Suppose you add the two peptide sequences shown below to the enzyme chymotrypsin
ID: 902590 • Letter: S
Question
Suppose you add the two peptide sequences shown below to the enzyme chymotrypsin in a buffered solution that is composed of entirely of 'heavy' water (H2 18O)*. After adding the peptides to the chymotrypsin solution, using a mass spectrometer you observe that the mass of peptide 1 has shifted by two mass units (Daltons) relative to its normal mass. The mass of peptide 2 is unaffected. If you add these peptides to the same heavy water buffer without chymotrypsin present, neither peptide shows a mass shift. Explain this result based on what we know about chymotrypsin.
1) H2N-Ala-His-Thr-Phe-COOH
2) H2N-Ala-His-Thr-Lys-COOH
*Heavy water is exactly like normal water, except the oxygen incorporated into the water molecule exists as a stable isotopic variant, having two more neutrons (10 total) compared to normal oxygen (8 total). These two extra neutrons give heavy water a molecular weight two daltons more than normal water.
Explanation / Answer
Two atoms of heavy oxygen derived from heavy water are introduced at the carboxy termini of each peptide generated as a protein is cleaved. The first atom is introduced upon the the cleavage of the peptidic amide bond and the second atom is catalyzed When the cleaved peptide Is bound as an intermediate To the enzyme And this second bond is cleaved In presence of heavy water.