PROTEINS and ENZYMES PROTEINS - Elementary Idea of -amino acids, peptide bond, polypeptides. Proteins – primary: NEET MCQs with Answers

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Syllabus covered: primary, secondary, tertiary, and quaternary structure (qualitative idea only), denaturation of proteins, enzymes.

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Question 1

In strongly acidic solution lysine exists as the dication HOOC–CH(NH₃⁺)–CH₂CH₂CH₂CH₂–NH₃⁺. Call the ammonium group at the end of the side chain P, the carboxyl group Q, and the ammonium group on the alpha carbon R. What is the decreasing order of acid strength of these three groups?
  1. P > R > Q
  2. Q > P > R
  3. Q > R > P
  4. R > Q > P
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Answer: (C) Q > R > P
Explanation: The carboxyl group loses its proton first because the carboxylate ion formed is stabilised by resonance, so Q is the strongest acid. The alpha ammonium group is close to the electron-withdrawing carboxyl group, and this -I effect makes its proton easier to remove. The side-chain ammonium group lies four CH₂ units further away, receives almost no such effect, and is the weakest acid of the three.

Question 2

Consider the two statements. Statement I: Maltase is named after the compound on which it acts, whereas oxidoreductases are named after the reaction they catalyse. Statement II: The enzyme sucrase hydrolyses sucrose by a path of higher activation energy than acid hydrolysis does. Which option is correct?
  1. Statement I is incorrect but Statement II is correct
  2. Both statements are incorrect
  3. Both statements are correct
  4. Statement I is correct but Statement II is incorrect
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Answer: (D) Statement I is correct but Statement II is incorrect
Explanation: Enzymes are generally named after their substrate, as maltase is for maltose, and sometimes after the reaction, as oxidoreductases are for simultaneous oxidation of one substrate and reduction of another. Statement II is wrong because enzymes reduce the activation energy. The value for sucrose hydrolysis falls from 6.22 kJ mol⁻¹ with acid to 2.15 kJ mol⁻¹ with sucrase.

Question 3

One molecule of glycine and one molecule of alanine are joined by a single peptide bond. How many different dipeptides are possible, and which residue carries the free –NH₂ group in glycylalanine?
  1. Four dipeptides; the alanine residue
  2. Two dipeptides; the alanine residue
  3. One dipeptide; the glycine residue
  4. Two dipeptides; the glycine residue
Show answer
Answer: (D) Two dipeptides; the glycine residue
Explanation: The –COOH of either amino acid can react with the –NH₂ of the other, so two dipeptides are possible: glycylalanine and alanylglycine. Glycylalanine forms when the carboxyl group of glycine combines with the amino group of alanine. Its free –NH₂ group therefore remains on the glycine residue and its free –COOH on the alanine residue.

Question 4

Glycine (H₂N–CH₂–COOH) has a much higher melting point than chloroacetic acid (Cl–CH₂–COOH). What is the best explanation?
  1. The C–N bond of glycine is stronger than the C–Cl bond of chloroacetic acid
  2. Chloroacetic acid has no –COOH group able to form hydrogen bonds with water
  3. Glycine has a chiral α-carbon atom, which allows its molecules to pack more tightly
  4. Glycine exists as a dipolar ion, so its particles are held by strong salt-like electrostatic attractions
Show answer
Answer: (D) Glycine exists as a dipolar ion, so its particles are held by strong salt-like electrostatic attractions
Explanation: Glycine has an acidic –COOH group and a basic –NH₂ group in the same molecule, so the carboxyl proton shifts to the amino group to give a zwitter ion. Amino acids therefore behave like salts rather than simple amines or carboxylic acids, with strong electrostatic attractions between their particles. This explains the high melting point. Chloroacetic acid has no such dipolar form, and glycine itself is not chiral.

Question 5

A tripeptide is formed from one molecule each of glycine (molecular mass 75 u), alanine (89 u) and valine (117 u). What is its molecular mass?
  1. 227 u
  2. 245 u
  3. 263 u
  4. 281 u
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Answer: (B) 245 u
Explanation: Each peptide bond forms by elimination of one water molecule, and a tripeptide has two peptide bonds. The sum of the amino acid masses is 75 + 89 + 117 = 281 u. Removing two water molecules gives 281 − 2 × 18 = 245 u. The sequence of the residues does not alter this mass.

Question 6

A little urease is added to a neutral urea solution containing a few drops of phenolphthalein, and the colourless solution slowly turns pink. Why does this happen?
  1. Urease converts urea into uric acid, which makes the solution alkaline
  2. Urease hydrolyses urea to ammonia and carbon dioxide, and the ammonia makes the solution alkaline
  3. Urease oxidises urea to nitric acid, which reacts with the indicator
  4. Urease joins urea molecules into a coloured protein
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Answer: (B) Urease hydrolyses urea to ammonia and carbon dioxide, and the ammonia makes the solution alkaline
Explanation: Urease is an enzyme that catalyses the hydrolysis of urea: NH₂CONH₂ + H₂O → 2NH₃ + CO₂. Ammonia is basic and raises the pH of the solution. Phenolphthalein is colourless in neutral solution and pink in alkaline solution.

Question 7

In strongly acidic solution glutamic acid exists as HOOC–CH(NH3+)–CH2CH2–COOH. Call the carboxyl group on the alpha carbon X, the carboxyl group at the end of the side chain Y, and the ammonium group Z. What is the decreasing order of acid strength of these groups?
  1. X > Y > Z
  2. Z > X > Y
  3. Y > Z > X
  4. Y > X > Z
Show answer
Answer: (A) X > Y > Z
Explanation: Carboxyl groups are much stronger acids than an ammonium group, so Z is the weakest of the three. The alpha carboxyl group lies next to the positively charged NH3+ group, whose strong electron-withdrawing inductive effect stabilises the carboxylate anion. The side-chain carboxyl group is too far away to feel this effect much. Hence the alpha COOH loses its proton most easily, giving X > Y > Z.
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Question 8

A tetrapeptide contains one residue each of Gly, Val, Ala and Phe. Partial hydrolysis gives the dipeptides Val-Ala, Gly-Val and Ala-Phe, each written from the free amino end to the free carboxyl end. Which residue carries the free α-carboxyl group of the tetrapeptide?
  1. Gly
  2. Phe
  3. Ala
  4. Val
Show answer
Answer: (B) Phe
Explanation: Overlapping the fragments puts Gly-Val before Val-Ala and Val-Ala before Ala-Phe. The sequence is therefore Gly-Val-Ala-Phe. Gly is at the free amino end and Phe is at the free carboxyl end.

Question 9

Egg white sets on boiling, and milk turns to curd when bacteria grow in it. Which statement correctly applies to both changes?
  1. Peptide bonds are hydrolysed, so proteins are converted to free amino acids
  2. The original protein structure returns fully on cooling or on neutralising the mixture
  3. The change is caused by lactic acid formed in the food
  4. Proteins are denatured, with secondary and tertiary structure disturbed while the primary structure stays intact
Show answer
Answer: (D) Proteins are denatured, with secondary and tertiary structure disturbed while the primary structure stays intact
Explanation: Heat and change in pH disturb the hydrogen bonds that hold the folded shape of a protein, so it uncoils and clumps together. This is denaturation, and the amino acid sequence is unchanged. Lactic acid from bacteria is the cause only in curdling, while heat is the cause with egg white. Neither change is readily reversed.

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