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?
ACompetitive inhibition
BNon-competitive inhibition
CUncompetitive inhibition
DMixed inhibition
Correct Answer:
B. Non-competitive inhibition
EXPLANATION
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.
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?
ARough endoplasmic reticulum and Golgi apparatus
BMitochondrial matrix
CCytoplasm
DLysosomal lumen
Correct Answer:
A. Rough endoplasmic reticulum and Golgi apparatus
EXPLANATION
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.
Which proteolytic enzyme is responsible for activating trypsinogen to trypsin in the small intestine?
AEnteropeptidase
BChymotrypsin
CElastase
DCarboxypeptidase A
Correct Answer:
A. Enteropeptidase
EXPLANATION
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.
In a temperature vs. enzyme activity graph, why does enzyme activity decrease above the optimal temperature?
AThe tertiary structure denatures and active site geometry is lost
BSubstrate concentration decreases
CCofactors are oxidized
DThe enzyme converts to its inactive zymogen form
Correct Answer:
A. The tertiary structure denatures and active site geometry is lost
EXPLANATION
Above optimal temperature, increased thermal energy disrupts hydrogen bonds and hydrophobic interactions maintaining the 3D structure, causing denaturation and loss of catalytic activity.
Which prosthetic group is found in cytochrome c oxidase?
AHeme a and copper centers
BOnly nicotinamide adenine dinucleotide
CFlavin adenine dinucleotide exclusively
DIron-sulfur clusters only
Correct Answer:
A. Heme a and copper centers
EXPLANATION
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.
Which enzyme belongs to the transferase class (EC 2)?
AAspartate aminotransferase (AST)
BAmylase
CCatalase
DHexokinase
Correct Answer:
A. Aspartate aminotransferase (AST)
EXPLANATION
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.
The Ramachandran plot is used to validate which aspects of protein structure?
APhi (φ) and psi (ψ) dihedral angles of the backbone
BSide chain rotamer configurations
CDisulfide bond angles
DHydrogen bonding patterns
Correct Answer:
A. Phi (φ) and psi (ψ) dihedral angles of the backbone
EXPLANATION
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.
Which amino acid's deficiency in the diet can lead to kwashiorkor in children?
AMethionine
BLysine
CAll essential amino acids proportionally
DProline
Correct Answer:
C. All essential amino acids proportionally
EXPLANATION
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.
In competitive enzyme inhibition, which Michaelis-Menten parameter is affected?
AKm increases while Vmax remains unchanged
BVmax decreases while Km remains unchanged
CBoth Km and Vmax decrease proportionally
DNeither Km nor Vmax are affected
Correct Answer:
A. Km increases while Vmax remains unchanged
EXPLANATION
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.
