Please help me. I get that PHE is in the hydrophobic pocket, but why would this

Question

Please help me. I get that PHE is in the hydrophobic pocket, but why would this alter the mass units of the pepetide? Thanks

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(H218O)*. 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. H2N-Ala-His-Thr-Phe- -OH H2N-Ala-His-Thr-LYS- -OH *Heavy water is exactly like normal water, except the oxygen incorporated in to 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

The mass shift is due to the substitution of N-H to N-D and O-H (of threonine) to O-D of peptide bond adjescent to Phe residue. As in the 2nd peptide, donot have Phe, the enzyme can't act on it. therefore no mass change.

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