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MCQEasyJEE 2026Proteins (Amino Acids, Structure, Denaturation)

JEE Chemistry 2026 Question with Solution

In the given pentapeptide, find out an essential amino acid (YY) and the sequence present in the pentapeptide.

Pentapeptide structure shown from left to right with H2N at the N-terminus and COOH at the C-terminus, containing side chains corresponding to threonine, serine, aspartic acid, glycine, and alanine residues.

Choose the correct answer from the options given below:

  • A

    Y=Y = Threonine;\quad Sequence: Thr–Ser–Asp–Gly–Ala

  • B

    Y=Y = Serine;\quad Sequence: Ser–Asp–Thr–Ala–Gly

  • C

    Y=Y = Serine;\quad Sequence: Thr–Ser–Asp–Ala–Gly

  • D

    Y=Y = Threonine;\quad Sequence: Ser–Thr–Asp–Gly–Ala

Answer

Correct answer:A

Step-by-step solution

Standard Method

Given: A pentapeptide structure is provided.

Find: The essential amino acid YY and the correct sequence present in the pentapeptide.

Concept: A peptide sequence is always read from the N-terminus (NH2\mathrm{NH_2} end) to the C-terminus (COOH\mathrm{COOH} end). Each amino acid residue is identified by its characteristic side chain (RR-group).

Step 1: Identify the direction of the peptide

The left end shows H2N\mathrm{H_2N}, so it is the N-terminus. The right end shows COOH\mathrm{COOH}, so it is the C-terminus. Therefore, the sequence is read from left to right.

Step 2: Identify each amino acid residue from the side chain

  • First residue: side chain −CH(OH)CH3-\mathrm{CH(OH)CH_3} ⇒\Rightarrow Threonine (Thr)
  • Second residue: side chain −CH2OH-\mathrm{CH_2OH} ⇒\Rightarrow Serine (Ser)
  • Third residue: side chain −CH2COOH-\mathrm{CH_2COOH} ⇒\Rightarrow Aspartic acid (Asp)
  • Fourth residue: side chain −H-\mathrm{H} ⇒\Rightarrow Glycine (Gly)
  • Fifth residue: side chain −CH3-\mathrm{CH_3} ⇒\Rightarrow Alanine (Ala)

Step 3: Write the peptide sequence

Reading from N-terminus to C-terminus:

Thr–Ser–Asp–Gly–Ala\text{Thr–Ser–Asp–Gly–Ala}

Step 4: Identify the essential amino acid

Among these amino acids, Threonine is essential, whereas Serine, Aspartic acid, Glycine, and Alanine are non-essential.

So,

Y=ThreonineY = \text{Threonine}

Conclude: The essential amino acid is Threonine and the sequence is Thr–Ser–Asp–Gly–Ala. Therefore, the correct option is A.

Residue Identification by Side Chains

Given: The peptide structure contains five amino acid residues joined by peptide bonds.

Find: Which residue is the essential amino acid and what is the order of residues.

The key is to match each side chain with the corresponding amino acid:

  • −CH(OH)CH3-\mathrm{CH(OH)CH_3} corresponds to Threonine
  • −CH2OH-\mathrm{CH_2OH} corresponds to Serine
  • −CH2COOH-\mathrm{CH_2COOH} corresponds to Aspartic acid
  • −H-\mathrm{H} corresponds to Glycine
  • −CH3-\mathrm{CH_3} corresponds to Alanine

Since the structure starts with H2N\mathrm{H_2N} and ends with COOH\mathrm{COOH}, the order is read from left to right as:

Thr–Ser–Asp–Gly–Ala\text{Thr–Ser–Asp–Gly–Ala}

Among these, only Threonine is an essential amino acid.

Conclude: The correct option is A.

Common mistakes

  • Reading the peptide from the C-terminus to the N-terminus gives the wrong sequence. Always start from the end containing NH2\mathrm{NH_2} and move toward the COOH\mathrm{COOH} end.

  • Confusing the side chain −CH(OH)CH3-\mathrm{CH(OH)CH_3} with serine is incorrect. Threonine has the extra CH3\mathrm{CH_3} group, whereas Serine has only −CH2OH-\mathrm{CH_2OH}.

  • Treating all identified amino acids as essential is wrong. Only Threonine is essential here; Serine, Aspartic acid, Glycine, and Alanine are non-essential.

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