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Chapter 5: Q33P. (page 129)

Treatment of a polypeptide with 2-mercaptoethanol yields two polypeptides:

1. Ala鈥揤al鈥揅ys鈥揂rg鈥揟hr鈥揋ly鈥揅ys鈥揕ys鈥揂sn鈥揚he鈥揕eu

2. Tyr鈥揕ys鈥揅ys鈥揚he鈥揂rg鈥揌is鈥揟hr鈥揕ys鈥揅ys鈥揝er

Treatment of the intact polypeptide with trypsin yields fragments with the following amino acid compositions:

3. (Ala, Arg, Cys2, Ser, Val)

4. (Arg, Cys2, Gly, Lys, Thr, Phe)

5. (Asn, Leu, Phe)

6. (His, Lys, Thr)

7. (Lys, Tyr)

Indicate the positions of the disulfide bonds in the intact polypeptide.

Short Answer

Expert verified

The disulfide bond formed between the two peptides is shown below.

Step by step solution

01

Treatment with 2 – Mercaptoethanol

The Cysteine residues contain a sulfur group in their side chain. Two polypeptide chains are linked when cysteine residue of one chain forms a disulfide bond with another cysteine residue of another chain. For determining the sequence of protein, these dislufide bonds must be broken to get two separate linear chains. 2鈥揗ercaptoethanol is used to separate disulfide bonds formed between the peptide chains.

02

Cleavage by trypsin

Trypsin is an endopeptidase isolated from Bovine pancreas.Only if the next residue is not Proline (Pro), Trypsin breaks the peptide bond the on the C side (carboxyl terminus) of positively charged amino acids Lysine (Lys) and Arginine (Arg).

03

Explanation

When the given polypeptide is treated with 2 - mercaptoethanol, it yields two separate polypeptide chains. When the same polypeptide is treated with trypsin, it will yield several small fragments having the composition given in the question. This composition will only be obtained when disulfide bond is formed between the first Cys of 1 polypeptide and second Cys residue of 2 polypeptides; and between second Cys of 1 polypeptide and first Cys of 2 polypeptides.

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Most popular questions from this chapter

(a) In what order would the amino acids Arg, His, and Leu be eluted from a carboxymethyl column at pH6 ?

(b) In what order would Glu, Lys, and Val be eluted from a diethylaminoethyl column at pH 8?

What fractionation procedure could be used to purify protein 1 from a mixture of three proteins whose amino acid compositions are as follows?

1. 25% Ala, 20% Gly, 20% Ser, 10% Ile, 10% Val, 5% Asn, 5% Gln, 5% Pro

2. 30% Gln, 25% Glu, 20% Lys, 15% Ser, 10% Cys

3. 25% Asn, 20% Gly, 20% Asp, 20% Ser, 10% Lys, 5% Tyr

All three proteins are similar in size and pI, and there is no antibody available for protein 1.

You wish to sequence the light chain of a protease inhibitor from the Brassica nigra plant. Cleavage of the light chain by trypsin and chymotrypsin yields the following fragments. What is the sequence of the light chain?

Chymotrypsin

1. Leu鈥揌is鈥揕ys鈥揋ln鈥揂la鈥揂sn鈥揋ln鈥揝er鈥揋ly鈥揋ly鈥揋ly鈥揚ro鈥揝er

2. Gln鈥揋ln鈥揂la鈥揋ln鈥揌is鈥揕eu鈥揂rg鈥揂la鈥揅ys鈥揋ln鈥揋ln鈥揟rp

3. Arg鈥揑le鈥揚ro鈥揕ys鈥揅ys鈥揂rg鈥揕ys鈥揚he

Trypsin

4. Arg

5. Ala鈥揅ys鈥揋ln鈥揋ln鈥揟rp鈥揕eu鈥揌is鈥揕ys

6. Cys鈥揂rg

7. Gln鈥揂la鈥揂sn鈥揋ln鈥揝er鈥揋ly鈥揋ly鈥揋ly鈥揚ro鈥揝er

8. Phe鈥揋ln鈥揋ln鈥揂la鈥揋ln鈥揌is鈥揕eu鈥揂rg

9. Ile鈥揚ro鈥揕ys

10. Lys

Explain why a certain protein has an apparent molecular mass of 90kDwhen determined by gel filtration and 60kDwhen determined by SDS-PAGE in the presence or absence of 2-mercaptoethanol. Which molecular mass determination is more accurate?

Consult Table 5-1 to complete the following: (a) On a plot of absorbance at 280 nm versus elution volume, sketch the results of gel filtration of a mixture containing human cytochrome c and bacteriophage T7 RNA polymerase and identify each peak. (b) Sketch the results of SDS-PAGE of the same protein mixture showing the direction of migration and identifying each band.
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