13. You are performing site-directod for a protelin\'s function. Which of each p

Question

13. You are performing site-directod for a protelin's function. Which of each pair of amino acid substitutions tisted below would you enp mutagenesis to test predictions about which residues are essential to disrupt protein function the mosr? Circle one for each (a-d).(4 points) (a) Val replaced by Ala or Phe (c) Gln replaced by Glu or Asn (b) Lys replaced by Asp or Arg (d) Pro replaced by His or Gly 14. On the diagram of a part of an extended polypeptide chain given below.(3 points) Draw the four backbone hydrogen bonds which would be formed the peptide were part of an -helix. Indicate the direction of the helix dipole moment with an arrow drawn from positive to a. among the residues shown if b. Lys 15. A globular protein undergoes a subtle conformational change in which the most significant event is movement of three closely packed side chains--leucine, alanine, and phenylalanine-from the imerior of the protein to the exterior, where they are immersed in water. Assuming that this is the only thermodynamically significant event in the conformational change, describe the effect on AG (Gibbs free energy): increase, decrease, stay the same. Why? (3 poits) 16. A hypothetical protein has the sequence: (4 points) NHs-ARVSMKIEAKGDWTG QMTGDANFRASVEL-CO0 The protein is found to form a very stable secondary structure and exists in free solution as a dimer. Speculate on the secondary structure of this protein. Why does it form a dimer in free solution? a. b. What is the length of a single monomer (in Angstrôms) of this protein? CalCulation

Explanation / Answer

13)

(a)

Phe. Phenylalanine has a benzene ring, so even though both alanine and phenylalanine are hydrophobic, the larger size of phenylalanine’s R side chain may not fit as well in place of valine in comparison to alanine so it would disrupt the structure.

(b)

Asp-Lys and Arg both are basic amino acid while Asp is acidic.

(c)

Glu-Glutamic acid (Glu) has acidic properties and carries a negative charge in its R group. Gln, although polar, has an amide group as opposed to a carboxylate. Therefore, asparagine’s amide-containing R group would be better suited to substituted for Gln than Glu.

(d)

His.-Histidine is a hydrophilic amino acid with basic properties and a positive charge due to its side R group. Moreover, proline is hydrophobic and has a constrained geometry due to the heterocyclic ring. Therefore, a protein is better off having an amino acid with no side chain to replace proline than to have an amino acid side chain that is bulkier.

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