INI-CET 2024 — Biochemistry
10 Previous Year Questions with Answers & Explanations
Select the correct sequence of events in the cAMP signaling pathway.
Which of the following is not an amino acid-derived neurotransmitter?
Which is not a product of heme catabolism?
Which of the following minerals is needed for fertility?
Which nutrient deficiency directly impairs hydroxylation during collagen synthesis?
Congenital lactic acidosis is due to the defect of:
Which gene defect causes familial hypercholesterolemia?
In an infant presenting with doll-like facies, which enzyme is deficient in Von Gierke disease?
Which type of mutation can act as a suppressor to restore the wild-type phenotype in organisms carrying a mutant gene?
Which of the following is true about non-competitive inhibition?
INI-CET 2024 - Biochemistry INI-CET Practice Questions and MCQs
Question 1: Select the correct sequence of events in the cAMP signaling pathway.
- A. Adenylyl cyclase converts ATP to cAMP, which activates PKA. (Correct Answer)
- B. PKA is activated before cAMP is formed.
- C. Adenylyl cyclase activates PKA before producing cAMP.
- D. cAMP directly activates adenylyl cyclase to produce more cAMP.
Explanation: ***Adenylyl cyclase converts ATP to cAMP, which activates PKA.*** - **Adenylyl cyclase** is an enzyme that catalyzes the conversion of **ATP (adenosine triphosphate)** into **cyclic AMP (cAMP)**, a crucial second messenger. - Subsequently, **cAMP** binds to and activates **Protein Kinase A (PKA)**, which then phosphorylates various target proteins to mediate cellular responses. *PKA is activated before cAMP is formed.* - **cAMP formation** is a prerequisite for **PKA activation**; PKA cannot be activated independently before cAMP is produced. - The binding of **cAMP** to the regulatory subunits of **PKA** is what causes the dissociation and activation of its catalytic subunits. *Adenylyl cyclase activates PKA before producing cAMP.* - **Adenylyl cyclase's** sole function in this pathway is to synthesize **cAMP** from ATP; it does not directly activate PKA. - **PKA activation** is mediated by **cAMP**, not directly by adenylyl cyclase. *cAMP directly activates adenylyl cyclase to produce more cAMP.* - While **cAMP** is a critical messenger, it does not directly activate **adenylyl cyclase** to produce more of itself in a positive feedback loop. - **Adenylyl cyclase** is typically activated by **G-protein coupled receptors (GPCRs)** binding to their ligands, which then stimulate G proteins to activate adenylyl cyclase.
Question 2: Which of the following is not an amino acid-derived neurotransmitter?
- A. Dopamine
- B. GABA
- C. Serotonin
- D. Creatine (Correct Answer)
Explanation: ***Creatine*** - **Creatine** is an organic compound that helps supply energy to cells, primarily muscle cells, but it does **not** function as a neurotransmitter. - It is synthesized from **amino acids** like arginine, glycine, and methionine, but it is not itself an amino acid-derived neurotransmitter. *Dopamine* - **Dopamine** is a **catecholamine neurotransmitter** derived from the amino acid **tyrosine**. - It plays crucial roles in **motivation**, reward, motor control, and various other brain functions. *GABA* - **GABA** (gamma-aminobutyric acid) is the **primary inhibitory neurotransmitter** in the central nervous system, derived from the amino acid **glutamate**. - It works to reduce neuronal excitability throughout the nervous system. *Serotonin* - **Serotonin** (5-hydroxytryptamine or 5-HT) is a **monoamine neurotransmitter** derived from the amino acid **tryptophan**. - It regulates mood, appetite, sleep, and numerous other physiological processes.
Question 3: Which is not a product of heme catabolism?
- A. Aminolevulinic acid (Correct Answer)
- B. Ferrous ion
- C. Biliverdin
- D. Carbon monoxide
Explanation: ***Aminolevulinic acid*** - **Aminolevulinic acid (ALA)** is a precursor in the **heme biosynthetic pathway**, not a product of its degradation. - The formation of ALA is the **rate-limiting step** in heme synthesis, catalyzed by ALA synthase. *Ferrous ion* - During heme catabolism, the **iron atom (Fe2+)** is released from the porphyrin ring. - This **ferrous ion** is then recycled or stored, as it is a product of heme degradation. *Biliverdin* - **Biliverdin** is the first green-colored product formed when heme oxygenase cleaves the **porphyrin ring** of heme. - It is an intermediate in the conversion of heme to **bilirubin**, making it a direct product of heme catabolism. *Carbon monoxide* - The oxidative cleavage of the heme ring by **heme oxygenase** liberates one molecule of **carbon monoxide (CO)**. - This CO is an important signaling molecule and has vasodilatory effects, making it a product of heme degradation.
Question 4: Which of the following minerals is needed for fertility?
- A. Copper
- B. Iron
- C. Zinc (Correct Answer)
- D. Selenium
Explanation: ***Zinc*** - Zinc is crucial for **reproductive health** in both men and women, impacting **testosterone synthesis**, **spermatogenesis**, egg quality, and **hormone regulation**. - Essential for **gonadal development** and function in both sexes. - Deficiency leads to **hypogonadism**, reduced fertility, impaired sperm production, and increased risk of **miscarriage**. - Most commonly deficient mineral affecting fertility globally. *Selenium* - Selenium is also **essential for male fertility**, being a critical component of **glutathione peroxidase** in sperm mitochondria. - Required for **sperm motility**, morphology, and structural integrity of the sperm midpiece. - Deficiency can cause male infertility due to impaired sperm function. - However, zinc deficiency is more prevalent and has broader effects across both male and female reproductive systems. *Iron* - Iron is vital for **red blood cell formation** and preventing **anemia**. - Severe iron deficiency anemia can **impair ovulation** and indirectly affect fertility in women. - Not directly involved in reproductive processes at the cellular level like zinc. *Copper* - Essential for various enzymatic functions but not primarily associated with fertility. - **Excessive copper** can negatively impact fertility and cause hormonal imbalances. - Deficiency is rare and not a primary cause of infertility.
Question 5: Which nutrient deficiency directly impairs hydroxylation during collagen synthesis?
- A. A. Vitamin D
- B. B. Copper
- C. C. Vitamin E
- D. D. Vitamin C (Correct Answer)
Explanation: ***Vitamin C*** - **Vitamin C** (ascorbic acid) is a crucial **cofactor** for the enzymes **prolyl hydroxylase** and **lysyl hydroxylase**, which are essential for **collagen cross-linking and stability**. - Its deficiency leads to **scurvy**, characterized by weakened connective tissue, impaired wound healing, and fragile blood vessels due to **defective collagen synthesis**. *Vitamin D* - **Vitamin D** is primarily involved in **calcium and phosphate homeostasis**, which are vital for bone mineralization. - Its deficiency can lead to **rickets** in children and **osteomalacia** in adults, conditions of weakened bones, but not directly to collagen defects. *Copper* - **Copper** is a cofactor for **lysyl oxidase**, an enzyme that cross-links collagen and elastin, contributing to the tensile strength of connective tissues. - While copper deficiency can affect collagen structure, **Vitamin C deficiency** has a more direct and severe impact on the initial synthesis and hydroxylation steps of collagen, making it the primary answer for collagen defects. *Vitamin E* - **Vitamin E** is a fat-soluble antioxidant that protects cell membranes from **oxidative damage**. - Its deficiency is associated with neurological symptoms and hemolytic anemia but does not directly cause defects in **collagen synthesis or structure**.
Question 6: Congenital lactic acidosis is due to the defect of:
- A. Transaldolase
- B. Pyruvate dehydrogenase (Correct Answer)
- C. Alpha-ketoglutarate dehydrogenase
- D. Branched chain alpha-ketoacid dehydrogenase
Explanation: ***Pyruvate dehydrogenase*** - A defect in **pyruvate dehydrogenase (PDH)** is the most common cause of **congenital lactic acidosis** - PDH is a crucial enzyme complex that converts **pyruvate to acetyl-CoA**, linking glycolysis to the citric acid cycle - When PDH is deficient, **pyruvate accumulates** and is shunted to **lactate** via lactate dehydrogenase, causing persistent elevation of blood lactate levels - Clinical features include **neurological dysfunction, developmental delay, and metabolic acidosis** from birth or early infancy *Transaldolase* - **Transaldolase** is an enzyme in the **pentose phosphate pathway** - Its deficiency primarily affects **NADPH production and ribose-5-phosphate synthesis**, not lactate metabolism - Transaldolase deficiency causes hepatosplenomegaly and liver dysfunction, but is **not a direct cause of congenital lactic acidosis** *Alpha-ketoglutarate dehydrogenase* - **Alpha-ketoglutarate dehydrogenase** is part of the **citric acid cycle (TCA cycle)** - Its deficiency would impair energy production and lead to accumulation of **alpha-ketoglutarate**, not lactate - Defects cause **neurological dysfunction** but do not primarily present with **lactic acidosis** *Branched chain alpha-ketoacid dehydrogenase* - **Branched chain alpha-ketoacid dehydrogenase (BCKDH)** metabolizes **branched-chain amino acids** (leucine, isoleucine, valine) - Deficiency causes **maple syrup urine disease (MSUD)**, characterized by accumulation of **branched-chain keto acids** and their corresponding amino acids - Presents with characteristic maple syrup odor in urine, neurological symptoms, but **not lactic acidosis**
Question 7: Which gene defect causes familial hypercholesterolemia?
- A. LDL Receptor (Correct Answer)
- B. Apo E
- C. Apo CII
- D. Apo B48
Explanation: ***LDL Receptor*** - Familial hypercholesterolemia (FH) is primarily caused by mutations in the **LDL receptor (LDLR) gene**, which leads to impaired clearance of **low-density lipoprotein (LDL)** from the blood. - This defect results in significantly elevated levels of **LDL cholesterol** and an increased risk of premature cardiovascular disease. *Apo E* - Mutations in the **Apo E gene** are associated with **Type III hyperlipoproteinemia (dysbetalipoproteinemia)**, characterized by elevated **chylomicron remnants** and **VLDL remnants**. - This condition presents with xanthomas and premature atherosclerosis, but is distinct from the primary defect in FH. *Apo CII* - **Apo CII** is a cofactor for **lipoprotein lipase (LPL)**, an enzyme essential for the breakdown of **triglycerides** in chylomicrons and VLDL. - Deficiency in Apo CII or LPL causes **Type I hyperlipoproteinemia (familial chylomicronemia syndrome)**, leading to marked **hypertriglyceridemia**, not hypercholesterolemia. *Apo B48* - **Apo B48** is a structural component of **chylomicrons**, which are responsible for transporting dietary fats from the intestines. - It is not directly involved in the primary defect of **LDL clearing** that characterizes familial hypercholesterolemia.
Question 8: In an infant presenting with doll-like facies, which enzyme is deficient in Von Gierke disease?
- A. Fructose 1,6 bisphosphatase
- B. Debranching enzyme
- C. Glucose 6 phosphatase (Correct Answer)
- D. Phosphorylase
Explanation: ***Glucose 6 phosphatase*** - **Von Gierke disease (Type I glycogen storage disease)** is characterized by a deficiency of **glucose-6-phosphatase**, an enzyme crucial for the final step of gluconeogenesis and glycogenolysis. - This enzyme's deficiency leads to the inability to release free glucose from the liver and kidneys, resulting in **hypoglycemia**, hepatomegaly, and the characteristic **doll-like facies** due to fat deposits. *Fructose 1,6 bisphosphatase* - This enzyme is involved in **gluconeogenesis**, catalyzing the conversion of fructose-1,6-bisphosphate to fructose-6-phosphate. - **Fructose-1,6-bisphosphatase deficiency** is a distinct metabolic disorder causing hypoglycemia, lactic acidosis, and hepatomegaly, but it does not present with the characteristic features of Von Gierke disease. *Debranching enzyme* - A deficiency in the **debranching enzyme** (**amylo-1,6-glucosidase**) is characteristic of **Cori's disease (GSD III)**. - While it also causes hepatomegaly and hypoglycemia, it typically presents with milder symptoms and a different metabolic profile than Von Gierke disease. *Phosphorylase* - **Glycogen phosphorylase** deficiency is associated with **McArdle's disease (GSD V)** in muscle and **Hers' disease (GSD VI)** in the liver. - These conditions primarily cause muscle weakness and cramping (McArdle's) or mild hypoglycemia and hepatomegaly (Hers'), but not the severe hypoglycemia and characteristic findings of Von Gierke disease.
Question 9: Which type of mutation can act as a suppressor to restore the wild-type phenotype in organisms carrying a mutant gene?
- A. Frameshift mutation of coding gene
- B. Mutation of tRNA (Correct Answer)
- C. Deletion of mutant gene
- D. Addition of another normal gene
Explanation: ***Mutation of tRNA*** - A **tRNA suppressor mutation** can alter its anticodon, allowing it to recognize a **stop codon** (nonsense suppressor) or a missense codon, and insert an amino acid, thereby suppressing the original mutation. - This is a classic example of an **intergenic suppressor mutation** that acts at a different genetic locus from the original mutation. - These suppressors are particularly effective for **nonsense mutations** (premature stop codons) and certain missense mutations by correcting the decoding error during translation. *Frameshift mutation of coding gene* - A single frameshift mutation causes a shift in the **reading frame**, leading to a completely different protein sequence downstream and often a premature stop codon, which would worsen the phenotype. - While a **second compensating frameshift** mutation in the same gene could theoretically restore the reading frame (acting as an intragenic suppressor), this is context-dependent and less reliable than tRNA suppressors. - The question asks for mutations that "can act as a suppressor," and **tRNA mutations are the more universally recognized and reliable suppressor mechanism** in classical genetics. *Deletion of mutant gene* - **Deleting the mutant gene** removes the genetic information entirely but does not restore wild-type function; instead, it typically results in **loss of function** or complete absence of the protein. - This would lead to a **null phenotype** rather than restoration of wild-type phenotype, especially if the gene is essential. *Addition of another normal gene* - The **addition of another normal (wild-type) gene copy** provides a functional protein that can compensate for the mutant gene's deficiency. - While this can restore a wild-type phenotype, it represents **gene complementation** or gene therapy, not a true suppressor mutation that modifies the interpretation or expression of the existing mutant allele.
Question 10: Which of the following is true about non-competitive inhibition?
- A. Km increases, Vmax remains same
- B. Km decreases, Vmax increases
- C. Km increases, Vmax increases
- D. Km remains same, Vmax decreases (Correct Answer)
Explanation: ***Km remains same, Vmax decreases*** - In **non-competitive inhibition**, the inhibitor binds to an allosteric site on the enzyme, altering its conformation, thereby **reducing its catalytic efficiency**. - This binding does not affect the **enzyme's affinity for the substrate (Km remains the same)**, but it **reduces the maximum reaction rate (Vmax decreases)** because fewer enzyme molecules are able to perform catalysis per unit time. *Km increases, Vmax remains same* - This describes **competitive inhibition**, where the inhibitor competes with the substrate for the enzyme's active site. - While it **increases the apparent Km** (more substrate needed to reach half Vmax), **Vmax remains unchanged** as high substrate concentrations can overcome the inhibition. *Km decreases, Vmax increases* - This scenario would imply an activation rather than inhibition, where both enzyme affinity and catalytic efficiency are enhanced. - This is not characteristic of any standard **enzyme inhibition mechanism**. *Km increases, Vmax increases* - This combination is not observed in any typical **enzyme inhibition pattern**. - An increase in **Vmax** implies enhanced catalytic activity, while an increase in **Km** suggests reduced substrate affinity, which are contradictory effects for a single inhibitor.