Of the following, the correct statements about carboxypeptidase-A are

Correct option is A

​Carboxypeptidase A (CPA) is a pancreatic metalloenzyme which catalyses the cleavage of a peptide link in a polypeptide chain. The site of cleavage is specific in two ways: it occurs at the C-terminal amino acid, and it exhibits a high selectivity for substrates in which the C- terminal amino acid contains a large aliphatic or Ph substituent. The latter arises from the presence, near to the active site, of a hydrophobic pocket in the protein which is compatible with the accommodation of, for example, a Ph group 

Carboxypeptidase A is monomeric 

and exists in three forms 

which contain 307, 305 and 300 amino acids respectively. Near the surface of the protein lies a pocket in which a Zn²⁺ion is bound to the protein backbone by one bidentate Glu and two His residues. A 5-coordinate coordination sphere is completed by a water molecule
In the first step, the peptide to be cleaved is ‘manoeuvred’ into position close to the Zn²⁺ site; the dominant substrate–protein interactions involved at this stage are:

These interactions may be supplemented by hydrogen bond formation between the OH group of Tyr-248 and the N-H group indicated in the figure, and between Arg-127 and the C=O group adjacent to the peptide cleavage site. This latter interaction polarizes the carbonyl group, activating it towards nucleophilic attack. The nucleophile is the H₂O ligand coordinated to Zn²⁺.The Lewis acidity of the metal ion polarizes the O-H bonds, and (although this is not a unique proposal) it is likely that the carboxylate group of Glu-270 assists in the process by removing H⁺ from the H₂O ligand. The cleavage of the peptide C-N bond for which H⁺ is probably provided by Glu-270. It appears likely that the second H⁺ required for the formation of the [NH₃]⁺ group on the departing terminal amino acid comes from the terminal CO₂H group of the remaining portion of the substrate.Glu-72 bound in a monodentate manner to the Zn²⁺centre, whereas in the rest state, a bidentate mode has been confirmed. A change from a bi- to monodentate coordination appears to be associated with the formation of the 

the Zn²⁺ ion being able to move towards Arg-127 as the interaction develops. To complete the catalytic cycle, an H₂O ligand refills the vacant site on the Zn²⁺ centre.

A practical disadvantage of working with metalloproteins containing Zn²⁺ is the d¹⁰ configuration of the ion. The metal site cannot be probed by using UV-VIS or EPR spectroscopies or by magnetic measurements. Such methods were especially important before protein crystallography became a widely applied technique. Studies involving Co²⁺-for-Zn²⁺ substitution provide a metal centre that is amenable to investigation by spectroscopic and magnetic techniques (Co²⁺ is a d⁷ ion), the choice of Co²⁺ being because:

The Zn²⁺ ion can be removed from carboxypeptidase A by treatment with bipyridine, and after insertion of Co²⁺ , the model metalloenzyme [PCo²⁺] is found to be active (actually more so than native carboxypeptidase A) with respect to peptide cleavage.