Q. 17.67 - An Introduction to General, Organic, and Biological Chemistry

Question

a. A point mutation changes a codon in the mRNA for an enzyme from GCC to GCA. Why is there no change in the amino acid order in the protein?

b. In sickle-cell anemia, a point mutation in the mRNA for hemoglobin results in the replacement of glutamate with valine in the resulting hemoglobin molecule. Why does the replacement of one amino acid cause such a drastic change in biological function?

Step-by-Step Solution

Answer

a. When a point mutation changes a codon in the mRNA for an enzyme from GCC to GCA there no change in the amino acid order in the protein because of silent mutation.

b. When polar glutamic acid is replaced with nonpolar valine in the tertiary structure of haemoglobin, a crucial interaction is lost. Hemoglobin becomes misshapen and less capable of transporting oxygen as a result.

Part(a) Step 1: Given information

We need to explain that why there is no change in the amino acid order in the protein when a point mutation changes a codon in the mRNA for an enzyme from GCC to GCA  .

Part(a) Step 2: Explanation

Because of silent mutation, when a point mutation changes a codon in an enzyme's mRNA from GCC to GCA, there is no change in the amino acid order in the protein.

When a point mutation (single nucleotide alteration) occurs, the mRNA codon (trio sequence that codes for amino acid) changes, but it has no effect on the amino acid it codes for. For example, if a point mutation causes UUU to become UUC, this is a silent mutation because both codons are affected.

Part(b) Step 1: Given information

We need to explain that why does the replacement of one amino acid cause such a drastic change in biological function 

Part(b) Step 2: Explanation

During the replacement of glutamate with valine in the resulting hemoglobin molecule a key interaction is lost when polar glutamic acid is substituted with nonpolar valine in haemoglobin's tertiary structure. As a result, haemoglobin becomes distorted and less capable of delivering oxygen.