IMAT 2020 Q30 [Mutated Protein]

A single base mutation occurs in a gene.

Which of the following is/are possible effects on the protein produced as a result of this

  1. The protein has an altered tertiary structure.
  2. The protein is shorter in length.
  3. The protein is unchanged.

A. 2 and 3 only
B. 1, 2 and 3
C. 1 and 2 only
D. 1 and 3 only
E. 1 only

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The correct answer is B.

To understand this question, we have to know what the different structures of a protein are. So let’s review them one by one.

First, the primary structure : it is the sequence of amino-acids, linked to each other by peptide bonds. It is a covalent chemical bond that is found between 2 successive amino-acids, more precisely, it is formed between the carboxyl and amino group of 2 distinct molecules. It has a defined polarity : the first amino-acid is, by convention, the one that has the free amino-function : it is the N-term of the protein. On the other end is the C-term, or free carboxyl group.

Secondary structure is the local folding of the protein chain. This phenomenon is a consequence of the different patterns of the hydrogen bonds. They result in the formation of different forms of proteins : helix, strand and sheets.

The tertiary structure is the protein’s 3D structure. For example, the tertiary structure of the hemoglobin is its globular structure. The integrity of the structure is ensured by different interactions :

  1. covalent interactions in the form of disulfide bridges between 2 cysteine
  2. electrostatic interactions in the form of ionic and hydrogen bonds
  3. van der Waal interactions, which is a distant dependent interaction between 2 molecules.

Additionally, binding sites are little “pockets” that are found on the tertiary structure. Like the name suggests, its purpose is to bind another molecule or also known as ligand. It could be hormones, second messengers, enzyme substrates etc.

Lastly, quaternary structure refers to the way each protein chain is organized ; in other words, it is the combination of 2 or more subunits. Hemoglobin is an example of that : it is made of 4 sub-units, α and β.

What about the question ?

One thing we have to understand is the redundancy of the genetic code : an amino-acid can be specified by more than one codon. This means that the protein could or could not be changed if a single base mutation occurs.

In addition, because a single base mutation would change the primary structure of the protein, it could lead to an altered tertiary structure.

Similarly, there are more than one STOP codons : UAA, UGA, UAG are all STOP codons. Their purpose is to halt the protein translation. Therefore, if a single base mutation occurs, it could become a STOP codon and shorten the length of the protein.