Discussion
Chymotrypsin cleaves polypeptides preferentially on the carbonyl side of aromatic, and some other bulky nonpolar residues. Trypsin cleaves on the carbonyl side of positively charged residues. Digestion of a polypeptide having the composition-Ala2, Gly, Lys2, Phe, Thr, Trp, Val, independently by these two proteases yielded peptide fragments as shown in the figure below.
| Trypsin | Chymotrypsin |
|---|---|
| 1. Ala-Ala-Trp-Gly-Lys 2. Thr-Phe-Val-Lys | 1. Gly-Lys 2. Thr-Phe 3. Val-Lys-Ala-Ala-Trp |
Which one of the following is the correct primary structure of the polypeptide?
Answer: B
The sequence is Thr-Phe-Val-Lys-Ala-Ala-Trp-Gly-Lys.
Both trypsin fragments end in Lys, and one chymotrypsin fragment, Gly-Lys, does not end in an aromatic residue, so it must be the C-terminus. The chymotrypsin fragment Val-Lys-Ala-Ala-Trp spans a trypsin cut: it shows that ...Val-Lys is followed by Ala-Ala-Trp.... So the trypsin fragment ending in Val-Lys (Thr-Phe-Val-Lys) comes first and Ala-Ala-Trp-Gly-Lys second.
Check against both enzymes: trypsin cuts after Lys4 to give Thr-Phe-Val-Lys and Ala-Ala-Trp-Gly-Lys; chymotrypsin cuts after Phe2 and Trp7 to give Thr-Phe, Val-Lys-Ala-Ala-Trp and Gly-Lys. Both match.
The other sequences fail the test: the first would release a free Lys and Thr-Phe-Val with trypsin, the third would give Val-Lys with trypsin, and the last would give Thr-Phe-Ala-Ala-Trp-Val-Lys.
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