You are sequencing a protein. Previous work indicates that this protein consists

Question

You are sequencing a protein. Previous work indicates that this protein consists of two polypeptide chains that are held together by a single disulfide bond. You perform two experiments and get the results indicated below. From this ‘experimental’ information:

Determine the position of the disulfide bond that links the two chains and explain how you obtain this?

Knowing the sequence of two chains:

Experiment: In this experiment, you leave the disulfide bond intact, and treat the protein with chymotrypsin again. After separating the fragments, you treat them with performic acid and then determine the amino acid composition of the two framents that contain cysteine. The percentages refer to mole percent, not weight percent:

Cysteine-bearing fragment 1

A: 7.7%
C: 15.4%
D: 7.7%
F: 7.7%
I: 7.7%
K: 7.7%
L: 7.7%
M: 7.7%
P: 15.4%
R: 7.7%
S: 7.7%

Cysteine-bearing fragment 2

A: 22.2%
C: 11.1%
I: 11.1%
K: 11.1%
L: 11.1%
N: 11.1%
S: 11.1%
Y: 11.1%

Explanation / Answer

the disulphide bond is generally between the sulfur containing amino acids ie cystein and methionine present in two polypeptide chains. Genearally the percentage of sulfur amino acids indicate the number of disulphide bond between the polypeptides. In the given data disulphide bonds will be formed at the second position of first fragment and the second position of the second fragment

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