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.42Medium
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.43Medium
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.44Medium
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.45Medium
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.
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Q.46Medium
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.47Medium
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.
Q.48Medium
The Maillard reaction occurs between reducing sugars and amino acids, leading to the formation of:
Answer: B
The Maillard reaction is a non-enzymatic browning reaction between carbonyl groups of reducing sugars and amino groups of proteins/amino acids, producing AGEs. This is significant in glycemic control and diabetes complications.
Q.49Medium
In hyperglycemia, excess glucose undergoes polyol pathway conversion. Which enzyme catalyzes the first step?
Answer: A
Aldose reductase catalyzes the reduction of glucose to sorbitol using NADPH. Sorbitol dehydrogenase catalyzes the oxidation of sorbitol to fructose in the second step of the polyol pathway.
Q.50Medium
Which of the following is a branching enzyme in glycogen synthesis?
Answer: B
Glycogen branching enzyme transfers segments of 6-7 glucose residues from the outer chains to create α(1→6) branch points, increasing solubility and accessibility for glycogen phosphorylase.
Q.51Medium
Cori cycle involves recycling of lactate from muscles to liver. Which enzyme is key in converting lactate back to glucose in the liver?
Answer: B
Pyruvate carboxylase catalyzes the first committed step of gluconeogenesis, converting pyruvate to oxaloacetate. Lactate is converted to pyruvate by LDH, then to glucose via gluconeogenesis.
Q.52Medium
The anomeric carbon in glucose is:
Answer: A
The anomeric carbon (C1 in glucose) is the carbon that becomes chiral when the open-chain form cyclizes to form the hemiacetal. This is the site of α/β isomerism.
Q.53Medium
HbA1c levels are used to monitor long-term glycemic control because glucose binds to hemoglobin through:
Answer: B
HbA1c is formed through non-enzymatic glycation of hemoglobin by glucose, reflecting average blood glucose over 2-3 months (RBC lifespan). Normal HbA1c <5.7%, diagnostic for diabetes ≥6.5%.
Q.54Medium
Cellulose differs from starch in that it contains:
Answer: B
Cellulose contains β(1→4) glycosidic bonds linking glucose units, making it indigestible to humans (no cellulase enzyme). Starch contains α(1→4) bonds, which are digestible.
Q.55Medium
In the pentose phosphate pathway, the oxidative phase primarily generates:
Answer: B
The oxidative phase of PPP generates 2 NADPH per glucose-6-phosphate, essential for fatty acid synthesis, cholesterol synthesis, and antioxidant defense (glutathione reduction). This is particularly important in adipose tissue, liver, and RBCs.
Q.56Medium
In diabetes mellitus type 2, impaired fasting glucose (100-125 mg/dL) is associated with:
Answer: B
Type 2 diabetes is characterized by insulin resistance (defective glucose uptake and utilization) and progressive β-cell dysfunction. Impaired fasting glucose (100-125 mg/dL) is a prediabetic state. Fasting glucose ≥126 mg/dL on 2 occasions is diagnostic for diabetes.
Q.57Medium
Which of the following statements is TRUE about the Cori cycle?
Answer: B
The Cori cycle describes lactate produced in muscles during anaerobic glycolysis being transported to the liver where it is converted back to glucose via gluconeogenesis, which is then returned to muscles.
Q.58Medium
A patient presents with lactic acidosis after consuming alcohol. Which enzyme's inhibition by acetaldehyde is primarily responsible for this condition?
Answer: B
Ethanol metabolism consumes NAD+, reducing the NAD+/NADH ratio. This inhibits NAD+-dependent dehydrogenases including those in gluconeogenesis and lactate oxidation, leading to lactate accumulation.
Q.59Medium
Which monosaccharide is the primary carbon source for synthesis of heparin and other glycosaminoglycans?
Answer: C
Glucuronic acid, derived from glucose, is a key component of glycosaminoglycans like heparin, hyaluronic acid, and chondroitin sulfate, providing their acidic properties.
Q.60Medium
In the pentose phosphate pathway, which intermediate can directly feed into glycolysis as glyceraldehyde-3-phosphate?
Answer: C
Through the non-oxidative phase of the pentose phosphate pathway, sugars are rearranged to produce fructose-6-phosphate and glyceraldehyde-3-phosphate, which directly enter glycolysis.
