Question

Which of these tripeptides are hydrolyzed by trypsin? by chymotrypsin? (a) Tyr-Gln-Val (b) Thr-Phe-Ser (c) Thr-Ser-Phe

Short answer

Answer: (b) Thr-Phe-Ser is hydrolyzed by chymotrypsin. Tripeptides (a) and (c) are not hydrolyzed by either trypsin or chymotrypsin.

Step-by-step solution

Identify trypsin cleavage sites

Trypsin cleaves peptides on the carboxyl side of arginine (Arg) and lysine (Lys) residues. Therefore, we will look for these amino acids in the given tripeptides.

Identify chymotrypsin cleavage sites

Chymotrypsin cleaves peptides on the carboxyl side of phenylalanine (Phe), tryptophan (Trp), and tyrosine (Tyr) residues. We will look for these amino acids in the given tripeptides. Now we will examine each tripeptide for the presence of amino acids where the enzymes trypsin and chymotrypsin will cleave.

Analyze tripeptide (a) Tyr-Gln-Val

There are no arginine (Arg) or lysine (Lys) residues, so trypsin will not cleave this peptide. There is a tyrosine (Tyr) at the amino end of the tripeptide, but cleavage would require for the residue to be in the middle or towards the carboxyl end, thus chymotrypsin will not cleave this peptide either. Result: (a) hydrolyzed by neither trypsin nor chymotrypsin.

Analyze tripeptide (b) Thr-Phe-Ser

There are no arginine (Arg) or lysine (Lys) residues, so trypsin will not cleave this peptide. There is a phenylalanine (Phe) in the middle of the peptide, which is a cleavage site for chymotrypsin. Thus, chymotrypsin will cleave this peptide. Result: (b) hydrolyzed by chymotrypsin.

Analyze tripeptide (c) Thr-Ser-Phe

There are no arginine (Arg) or lysine (Lys) residues, so trypsin will not cleave this peptide. There is a phenylalanine (Phe) at the carboxyl end of the tripeptide. Chymotrypsin will not cleave this peptide as the Phe residue needs to be in the middle or towards the amino end. Result: (c) hydrolyzed by neither trypsin nor chymotrypsin. In conclusion: -Tripeptide (a) Tyr-Gln-Val is not hydrolyzed by either trypsin or chymotrypsin. -Tripeptide (b) Thr-Phe-Ser is hydrolyzed by chymotrypsin. -Tripeptide (c) Thr-Ser-Phe is not hydrolyzed by either trypsin or chymotrypsin.