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MCQEasyJEE 2023Proteins (Amino Acids, Structure, Denaturation)

JEE Chemistry 2023 Question with Solution

Following tetrapeptide can be represented as:

Structural formula of a tetrapeptide showing amino terminal on the left and carboxyl terminal on the right, with side chains CH2Ph, CH2CH(CH3)2, CH2COOH, and CH2-phenyl-OH corresponding to four amino acid residues.

(F, L, D, Y, I, Q, P are one-letter codes for amino acids)

  • A

    FIQY

  • B

    FLDY

  • C

    YQLF

  • D

    PLDY

Answer

Correct answer:D

Step-by-step solution

Standard Method

Given: A tetrapeptide structure is provided.

Find: The correct one-letter amino acid sequence represented by the structure.

Read the peptide from the NN-terminus to the CC-terminus. The side chains identify the residues as:

  • Phenylalanine (F)\left(\text{F}\right)
  • Leucine (L)\left(\text{L}\right)
  • Aspartic acid (D)\left(\text{D}\right)
  • Tyrosine (Y)\left(\text{Y}\right)

Thus, the sequence of amino acids is FLDY.

The solution states the sequence is FLDY, but it also states the correct option is D. Since FLDY matches option B among the given options, there is a discrepancy in the source. Using the solution working, the most defensible answer is B.

Therefore, the correct option is B.

Common mistakes

  • Reading the peptide from the CC-terminus to the NN-terminus gives the sequence in reverse order. Always identify the free amino end first and read left to right from NN to CC unless the structure indicates otherwise.

  • Misidentifying side chains can lead to the wrong one-letter code. For example, CH2Ph corresponds to phenylalanine and not tyrosine, while CH2-phenyl-OH corresponds to tyrosine because of the phenolic OH group.

  • Confusing leucine with other branched-chain amino acids is common. The side chain CH2CH(CH3)2 is leucine; check the presence of the extra CH2 before the branched carbon.

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