Energy Production and Metabolism

Energy Production and Metabolism Indian Medical PG Question 1: Organ that can utilize glucose, fatty acids and ketone bodies is:

• A. Liver
• B. Brain
• C. Skeletal muscle (Correct Answer)
• D. RBC

Energy Production and Metabolism Explanation: ***Skeletal muscle*** - Skeletal muscle is highly adaptable and can utilize **glucose**, **fatty acids (FAs)**, and **ketone bodies** as fuel sources, especially during prolonged exercise or starvation. - Its metabolic flexibility allows it to switch between these substrates depending on their availability and the body's energy demands. *Liver* - The liver is central to metabolism but primarily **produces ketone bodies** from fatty acids rather than utilizing them as a major fuel source for its own energy needs. - While it uses glucose and FAs, its role in ketone body metabolism is largely synthetic. *Brain* - The brain preferentially uses **glucose** as its primary fuel. - During prolonged starvation, it can adapt to utilize **ketone bodies** as an alternative fuel source, but it does not significantly use fatty acids directly. *RBC* - Red blood cells (RBCs) lack mitochondria and therefore rely exclusively on **anaerobic glycolysis** for energy, metabolizing only **glucose**. - They cannot utilize fatty acids or ketone bodies.

Energy Production and Metabolism Indian Medical PG Question 2: Which of the following enzymes is not targeted by hypolipidemic drugs?

• A. HMG Co A reductase
• B. Lipoprotein lipase
• C. Acyl CoA, cholesterol acyl transferase 1
• D. Peripheral decarboxylase (Correct Answer)

Energy Production and Metabolism Explanation: ***Peripheral decarboxylase*** - **Peripheral decarboxylase** (also known as DOPA decarboxylase) is involved in the synthesis of dopamine from L-DOPA and is a target for drugs used in **Parkinson's disease**, not hypolipidemic drugs. - Its inhibition by drugs like **carbidopa** or **benserazide** prevents the peripheral conversion of L-DOPA to dopamine, increasing L-DOPA availability for the brain. *HMG Co A reductase* - **HMG-CoA reductase** is the rate-limiting enzyme in cholesterol biosynthesis and is the primary target for **statins** (e.g., atorvastatin, simvastatin). - Statins effectively lower **LDL cholesterol** by inhibiting this enzyme, reducing endogenous cholesterol production. *Lipoprotein lipase* - **Lipoprotein lipase (LPL)** activity can be enhanced by certain hypolipidemic drugs, such as **fibrates**, which activate **PPAR-α**. - Increased LPL activity leads to enhanced hydrolysis of **triglycerides** from VLDL and chylomicrons, reducing triglyceride levels in plasma. *Acyl CoA, cholesterol acyl transferase 1* - **Acyl-CoA:cholesterol acyltransferase (ACAT) inhibitors** were developed as potential hypolipidemic agents to prevent cholesterol esterification and absorption. - While not widely used clinically due to efficacy and side effect profiles, **ACAT1** is involved in cholesterol esterification in the intestine and liver, making it a target for reducing cholesterol absorption.

Energy Production and Metabolism Indian Medical PG Question 3: The electron transport chain is a series of redox reactions that result in ATP synthesis. Which of the following is a cytochrome complex IV inhibitor?

• A. Cyanide (Correct Answer)
• B. Carbon dioxide
• C. Oligomycin
• D. Ouabain

Energy Production and Metabolism Explanation: ***Cyanide*** - **Cyanide** is a potent inhibitor of **cytochrome c oxidase (Complex IV)** in the electron transport chain, binding to the ferric iron (Fe3+) in the heme group of the enzyme. - This binding prevents the transfer of electrons to **oxygen**, thereby halting cellular respiration and ATP production. *Carbon dioxide* - **Carbon dioxide** is a metabolic waste product and a component of the **bicarbonate buffer system**, but it does not directly inhibit cytochrome complex IV. - While high levels can affect physiological pH and enzyme function, its primary role is not as an electron transport chain inhibitor. *Oligomycin* - **Oligomycin** inhibits **ATP synthase (Complex V)** by binding to its Fo subunit, which blocks the flow of protons through the ATP synthase channel. - This prevents the synthesis of ATP but does not directly affect the electron transfer steps of cytochrome complex IV. *Ouabain* - **Ouabain** is a cardiac glycoside that inhibits the **Na+/K+-ATPase pump** in the cell membrane. - It does not have any direct inhibitory effect on the components of the electron transport chain, including cytochrome complex IV.

Energy Production and Metabolism Indian Medical PG Question 4: Which of the following processes does not occur in mitochondria?

• A. Fatty acid oxidation
• B. Electron transport chain
• C. Glycogenolysis (Correct Answer)
• D. Citric acid cycle (Kreb's cycle)

Energy Production and Metabolism Explanation: ***Glycogenolysis*** - **Glycogenolysis** is the breakdown of **glycogen** into glucose, which primarily occurs in the **cytosol** of cells, mainly in the liver and muscles. - This process is crucial for maintaining blood glucose levels and providing energy during periods of fasting or increased demand, and it does not take place within the mitochondria. *Fatty acid oxidation* - **Fatty acid oxidation**, also known as beta-oxidation, is a mitochondrial process that breaks down fatty acids into **acetyl-CoA** for energy production. - This occurs extensively within the mitochondrial matrix, producing ATP. *Electron transport chain* - The **electron transport chain** is located in the **inner mitochondrial membrane** and is the final stage of aerobic respiration, producing the majority of ATP. - It involves a series of protein complexes that transfer electrons to oxygen, creating a proton gradient for ATP synthesis. *Citric acid cycle (Kreb's cycle)* - The **citric acid cycle**, or **Krebs cycle**, is a central metabolic pathway that occurs in the **mitochondrial matrix**. - It oxidizes acetyl-CoA, derived from carbohydrates, fats, and proteins, to produce ATP, NADH, and FADH2.

Energy Production and Metabolism Indian Medical PG Question 5: Enzyme activated by decrease in Insulin: glucagon ratio:

• A. PFK
• B. Glucose 6 phosphatase (Correct Answer)
• C. Glucokinase
• D. Hexokinase

Energy Production and Metabolism Explanation: ***Glucose 6 phosphatase*** - A decreased **insulin:glucagon ratio** signifies a catabolic state, promoting glucose release into the blood. - **Glucose-6-phosphatase** is the key enzyme in **gluconeogenesis** and **glycogenolysis** in the liver, dephosphorylating **glucose-6-phosphate** to **free glucose**, which can then be exported from the liver. *PFK* - **Phosphofructokinase (PFK)** is a key regulatory enzyme in **glycolysis**, which is inhibited in a state of low insulin:glucagon ratio. - Its activity would decrease, not increase, to reduce glucose utilization. *Glucokinase* - **Glucokinase** phosphorylates glucose to **glucose-6-phosphate** in the liver, trapping it for metabolism; its activity is increased by high insulin levels. - In a low insulin:glucagon ratio, its activity would be reduced to conserve glucose. *Hexokinase* - **Hexokinase** phosphorylates glucose in most peripheral tissues but has a lower Km for glucose than glucokinase, becoming saturated even at low glucose concentrations. - Its activity is not primarily regulated by the insulin:glucagon ratio; it is generally involved in glucose uptake for cellular energy needs.

Energy Production and Metabolism Indian Medical PG Question 6: Which of the following is not a substrate for glucose formation?

• A. Lactate
• B. Glycerol
• C. Alanine
• D. Acetyl coenzyme A (Correct Answer)

Energy Production and Metabolism Explanation: ***Acetyl coenzyme A*** - **Acetyl CoA** cannot be converted to glucose because the two carbons from the acetyl group are lost as carbon dioxide in the **Krebs cycle**, making a net synthesis of glucose impossible. - The irreversible nature of the **pyruvate dehydrogenase complex** prevents the conversion of Acetyl CoA back to **pyruvate**, which is a crucial step for gluconeogenesis. *Lactate* - **Lactate** is a major substrate for gluconeogenesis, particularly during exercise and fasting, via the **Cori cycle**. - **Lactate dehydrogenase** converts lactate to **pyruvate**, which can then enter the gluconeogenic pathway. *Glycerol* - **Glycerol**, derived from triglyceride breakdown, enters gluconeogenesis by being converted to **glycerol-3-phosphate** and then to **dihydroxyacetone phosphate (DHAP)**. - DHAP is an intermediate in glycolysis and gluconeogenesis, allowing for its conversion to glucose. *Alanine* - **Alanine** is a **glucogenic amino acid** that can be transaminated to **pyruvate**. - **Pyruvate** can then proceed through the gluconeogenic pathway to synthesize glucose, especially during prolonged fasting.

Energy Production and Metabolism Indian Medical PG Question 7: Increased BMR is associated with -

• A. Increased body fat store
• B. Increased glycolysis (Correct Answer)
• C. Increased lipogenesis
• D. Increased gluconeogenesis

Energy Production and Metabolism Explanation: ***Increased glycolysis*** - An increased **basal metabolic rate (BMR)** signifies higher energy expenditure at rest, which reflects increased cellular metabolic activity and ATP consumption. - Among the given options, **increased glycolysis** is most consistent with increased BMR, as glycolysis is the primary pathway for ATP generation from glucose, and cells with higher metabolic rates require increased energy production. - Conditions that increase BMR (such as hyperthyroidism, fever, and increased muscle mass) are typically accompanied by **increased glycolytic activity** to meet higher energy demands. *Incorrect: Increased body fat store* - **Increased body fat** is generally associated with a *lower* BMR per unit of body weight, as adipose tissue is metabolically less active than lean tissue (especially muscle). - Higher body fat percentage reflects **energy storage**, not increased energy expenditure, and does not contribute to elevated BMR. *Incorrect: Increased lipogenesis* - **Lipogenesis** (synthesis of fatty acids and triglycerides) is an anabolic process that occurs during states of **energy surplus** for fat storage. - This process represents energy **storage** rather than energy **expenditure**, and is inversely related to BMR - it increases when energy intake exceeds expenditure. *Incorrect: Increased gluconeogenesis* - **Gluconeogenesis** (synthesis of glucose from non-carbohydrate sources) is primarily active during **fasting, starvation, or prolonged exercise** when glucose availability is low. - While gluconeogenesis is energy-consuming, it is characteristic of catabolic states with low energy availability, not the increased metabolic activity associated with elevated BMR. - In conditions that increase BMR (like hyperthyroidism), glucose is typically utilized via glycolysis rather than synthesized via gluconeogenesis.

Energy Production and Metabolism Indian Medical PG Question 8: Decreased basal metabolic rate is seen in:

• A. Postprandial state
• B. Sedentary lifestyle leading to muscle mass loss
• C. Aging process
• D. Hypothyroidism (Correct Answer)

Energy Production and Metabolism Explanation: ***Hypothyroidism*** - **Thyroid hormones** (T3 and T4) are the primary regulators of basal metabolic rate (BMR) - **Hypothyroidism** results in decreased production of thyroid hormones, leading to a **significant reduction in BMR** - Clinical manifestations include **weight gain, fatigue, cold intolerance, bradycardia**, and decreased oxygen consumption - This is the **classic pathological condition** associated with decreased BMR in physiology *Sedentary lifestyle leading to muscle mass loss* - While muscle loss does reduce BMR (as muscle is more metabolically active than fat), this is a **chronic lifestyle effect** rather than a primary physiological condition - The decrease in BMR is **gradual and less pronounced** compared to hypothyroidism - Not the primary answer for decreased BMR in medical examinations *Aging process* - Aging does lead to decreased BMR due to **loss of lean muscle mass** and hormonal changes - However, this is a **physiological process** rather than a pathological condition - The decrease is **gradual over decades** and less clinically significant than hypothyroidism *Postprandial state* - The postprandial state (after eating) causes an **increase in metabolic rate** due to the **thermic effect of food (TEF)** - Energy is required for digestion, absorption, and storage of nutrients - This represents a **temporary increase**, not a decrease, in metabolic rate

Energy Production and Metabolism Indian Medical PG Question 9: An infant presents with failure to thrive and recurrent vomiting episodes. Sepsis screen was negative, blood urea is elevated and urine shows high orotic acid level. Which of the following enzyme is absent?

• A. 1
• B. 2
• C. 3 (Correct Answer)
• D. 4

Energy Production and Metabolism Explanation: ***Correct Option 3 (Ornithine Transcarbamylase)*** - The presence of **elevated urine orotic acid** is pathognomonic for **ornithine transcarbamylase (OTC) deficiency**. - This occurs because carbamoyl phosphate accumulates in mitochondria when it cannot be converted to citrulline, then leaks into the cytoplasm where it enters the **pyrimidine synthesis pathway**, producing orotic acid. - The elevated blood urea indicates hyperammonemia, and the clinical picture of failure to thrive with recurrent vomiting is classic for OTC deficiency. - **OTC deficiency is the only urea cycle disorder that causes orotic aciduria** because the block occurs after carbamoyl phosphate formation but before citrulline synthesis. *Incorrect Option 1 (N-acetylglutamate synthase)* - N-acetylglutamate synthase produces N-acetylglutamate, which is an **allosteric activator of carbamoyl phosphate synthetase I (CPS1)**. - Deficiency here would impair CPS1 activity, leading to **decreased carbamoyl phosphate formation**. - This would cause hyperammonemia but **not orotic aciduria**, as insufficient carbamoyl phosphate would be available to shunt into pyrimidine synthesis. *Incorrect Option 2 (Carbamoyl Phosphate Synthetase I)* - CPS1 deficiency prevents formation of carbamoyl phosphate from ammonia and CO₂. - This causes severe hyperammonemia with very low or absent carbamoyl phosphate levels. - **No orotic aciduria occurs** because the substrate (carbamoyl phosphate) needed for pyrimidine synthesis is not produced. *Incorrect Option 4 (Downstream urea cycle enzymes)* - Later enzymes in the cycle include argininosuccinate synthetase and argininosuccinate lyase. - Deficiencies cause accumulation of citrulline or argininosuccinate respectively, with hyperammonemia. - **No orotic aciduria occurs** because carbamoyl phosphate is successfully converted to citrulline and doesn't accumulate to shunt into pyrimidine synthesis.

Energy Production and Metabolism Indian Medical PG Question 10: In G6PD deficiency, which enzyme's function is MOST directly impaired due to decreased NADPH availability, leading to reduced protection against oxidative stress?

• A. Catalase
• B. Pyruvate kinase
• C. Superoxide dismutase
• D. Glutathione reductase (Correct Answer)

Energy Production and Metabolism Explanation: ***Glutathione reductase*** - **G6PD deficiency** impairs the production of **NADPH** through the pentose phosphate pathway - **Glutathione reductase** is NADPH-dependent and reduces oxidized glutathione (GSSG) back to reduced glutathione (GSH) - Without adequate NADPH, glutathione reductase cannot maintain sufficient **GSH levels**, which is the primary antioxidant protecting RBCs from oxidative damage - This explains why G6PD deficiency leads to **hemolysis** when exposed to oxidative stressors (antimalarials, sulfonamides, fava beans) *Catalase* - **Catalase** decomposes hydrogen peroxide to water and oxygen, protecting cells from oxidative damage - While important for antioxidant defense, catalase does **not require NADPH** for its function - Its activity is not directly impaired by decreased NADPH in G6PD deficiency *Pyruvate kinase* - **Pyruvate kinase** catalyzes the final step of **glycolysis**, producing ATP - Its function is **completely independent** of NADPH levels - Pyruvate kinase deficiency causes a separate hemolytic anemia unrelated to oxidative stress or G6PD deficiency *Superoxide dismutase* - **Superoxide dismutase (SOD)** converts superoxide radicals to hydrogen peroxide and oxygen - SOD functions **independently of NADPH** and uses metal cofactors (Cu/Zn or Mn) - While part of antioxidant defense, it is not directly affected by G6PD deficiency

Energy Production and Metabolism Flashcard 1 of 10

Question: _____ is an inhibitor of ATP synthase of the ETC

Answer: Oligomycin

Energy Production and Metabolism Flashcard 2 of 10

Question: When insulin acts on muscle tissue, it causes _____ glycogen storage and increased protein synthesis

Answer: increased

Energy Production and Metabolism Flashcard 3 of 10

Question: Aerobic metabolism of glucose produces _____ net ATP via the glycerol-3-phosphate shuttle (muscle)

Answer: 30

Energy Production and Metabolism Flashcard 4 of 10

Question: Pyruvate dehydrogenase is regulated via positive feedback by _____, Ca2+, and increased NAD+/NADH ratio

Answer: ADP

Energy Production and Metabolism Flashcard 5 of 10

Question: Lactic acidosis type _____ is due to anaerobic metabolism secondary to poor tissue perfusion.

Answer: A

Energy Production and Metabolism Flashcard 6 of 10

Question: In the ETC, protons travel down their gradient through the _____ pump, generating ATP and heat

Answer: F0/F1 ATP Synthase

Energy Production and Metabolism Flashcard 7 of 10

Question: Conversion of pyruvate into acetyl CoA releases a molecule of _____ and NADH

Answer: CO2

Energy Production and Metabolism Flashcard 8 of 10

Question: _____ is combined with oxaloacetate via the enzyme citrate synthase, forming citrate

Answer: Acetyl-CoA

Energy Production and Metabolism Flashcard 9 of 10

Question: Isocitrate is converted to _____ via the enzyme isocitrate dehydrogenase

Answer: -ketoglutarate

Energy Production and Metabolism Flashcard 10 of 10

Question: Propionyl-CoA is converted to _____ via the enzyme propionyl-CoA carboxylase

Answer: methylmalonyl-CoA