Question

give a brief account of chymotrypsin as a completely active enzyme

Answer 1

it is an endopeptidase enzyme thatcleaves peptides at the carboxylside of tyrosine, tryptophan, andphenylalanine amino acids...it doesnt reqire any activator and hence is present in active form from the origin.

Answer 2

Chymotrypsin which hydrolysis peptide bonds following bulky aromatic amino acid residues in polypeptides is actually synthesized in pancreas and through the pancreatic duct released into duodenum.
Nature has ensured that chymotrypsin and other proteolytic enzymes are synthesized as inactive harmless precursors known as zymogens which are activated when required only in duodenum, their site of activity, a process called in-situ activation. This activation results in an altered shape so that it maybe able to interact with substrate.
The inactive precursor enzyme is termed as chymotrypsinogen and fully active enzyme is called chymotrypsin. The enzyme chymotrypsin is made up of linear chain of 245 amino acids interrupted into 3 peptides- A,B and C.
The protein folds into a globular structure. In the 3-D structure of the enzyme, 3 important amino acid residues, his57, asp102 and ser195 come close together in space which allows a charge relay system to operate.
The negatively charged asp 102 is able to hydrogen bond with the adjacent his57 partially borrowing the hydrogen ion from the latter.
The his57 makes good it's partial hydrogen ion loss to aspartate by attracting a hydrogen ion from adjacent ser195, through the his57 residue much like a relay race where the baton is passed from one member to another, the difference here brings that the baton is a charge.
The hydroxyl group of a serine residue is not acidic (pKa 12) and this is true for all other serine residues of chymotrypsin; only ser195 become acidic due to the unique constellation of 3 amino acids residues because the protein has filter uniquely into space.
Importance of acidic residue:- the negatively charged oxygen anion is able to make a nucleophilic attack on the carbonyl carbon of the peptide bond of it's substrate, loosening it so that a water molecule can hydrolyse the bond
The specificity site of chymotrypsin is a large space created within the enzyme shape and lined by hydrophobic residues, which therefore only allow bulky aromatic, hydrophobic amino acids to bind.
This binding brings the susceptible peptide bond close to the attacking ser195 residue
In chymotrypsinogen, the substrate binding site is blocked and hence the enzyme is inactive.
In-site activation by trypsin involves a proteolytic cut chymotrypsinogen which results in conformational change, exposing the substrate binding pocket. (Active site)
Binding a useful folding pattern which can cause hydrolysis of protein substrates, it is repeated in variety of other enzymes. Trypsin, substilysin, thrombin and brain enzyme, acetyl choline esterase all have a reactive serine residue which is Central to catalytic mechanism.