forms an electrostatic interaction with arginine-145 (Figure 6.36).

Figure 6.36 A Simplified Representation of the Three-dimensional Structure of Carboxypeptidase A

Tyrosine-248 is located in such a position in the enzyme-substrate complex that it could donate a proton to the nitrogen of the peptide bond being hydrolysed, and a carboxyl group of glutamate-270 acts as a general base catalyst to make the attacking water molecule nucleophilic.

The amino acids Arginine in position 145, glutamate in 270, and tyrosine-248 are moving to the substrate and forcing water molecules out of active site, thereby creating a hydrophobic environment for the substrate to bind.