Which of the following amino acids is classified as nonpolar and hydrophobic?
Answer: A
Leucine is a nonpolar, hydrophobic amino acid with an isobutyl side chain. Serine and threonine are polar uncharged, while aspartate is acidic and polar.
Q.82Easy
Collagen's characteristic triple helix structure is stabilized by which type of bonding?
Answer: A
Collagen triple helix is stabilized by hydrogen bonds between the three polypeptide chains and covalent cross-links (lysine and hydroxylysine residues) between molecules, providing mechanical strength.
Q.83Easy
Which cofactor is essential for the catalytic activity of lactate dehydrogenase (LDH)?
Answer: A
LDH catalyzes the interconversion of lactate and pyruvate using NAD+ as an electron acceptor and NADH as an electron donor in the coupled redox reaction.
Q.84Easy
What is the primary function of chaperone proteins in the endoplasmic reticulum?
Answer: A
ER chaperones like BiP (heat shock protein 70) assist in protein folding, prevent aggregation, and aid in proper disulfide bond formation during synthesis.
Q.85Easy
Which statement about peptide bonds is correct?
Answer: A
Peptide bonds form through a condensation reaction between the carboxyl group (-COOH) of one amino acid and the amino group (-NH2) of another, releasing H2O. They are covalent and stable.
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Q.86Medium
In competitive enzyme inhibition, which Michaelis-Menten parameter is affected?
Answer: A
In competitive inhibition, the inhibitor competes with substrate for the active site. This increases the apparent Km (requires more substrate to reach half-maximal velocity) while Vmax remains unchanged.
Q.87Medium
Which amino acid's deficiency in the diet can lead to kwashiorkor in children?
Answer: C
Kwashiorkor results from severe protein malnutrition involving deficiency of all essential amino acids, leading to loss of muscle mass and edema despite adequate calorie intake.
Q.88Medium
The Ramachandran plot is used to validate which aspects of protein structure?
Answer: A
The Ramachandran plot shows allowed and disallowed combinations of φ and ψ angles for the polypeptide backbone, used to verify the stereochemical quality of 3D protein structures.
Q.89Medium
Which enzyme belongs to the transferase class (EC 2)?
Answer: A
AST catalyzes the transfer of an amino group from aspartate to α-ketoglutarate, making it a transferase. Amylase is a hydrolase, catalase is a lyase, and hexokinase is a ligase.
Q.90Medium
A patient has elevated serum creatine kinase (CK). Which tissue type is primarily affected?
Answer: A
CK is abundant in skeletal and cardiac muscle. Elevated CK indicates muscle damage from myocardial infarction, rhabdomyolysis, or muscular dystrophy.
Q.91Medium
Which prosthetic group is found in cytochrome c oxidase?
Answer: A
Cytochrome c oxidase (Complex IV) contains heme a, heme a3, and copper centers (CuA and CuB) essential for electron transfer and oxygen reduction to water.
Q.92Medium
In a temperature vs. enzyme activity graph, why does enzyme activity decrease above the optimal temperature?
Answer: A
Above optimal temperature, increased thermal energy disrupts hydrogen bonds and hydrophobic interactions maintaining the 3D structure, causing denaturation and loss of catalytic activity.
Q.93Medium
Which proteolytic enzyme is responsible for activating trypsinogen to trypsin in the small intestine?
Answer: A
Enteropeptidase (enterokinase), secreted by the duodenal mucosa, cleaves a specific peptide bond in trypsinogen to produce active trypsin, initiating the cascade of pancreatic protease activation.
Q.94Hard
A student observes that an enzyme shows sigmoidal kinetics instead of Michaelis-Menten kinetics. What does this indicate?
Answer: A
Sigmoidal (S-shaped) kinetics indicate positive cooperativity, typical of allosteric enzymes with multiple subunits (e.g., aspartate transcarbamoylase). Binding of substrate to one subunit increases affinity in others.
Q.95Hard
How does the proteasome recognize proteins marked for degradation in the ubiquitin-proteasome system?
Answer: A
E3 ubiquitin ligases catalyze the attachment of ubiquitin chains (primarily through Lys48 linkages) to lysine residues on target proteins. The 19S proteasomal subunit recognizes these polyubiquitin chains and unfolds the protein for degradation.
Q.96Hard
A mutation changes a hydrophobic valine residue to a charged aspartate in the hydrophobic core of a globular protein. What is the most likely consequence?
Answer: A
Substituting a nonpolar residue with a charged, hydrophilic one in the protein core disrupts critical hydrophobic interactions that stabilize the tertiary structure, leading to misfolding, aggregation, or degradation.
Q.97Hard
A protein exhibits a β-sheet structure rich in proline residues. Why is this structurally problematic?
Answer: A
Proline is an imino acid with its side chain bonded to the backbone nitrogen, eliminating the NH group needed for β-sheet hydrogen bonding between strands. High proline content disrupts β-sheet formation, commonly found in turns and loops instead.
Q.98Hard
Which scenario best describes non-competitive enzyme inhibition kinetically?
Answer: A
In non-competitive inhibition, the inhibitor binds to an allosteric site on both E and ES, preventing product formation. This decreases Vmax (fewer active enzymes) while Km remains unchanged (substrate binding affinity unaffected).
Q.99Medium
A researcher studying protein folding observes that a newly synthesized polypeptide chain contains multiple disulfide bonds between cysteine residues. Which cellular compartment is most likely responsible for facilitating the formation of these disulfide bonds?
Answer: A
Disulfide bonds are formed in oxidizing environments. The rough endoplasmic reticulum (RER) and Golgi apparatus maintain oxidizing conditions suitable for disulfide bond formation, unlike the reducing environment of the cytoplasm. The enzyme protein disulfide isomerase (PDI) facilitates this process in the ER lumen.
Q.100Medium
An enzyme exhibits a Km of 2 mM and Vmax of 100 μmol/min. When substrate concentration is 6 mM and an allosteric inhibitor is added, the Vmax decreases to 50 μmol/min while Km remains unchanged. What type of inhibition is occurring?
Answer: B
Non-competitive inhibition decreases Vmax while keeping Km constant. This occurs when an inhibitor binds to a site other than the active site (allosteric site), preventing product formation regardless of substrate concentration. The Km value remains unchanged because substrate binding affinity is unaffected.
