**Question:** Which one of the following can be homologous substitution for isoleucine in a protein in sequence?

A. Valine
B. Leucine
C. Threonine
D. Alanine

**Core Concept:** Homologous Substitution in Proteins

Homologous substitution in proteins refers to the replacement of one amino acid residue with another amino acid residue that has similar chemical and structural properties to the original residue. These substitutions occur due to genetic mutations that lead to changes in amino acid sequences. In proteins, amino acids are encoded by specific nucleotides in the DNA, and mutations can lead to changes in the amino acid sequence, resulting in homologous substitutions.

**Why the Correct Answer is Right:**

In this question, we are asked about the homologous substitution for isoleucine in a protein sequence. Isoleucine is an alpha-amino acid with the chemical formula CH3CH(CH2)4NH2. Among the given options, valine (CH3CH(CH2)3NH2) and leucine (CH3CH(CH2)4NH2) share similar chemical and structural properties with isoleucine. Both valine and leucine are branched-chain amino acids, which means they have a branched side chain, similar to isoleucine.

**Why Each Wrong Option is Incorrect:**

- Option C (Threonine): Threonine is an alpha-amino acid with the chemical formula CH3CH(OH)CH(CH2)2NH2. It lacks the branched side chain present in isoleucine, valine, and leucine. Hence, it is not a homologous substitution for isoleucine.
- Option D (alanine): Ala is an alpha-amino acid with the chemical formula CH3CH(CH2)2NH2. Alanine also lacks the branched side chain present in isoleucine, valine, and leucine, making it an incorrect homologous substitution for isoleucine.

**Clinical Pearl:**

Understanding homologous substitutions in proteins is crucial for understanding genetic mutations and their effects on protein function and structure. In some cases, these substitutions can lead to altered protein properties or even loss of protein function, resulting in genetic disorders or diseases. By recognizing homologous substitutions, clinicians can deduce the genetic basis of certain disorders and design appropriate treatments or interventions.

**Correct Answer:**

The correct homologous substitution for isoleucine is **valine**. Valine shares the same chemical structure (CH3CH(CH2)3NH2) as isoleucine, and both are branched-chain amino acids. Leucine (CH3CH(CH2)4NH2) is another branched-chain amino acid but differs from isoleucine and valine in having an additional methyl group at the 4th carbon atom of the amino acid chain.

**Why Option B (Leucine) is Incorrect:**

Leucine differs from isoleucine and valine due to the presence of an additional methyl group at the 4th carbon atom of the amino acid chain. This structural difference