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MCQEasyJEE 2023Proteins (Amino Acids, Structure, Denaturation)

JEE Chemistry 2023 Question with Solution

A protein 'X' with molecular weight of 70,000 u70{,}000 \, \text{u}, on hydrolysis gives amino acids. One of these amino acids is

  • A
    Structural formula showing NH2-CH2-CH(CH3)-CH2CH2COOH, with amino group on terminal carbon and carboxyl group at chain end.
  • B
    Structural formula showing CH3-CH(CH3)-CH2-CH(NH2)-COOH, an alpha-amino acid with amino group on carbon adjacent to carboxyl group.
  • C
    Structural formula showing CH3-CH(CH3)-CH(NH2)-CH2COOH, with amino group not on the alpha carbon relative to carboxyl group.
  • D
    Structural formula showing CH3-C(NH2)(CH3)-CH2-CH2COOH, a substituted amino acid structure with amino group away from the alpha carbon.

Answer

Correct answer:B

Step-by-step solution

Standard Method

Given: A protein 'XX' on hydrolysis gives amino acids.

Find: Which given structure is an amino acid obtained from protein hydrolysis.

Proteins on hydrolysis generally give α\alpha-amino acids. In an α\alpha-amino acid, the amino group is attached to the carbon adjacent to the carboxyl group.

From the provided solution, only option (2) has this α\alpha-amino acid arrangement. All the other options are not α\alpha-amino acids.

Therefore, the correct option is B.

Highlighted structural formula of option two showing CH3-CH(CH3)-CH2-CH(NH2)-COOH with amino group on alpha carbon next to carboxyl group.

Detailed Identification

Given: A protein of molecular weight 70,000 u70{,}000 \, \text{u} gives amino acids on hydrolysis.

Find: Identify the structure that can be one of those amino acids.

Step 1: Amino acids are identified by the presence of both a carboxyl group and an amino group.

Step 2: The structure shown in the second image matches the structure identified in the source solution as Leucine.

Step 3: The source solution states that Leucine is a branched-chain amino acid and can be released upon hydrolysis of a protein.

Thus, the correct answer is option B.

Structural formula labeled as leucine, showing CH3 substituent on side chain and NH2 attached to alpha carbon next to COOH group.

Common mistakes

  • Mistake: Choosing any compound that contains both −NH2-NH_2 and −COOH-COOH groups. Why wrong: proteins yield specifically α\alpha-amino acids on hydrolysis, not just any amino acid-like compound. Do instead: check whether −NH2-NH_2 is on the carbon adjacent to −COOH-COOH.

  • Mistake: Ignoring the position of the amino group in the carbon chain. Why wrong: the defining feature here is the α\alpha position, not merely the molecular formula. Do instead: first locate the carboxyl carbon, then inspect the next carbon for the amino group.

  • Mistake: Using the protein molecular weight 70,000 u70{,}000 \, \text{u} to compare directly with an amino acid option. Why wrong: hydrolysis breaks a protein into much smaller amino acid units, so the intact protein mass does not have to match the amino acid mass. Do instead: use the structural criterion for amino acids released from proteins.

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