Three different proteins, A, B, and C, from a plant were studied. In each case,
ID: 93749 • Letter: T
Question
Three different proteins, A, B, and C, from a plant were studied. In each case, a glycine residue was changed to an alanine. Changes in the biological activities of the mutated proteins were then measured and compared to wild-type activity of that protein, and the results are shown in the table below.
Which statement about structure–function relationships in proteins is supported by the data?
a. Each unique amino acid sequence results in a protein with a unique function.
b. A protein’s function changes only when changes in primary structure alter its secondary, tertiary, or quaternary structure.
c. Substitution of a nonpolar amino acid for another nonpolar amino acid is sufficient to allow a protein to retain its biological activity.
d. Amino acids located at the beginning of a polypeptide chain have less importance in determining biological function than amino acids in the middle or far end of the chain.
Protein Amino acid substitution Emect on biological activity A Glycine 15 alanine 15 No change from native protein Activity decreased 85% Glycine 78 alanine 78 C Glycine 109 alanine109 Activity decreased 45%Explanation / Answer
Answer d is correct because we can see from the table that the change of gly15 to Ala 15 has no change in the determination of the biological function of protein and on the hand the later ends amino acid such as 78 and 109 have the ability to determine the function of protein.
Option A is wrong because if each unique sequence led to the unique function then the gly15 to ala15 substitution must have the unique function but it is not the case as there is no change function in native protein' s biological function.
Option B is also wrong because the substitutions are same gly to alanine so there are no changes in their primary to secondary to tertiary to quaternary structures but still the biological functions are changing as the position of glycine changes.
C is also wrong because here all the changes are same that is glycine to alanine (both are nonpolar) and still the first substitution has no effect on the biological activity of the protein so clearly it is not sufficient to retain protein it's activity.
E is also wrong because first substitution doesn't make any difference in the protein function. So not every amino acid substitution in a protein's primary structure leads to a change in the protein's function.
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