Question

Histidine amino acids contain an imidazole group of 6 pKa, which can cause positive charges or free of charge at neutral pH. Because of this characteristic, histidine often functions as an important function in enzyme active sites. In the case of Chymotrypsin enzymes, histidine plays an important role in the active site, which is produced in the pancreas and secreted into the small intestine with a pH of about 8. At pH 8, what is the probability that the imidazole will be charged?

Answer 1

Chymotrypsin is digestive enzyme which is functional in all intestine where pH is around 8. Catalytic site of chymotrypsin is having triad of serine, aspartate and histidine.

Arspartate donates it's H+ to histidine which causes to increase its pka from 6 to 7-12. So histidine acts as general base which at pH 8 donates it's partial proton to act as nucleophile. And this nucleophile attack on peptide bind of aeromatic amino acids.

So at pH 8 histidine remains positive charge in chymotrypsin.

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