Biochemistry
10 questionsWhat metabolic changes occur during overnight fasting?
Which of the following best describes the difference between glucokinase and hexokinase?
Which one of the following statements concerning gluconeogenesis is correct?
Which of the following metabolites is involved in glycogenolysis, glycolysis and gluconeogenesis ?
What is the primary biochemical defect in alkaptonuria?
Which cofactor is primarily associated with the activity of glutamate dehydrogenase?
Carnitine is synthesised from -
Selenocysteine is associated with ?
Taurine is biosynthesized from which amino acid?
Enzyme deficient in Type I Hyperlipidemia?
NEET-PG 2012 - Biochemistry NEET-PG Practice Questions and MCQs
Question 391: What metabolic changes occur during overnight fasting?
- A. Blood glucose decreases slightly
- B. Fat breakdown increases
- C. Glucose production increases (Correct Answer)
- D. Ketone levels rise slightly
Explanation: ***Glucose production increases*** - During overnight fasting (typically 8-12 hours), the body's **primary metabolic priority** is to maintain **blood glucose homeostasis** to fuel the brain and other glucose-dependent tissues. - As **hepatic glycogen stores** become depleted, the liver significantly increases **gluconeogenesis** (glucose production from non-carbohydrate sources like amino acids, lactate, and glycerol) to supply glucose. - This represents the **most critical metabolic adaptation** during overnight fasting, as the brain requires a constant glucose supply (~120g/day) and cannot initially use alternative fuels. *Blood glucose decreases slightly* - During a normal overnight fast, blood glucose levels remain **relatively stable** (70-100 mg/dL) due to compensatory mechanisms. - The body's homeostatic mechanisms (increased glucose production, decreased glucose utilization by muscles) prevent any significant drop in blood glucose. - A significant decrease would indicate **hypoglycemia**, which is prevented by the metabolic changes described above. *Fat breakdown increases* - **Lipolysis** (fat breakdown) does indeed increase significantly during overnight fasting to provide **fatty acids** as an alternative fuel source for skeletal muscle, cardiac muscle, and liver. - This is an important metabolic change, but is **secondary to glucose production** in terms of priority, as it serves to spare glucose for the brain rather than directly maintaining glucose levels. - Increased fatty acid oxidation provides acetyl-CoA for **ketone body synthesis** and reduces glucose consumption by peripheral tissues (glucose-sparing effect). *Ketone levels rise slightly* - **Ketone body production** (acetoacetate, β-hydroxybutyrate) does begin to increase as fasting progresses beyond 8-12 hours. - However, during an *overnight* fast, ketone levels rise only **modestly** (typically <1 mM); clinically significant ketosis develops during **prolonged fasting** (24-72 hours), when ketone bodies become a major fuel source for the brain. - The overnight period represents the **transition phase** where glucose production remains the dominant metabolic response.
Question 392: Which of the following best describes the difference between glucokinase and hexokinase?
- A. Glucokinase has higher Km for glucose compared to hexokinase. (Correct Answer)
- B. Glucokinase is not inhibited by glucose-6-phosphate unlike hexokinase.
- C. Glucokinase has a low affinity for glucose.
- D. Glucokinase activity increases with glucose concentration while hexokinase remains saturated.
Explanation: ***Glucokinase has higher Km for glucose compared to hexokinase*** - **Glucokinase** has a **Km of ~10 mM** for glucose, while **hexokinase** has a **Km of ~0.1 mM**, making glucokinase's Km approximately **100-fold higher** - This **high Km** is the fundamental biochemical parameter that defines glucokinase's unique role as a **glucose sensor** in liver and pancreatic β-cells - The high Km means glucokinase activity is **proportional to blood glucose concentration** in the physiological range (5-15 mM), allowing it to regulate glucose metabolism in response to feeding - This is the **most precise biochemical descriptor** of the difference, from which other functional characteristics derive *Glucokinase has a low affinity for glucose* - While this statement is **correct** (high Km = low affinity), it is a **qualitative description** of what Km quantifies - Option stating "higher Km" is more specific and biochemically precise than simply stating "low affinity" *Glucokinase is not inhibited by glucose-6-phosphate unlike hexokinase* - This is a **correct and important regulatory difference** - **Hexokinase** is allosterically inhibited by its product **glucose-6-phosphate**, providing feedback regulation to prevent excessive glucose phosphorylation when cellular needs are met - **Glucokinase** lacks this product inhibition, allowing the liver to continue glucose uptake and storage even when G6P levels are high after meals - However, this describes a regulatory difference rather than the fundamental kinetic parameter *Glucokinase activity increases with glucose concentration while hexokinase remains saturated* - This statement is **correct** and describes the **functional consequence** of the different Km values - **Hexokinase** with its low Km (~0.1 mM) is saturated at normal blood glucose levels (5 mM), operating at Vmax - **Glucokinase** with its high Km (~10 mM) shows increasing activity as glucose rises from 5 to 15 mM postprandially - This is a physiological consequence rather than the fundamental biochemical parameter
Question 393: Which one of the following statements concerning gluconeogenesis is correct?
- A. It occurs primarily in the liver.
- B. It is stimulated by elevated levels of acetyl CoA.
- C. It is important in maintaining blood glucose during the normal overnight fast. (Correct Answer)
- D. It is primarily inhibited by insulin.
Explanation: ***It is important in maintaining blood glucose during the normal overnight fast.*** - **This is the BEST answer** as it emphasizes the **primary physiological role** of gluconeogenesis in human metabolism. - During the **overnight fast** (8-12 hours), hepatic glycogen stores become depleted, making gluconeogenesis the **critical mechanism** to maintain blood glucose for glucose-dependent tissues like the **brain** (requires ~120g glucose/day) and **red blood cells**. - Without gluconeogenesis, blood glucose would drop dangerously during fasting, leading to hypoglycemia and neurological dysfunction. *It occurs primarily in the liver.* - This statement is **technically correct** - the liver accounts for approximately **90%** of total gluconeogenesis under normal conditions. - However, the **kidney cortex** also contributes significantly (10% normally, up to 40% during prolonged fasting), and the **intestine** plays a minor role. - While true, this is more of a **anatomical fact** rather than highlighting the critical physiological importance of the pathway, making it a less comprehensive answer than Option 1. *It is stimulated by elevated levels of acetyl CoA.* - This statement is **biochemically correct** - **Acetyl-CoA** is an important **allosteric activator** of **pyruvate carboxylase**, the first committed enzyme of gluconeogenesis. - However, this represents just **one regulatory mechanism** at the enzymatic level, not the overall physiological significance. - Primary regulation occurs through **hormones** (glucagon, cortisol, epinephrine) that coordinate the entire pathway, making this a narrower answer than Option 1. *It is primarily inhibited by insulin.* - This statement is also **correct** - **Insulin** is the primary hormonal **inhibitor** of gluconeogenesis. - Insulin suppresses gluconeogenesis by inhibiting key enzymes (PEPCK, glucose-6-phosphatase) and decreasing transcription of gluconeogenic genes. - However, this describes **inhibition** rather than the positive physiological role, making it less representative of gluconeogenesis's essential function than Option 1. **Note:** All four statements are technically correct, but Option 1 best captures the **essential physiological importance** of gluconeogenesis in human metabolism, which is why it is the preferred answer for this question.
Question 394: Which of the following metabolites is involved in glycogenolysis, glycolysis and gluconeogenesis ?
- A. Glucose-6-phosphate (Correct Answer)
- B. Uridine diphosphoglucose
- C. Fructose-6-phosphate
- D. Galactose-1-phosphate
Explanation: ***Glucose-6-phosphate*** - In **glycogenolysis**, **glycogen phosphorylase** breaks down glycogen into **glucose-1-phosphate**, which is then converted into **glucose-6-phosphate** by **phosphoglucomutase**. - In **glycolysis**, **glucose-6-phosphate** is isomerized to **fructose-6-phosphate** by **phosphoglucose isomerase**, committing it to the glycolytic pathway. - In **gluconeogenesis**, **glucose-6-phosphate** is the final product formed from other precursors; it can then be dephosphorylated to free glucose by **glucose-6-phosphatase**. *Galactose-1-phosphate* - This is an intermediate specifically in **galactose metabolism**, not directly involved in the central common pathways of glycogenolysis, glycolysis, or gluconeogenesis. - It is converted to **glucose-1-phosphate** via the **Leloir pathway** (involving **galactose-1-phosphate uridylyltransferase**), which can then enter glycogen metabolism. *Uridine diphosphoglucose* - **UDP-glucose** is crucial for **glycogen synthesis** (**glycogenesis**), serving as the activated glucose donor. - It is not directly a metabolite in the catabolic process of glycogenolysis, nor is it a direct intermediate in glycolysis or gluconeogenesis. *Fructose-6-phosphate* - **Fructose-6-phosphate** is a key intermediate in **glycolysis** and **gluconeogenesis**, specifically downstream from **glucose-6-phosphate**. - However, it is not directly produced from glycogenolysis; **glucose-6-phosphate** is the direct link between glycogenolysis and glycolysis.
Question 395: What is the primary biochemical defect in alkaptonuria?
- A. FeCl3 test is negative
- B. Urine turns black immediately upon voiding
- C. Benedict's test is diagnostic for alkaptonuria
- D. Deficiency of homogentisate 1,2-dioxygenase (Correct Answer)
Explanation: ***Deficiency of homogentisate 1,2-dioxygenase*** - **Alkaptonuria** is an autosomal recessive disorder caused by the deficiency of **homogentisate 1,2-dioxygenase**, an enzyme in the **tyrosine degradation pathway**. - This deficiency leads to the accumulation of **homogentisic acid** in the body, which is excreted in urine and deposited in connective tissues. *Urine turns black immediately upon voiding* - While urine in alkaptonuria does **turn black**, it typically darkens upon **standing** and exposure to air, not immediately upon voiding. - The darkening is due to the oxidation of accumulated **homogentisic acid**. *FeCl3 test is negative* - The **ferric chloride (FeCl3) test** typically yields a **positive result** (transient green color) in the presence of homogentisic acid in the urine. - Therefore, a negative result would argue against a diagnosis of alkaptonuria. *Benedict's test is diagnostic for alkaptonuria* - **Benedict's test** is used to detect reducing sugars like glucose in urine and would not be used to diagnose alkaptonuria. - A positive Benedict's test in alkaptonuria is due to the reducing properties of homogentisic acid, but it is not specific or diagnostic.
Question 396: Which cofactor is primarily associated with the activity of glutamate dehydrogenase?
- A. NAD+ (Correct Answer)
- B. FAD
- C. FMN
- D. FADH2
Explanation: ***NAD+*** - Glutamate dehydrogenase catalyzes the oxidative deamination of **glutamate** to **α-ketoglutarate** and ammonia, and this reaction primarily uses **NAD+** as an electron acceptor. - In some organisms and contexts, it can also use **NADP+**, but **NAD+** is the more common and significant cofactor for its catabolic role. *FAD* - **FAD (flavin adenine dinucleotide)** is typically associated with **flavoproteins** and enzymes involved in oxidation-reduction reactions, such as those in the **electron transport chain** and the **Krebs cycle**. - Enzymes like **succinate dehydrogenase** use FAD, not glutamate dehydrogenase. *FMN* - **FMN (flavin mononucleotide)** is another flavin coenzyme, similar to FAD, and is found in various **flavoproteins** and enzymes of the **electron transport chain**, such as **NADH dehydrogenase (Complex I)**. - It does not serve as a primary cofactor for **glutamate dehydrogenase** activity. *FADH2* - **FADH2** is the reduced form of **FAD**, carrying high-energy electrons to the **electron transport chain** for ATP synthesis. - It's a product or reactant of various metabolic pathways, but not a direct cofactor for **glutamate dehydrogenase**.
Question 397: Carnitine is synthesised from -
- A. Lysine (Correct Answer)
- B. Histidine
- C. Choline
- D. Arginine
Explanation: ***Lysine*** - **Carnitine** is synthesized in the liver and kidneys from the amino acids **lysine** and methionine. - **Lysine provides the essential carbon backbone** for carnitine synthesis (trimethyllysine is the actual precursor formed from protein-bound lysine residues). - Methionine contributes methyl groups via S-adenosylmethionine (SAM), but lysine is the primary structural precursor. *Arginine* - **Arginine** is a precursor for **nitric oxide**, urea, and creatine, but not a direct precursor for carnitine synthesis. - While arginine is an amino acid, its metabolic pathways are distinct from those involved in carnitine formation. *Histidine* - **Histidine** is a precursor for **histamine** and contributes to protein synthesis, but is not involved in carnitine biosynthesis. - Its metabolic fate differs significantly from the pathway leading to carnitine. *Choline* - **Choline** is a precursor for **acetylcholine** and phospholipids, but not directly for carnitine. - Although both choline and carnitine contain methyl groups, they have different biosynthetic origins.
Question 398: Selenocysteine is associated with ?
- A. Carbonic anhydrase
- B. Catalase
- C. Transferase
- D. Deiodinase (Correct Answer)
Explanation: ***Deiodinase*** - Selenocysteine is a critical component of **iodothyronine deiodinases**, a family of enzymes that regulate **thyroid hormone metabolism**. - These enzymes catalyze the removal of iodine from thyroid hormones, converting **thyroxine (T4)** into the more active **triiodothyronine (T3)** or inactive forms. *Carbonic anhydrase* - This enzyme contains **zinc** as its essential metal cofactor and is involved in the interconversion of **carbon dioxide** and **bicarbonate**. - Its primary role is in pH regulation and CO2 transport, without any direct association with selenocysteine. *Catalase* - Catalase is an enzyme primarily found in **peroxisomes** and contains **iron-porphyrin** groups as its prosthetic group. - Its function is to convert **hydrogen peroxide** into water and oxygen, protecting cells from oxidative damage. *Transferase* - Transferases are a broad class of enzymes that catalyze the transfer of **functional groups** (e.g., methyl, glucose) from one molecule to another. - While essential for many metabolic processes, there is no inherent association of the general class of transferases with selenocysteine.
Question 399: Taurine is biosynthesized from which amino acid?
- A. Cysteine (Correct Answer)
- B. Valine
- C. Arginine
- D. Leucine
Explanation: ***Cysteine*** - **Taurine** is primarily synthesized from the amino acid **cysteine** through a pathway involving **cysteine sulfinic acid** and **hypotaurine**. - This pathway utilizes enzymes like **cysteine dioxygenase** and **cysteine sulfinic acid decarboxylase**. - The biosynthetic pathway: Cysteine → Cysteine sulfinic acid → Hypotaurine → Taurine. *Arginine* - **Arginine** is a precursor for **nitric oxide**, **urea**, and **creatine**, not taurine. - It is involved in various metabolic pathways, including the **urea cycle** and protein synthesis. *Valine* - **Valine** is a **branched-chain amino acid (BCAA)** involved in protein synthesis and energy production. - It is not a direct precursor for taurine biosynthesis. *Leucine* - **Leucine** is also a **branched-chain amino acid (BCAA)** crucial for protein synthesis and muscle metabolism. - It does not participate in the synthesis of taurine.
Question 400: Enzyme deficient in Type I Hyperlipidemia?
- A. HMG CoA reductase
- B. Lipoprotein lipase (Correct Answer)
- C. Peroxidase
- D. Cholesterol acyl transferase
Explanation: ***Lipoprotein lipase*** - **Type I hyperlipidemia**, also known as **familial hyperchylomicronemia**, is characterized by a deficiency in **lipoprotein lipase (LPL)**. - LPL is crucial for hydrolyzing triglycerides in **chylomicrons** and **VLDLs** into fatty acids and glycerol, allowing their uptake by tissues. *HMG CoA reductase* - This enzyme is involved in the **rate-limiting step of cholesterol synthesis** in the liver. - While it plays a role in lipid metabolism, its deficiency is not characteristic of **Type I hyperlipidemia**. *Peroxidase* - **Peroxidase** is an enzyme involved in various oxidative reactions, including the breakdown of **hydrogen peroxide**. - It is not directly involved in the metabolism of **chylomicrons** or **triglycerides**, and its deficiency is unrelated to hyperlipidemia. *Cholesterol acyl transferase* - This enzyme, often referring to **lecithin-cholesterol acyltransferase (LCAT)** or **acyl-CoA:cholesterol acyltransferase (ACAT)**, is involved in **cholesterol esterification**. - While important for cholesterol transport and storage, its deficiency is not the primary cause of **Type I hyperlipidemia**, which is marked by severe **chylomicronemia**.