3) (20 points) pKa perturbations. A water-soluble globular protein has 2 asparti

Question

3) (20 points) pKa perturbations. A water-soluble globular protein has 2 aspartic acid residues in different parts of the tertiary structure. Both are almost completely buried in the interior of the native protein. A. (10 points) The first Asp residue has a pKa of 7.4 in the native state, but a pKa of 4.4 in the denatured state. Explain the difference in pKa between the two states. B. (10 points) The other Asp residue, also buried in the native structure, has a pKa of 2.4 in the native state. Explain how this might be possible.

Explanation / Answer

3. a) At pKa 7.4 the aspartic acid in the protein’s side chain occurs as the negatively charged aspartate form, COO. However, at more acidic condition i.e., pKa 4.4 side group is in the COOH form.

Actually the side chain of the aspartic acid which bears an additional acidic moiety, are in different form and hence alter the form of the protein from native to denature or it is found at two different states in different condition.

b) At pKa 2.4 the side chain acidic group is in the COOH form and the alpha-amino group is in the –NH3+ form butit is in native state which as compared to the first Asp residue is in very different state. This is possible because the second residue resides at a different location of the protein and the condition and surronding of that part may vary from the location of first residue.

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