The metal present at the active site of the protein Carboxypeptidase A is Zinc.

Carboxypeptidases are enzymes that hydrolyze peptide bonds at the C-terminus (carboxyl end) of proteins and peptides. Carboxypeptidase A is one of the major forms of carboxypeptidases found in mammals and is involved in the digestion and processing of proteins.

Importance of Metal Ions in Enzyme Catalysis:
Metal ions play a critical role in the catalytic function of many enzymes. They can function as cofactors, which are essential for the enzyme's activity, or as activators/modulators, which enhance or modulate the enzyme's activity. In the case of carboxypeptidase A, the metal ion present at the active site acts as a cofactor, facilitating the enzyme's catalytic activity.

Role of Zinc in Carboxypeptidase A:
Zinc is the metal ion present at the active site of Carboxypeptidase A. It plays a crucial role in the enzyme's catalytic mechanism. The zinc ion coordinates with specific amino acid residues in the active site of the enzyme, forming a stable complex.

Coordination of Zinc Ion:
The zinc ion in carboxypeptidase A is coordinated by three histidine residues (His69, His196, and His69) and a water molecule. These residues, along with the zinc ion, form a tetrahedral coordination complex.

Stabilization of Transition State:
The presence of the zinc ion in the active site of carboxypeptidase A helps in the stabilization of the transition state during catalysis. The zinc ion can polarize the peptide bond, facilitating its hydrolysis. It also helps in the orientation and positioning of the substrate, enabling efficient catalysis.

General Function of Metal Ions in Enzymes:
Metal ions in enzymes can participate in various catalytic mechanisms, such as acid-base catalysis, redox reactions, and stabilization of reaction intermediates. They can also enhance the binding of substrates to the active site and promote conformational changes required for catalysis.

In summary, the metal ion present at the active site of Carboxypeptidase A is Zinc. It acts as a cofactor, facilitating the enzyme's catalytic activity by coordinating with specific amino acid residues and stabilizing the transition state during peptide bond hydrolysis.