Question

A protein chemist told a molecular geneticist that he had found a new mutant hemoglobin in which aspartate replaced lysine. The molecular geneticist expressed surprise and sent his friend scurrying back to the laboratory. (a) Why did the molecular geneticist doubt the reported amino acid substitution? (b) Which amino acid substitutions would have been more palatable to the molecular geneticist?

Answer everything but please focus on part b. I do not understand how to identify the amino acids that would’ve made for better substitutions.

Answer 1

Answer A: Codon in lysine cannot be changed to one for aspartate via single nucleotide mutation.

- We can also say that single nucleotide/base substitution won't lead to aspartate formation.

Explanation: There are different types of mutations like Silent, missense, non sense, frameshift.

- Mostly there is single nucleotide change that leads to these base mutations.

- They occur at only single base which can change the protein.

- The lysine and Aspartate has following codons that code for these amino acids.

Amino Acid	Codon 1	Codon 2
Lysine	TTT	TTC
Aspartate	CTA	CTG

- A single change like T --> C or C --> G won’t lead to formation of aspartate.

Answer B: Possible amino acid substitions: Thr, Arg, Asn, Gln, Glu, Ile, or Met.

Below are the names of the amino acids along with their abbreviations.

• Thr: threonine
• Arg: arginine
• Asn: asparagine
• Gln: glutamine
• Glu: glutamate or glutamic acid
• Ile: Isoleucine
• Met: Methionine

Explanation: These are the palatable ones because there is single nucleotide change in the below amino acids.

- Below is the possible single base mutations which can form the above mentioned amino acids.

- The possible codons are taken from the genetic codon table. The referenced codons are the DNA base triplets.

- Out of 21 amino acids, each single change would lead to these 7 amino acids.

Note: For your convenience the changes have been highlighted with yellow background color.

(1): Change at base number 2.

Amino Acid	Codon 1	Codon 2
Lysine	TTT	TTC
Threonine	TGT	TGC
Arginine	TCT	TCC
Isoleucine	TAT
Methionine		TAC

(2): Change at base number 1:

Amino Acid	Codon 1	Codon 2
Lysine	TTT	TTC
Glutamine	GTT	GTC
Glutamate	CTT	CTC

(3): Change at base number 3:

Amino Acid	Codon 1	Codon 2
Lysine	TTT	TTC
Asparagine	TTA	TTG