Which of the following is true regarding enzyme specificity?
Answer: B
Enzyme specificity varies: absolute (one substrate only), group (substrates with similar functional groups), linkage (specific types of bonds), and stereochemical (stereoisomers). Specificity results from 3D active site structure and orientation of catalytic residues.
Q.22Hard
What is the physiological significance of the Cori cycle?
Answer: B
The Cori cycle (glucose-lactate cycle) allows muscles undergoing anaerobic glycolysis to produce lactate, which is transported to liver and converted back to glucose via gluconeogenesis, maintaining blood glucose homeostasis during intense exercise.
Q.23Hard
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.24Hard
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.25Hard
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.
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Q.26Hard
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.27Hard
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.28Hard
A patient with galactosemia has a deficiency in which enzyme?
Answer: B
Classic galactosemia results from deficiency of galactose-1-phosphate uridyltransferase (GALT), preventing conversion of galactose-1-phosphate to UDP-galactose. Leads to accumulation of galactose-1-phosphate and galactitol (in polyol pathway), causing cataracts, intellectual disability, and liver damage.
Q.29Hard
Pasteur effect refers to the inhibition of glycolysis by oxidative phosphorylation. In terms of ATP and citrate, which statement is correct?
Answer: D
PFK is inhibited by high ATP, citrate (signal of sufficient acetyl-CoA), and low pH. In aerobic conditions, efficient oxidative phosphorylation produces sufficient ATP, reducing need for glycolysis (Pasteur effect). Citrate is an allosteric inhibitor of PFK.
Q.30Hard
In glycolysis, which step is irreversible under physiological conditions and requires a different enzyme during gluconeogenesis?
Answer: B
The PFK reaction is highly exergonic (ΔG°' = -14.2 kJ/mol) and is essentially irreversible. During gluconeogenesis, fructose-1,6-bisphosphatase catalyzes the reverse reaction. This is a major control point in carbohydrate metabolism.
Q.31Hard
The Warburg effect describes increased glycolysis in cancer cells even in the presence of oxygen. Which enzyme is typically upregulated in cancer cells to support this?
Answer: C
In the Warburg effect, PFK-2 and PKM2 are upregulated. PFK-2 produces fructose-2,6-bisphosphate (a potent PFK-1 activator), while PKM2 (pyruvate kinase isoform) is upregulated in cancer cells. PKM2 also has non-glycolytic functions in cancer metabolism.
Q.32Hard
A 6-month-old infant develops hypoglycemia, hepatomegaly, and lactic acidosis after feeding. Genetic testing reveals glucose-6-phosphatase deficiency (Type I Glycogen Storage Disease). Why does this cause lactic acidosis?
Answer: A
G6Pase is the final enzyme in both gluconeogenesis and glycogenolysis. Its deficiency traps glucose-6-phosphate, forcing it through glycolysis and the pentose phosphate pathway, increasing pyruvate and lactate production.
Q.33Hard
A competitive athlete is found to have a deficiency in muscle phosphorylase (McArdle disease). During intense exercise, which metabolic consequence is PRIMARY?
Answer: A
Muscle phosphorylase deficiency prevents glycogen breakdown, depriving muscles of glucose-1-phosphate during exercise, causing severe energy crisis, fatigue, cramps, and myoglobinuria.
Q.34Hard
A patient with hemoglobin C disease (defect in β-globin) shows increased levels of fetal hemoglobin (HbF). Why might elevated HbF reduce hemolysis compared to HbS?
Answer: B
While this is primarily a hemoglobin question, HbF (with γ-chains instead of β-chains) does not polymerize like HbS or aggregate like HbC, reducing hemolysis and RBC sickling/crystallization.
Q.35Hard
A 3-year-old child presents with hepatomegaly, growth retardation, and elevated liver transaminases. Enzyme assay shows deficiency of lysosomal acid glucosidase (Pompe disease/GSD Type II). Which carbohydrate accumulates PRIMARILY in lysosomes?
Answer: C
In Pompe disease, acid α-glucosidase deficiency prevents lysosomal glycogen hydrolysis. Normally structured glycogen accumulates in lysosomes (unlike the abnormal structures seen in Type IV GSD), causing lysosomal dysfunction and cellular damage.
Q.36Hard
Which of the following correctly pairs a glycogen storage disease with its enzyme defect and primary organ affected?
Which monosaccharide cannot be directly metabolized by red blood cells due to lack of specific enzymes?
Answer: C
RBCs lack galactokinase and UDP-galactose-4-epimerase, making them unable to utilize galactose. They can metabolize glucose, fructose (via hexokinase), and mannose. This is relevant to understanding galactosemia pathophysiology where galactose accumulates in RBCs.
Q.38Hard
In the Pasteur effect, the inhibition of glycolysis by oxidative phosphorylation is primarily mediated by which molecule(s)?
Answer: D
The Pasteur effect describes how aerobic respiration inhibits glycolysis through multiple mechanisms: increased ATP/AMP ratio (inhibiting PFK-1), increased NADH/NAD⁺ ratio (inhibiting GAPDH), and increased citrate (allosteric inhibitor of PFK-1). This explains why cells prefer oxidative metabolism when oxygen is available.
Q.39Hard
A newborn presents with jaundice, hepatomegaly, and infantile cataracts. Laboratory findings show elevated galactose in blood and urine. Which enzyme deficiency is most likely?
Answer: C
Classical galactosemia results from galactose-1-phosphate uridylyltransferase (GALT) deficiency, causing accumulation of galactose-1-phosphate which is toxic to liver, brain, and lens. This leads to the classic triad of neonatal jaundice, hepatomegaly, and cataracts. Early dietary restriction of lactose prevents complications.
Q.40Hard
A 45-year-old male presents with chronic hyperuricemia and gout. Testing reveals elevated lactic acid and hepatomegaly. Which GSD is most likely?
Answer: B
Von Gierke disease causes hepatomegaly, lactic acidosis, and hyperuricemia due to glucose-6-phosphatase deficiency, leading to increased glycolysis and purine metabolism.
