Metabolic Integration Practice Questions

Q: What happens to excess proteins in the body?

Converted to energy or stored as fat. ***Converted to energy or stored as fat*** - When protein intake exceeds the body's needs for structural and functional protein synthesis, excess **amino acids** are deaminated. - The carbon skeletons can then be converted into **glucose** (via gluconeogenesis) or **fatty acids** (which are stored as triglycerides). *Used to build structural proteins* - While proteins are essential for building structural components like **collagen** and **keratin**, the body has a specific need for this, and excess intake doesn't lead to endless structural growth. - Building structural proteins is a regulated process that depends on tissue repair and growth demands, not solely on protein availability. *Used to synthesize functional proteins* - Functional proteins, such as **enzymes** and **hormones**, are synthesized as needed to maintain specific metabolic processes and cellular functions. - The body doesn't synthesize an unlimited amount of these proteins just because there's an excess of amino acids; synthesis is regulated based on physiological demand. *Stored as amino acids in tissues* - The body has a very limited capacity to store free amino acids; there is no dedicated storage compartment akin to **glycogen** for carbohydrates or **triglycerides** for fats. - Instead, unused amino acids are rapidly catabolized rather than accumulated in significant quantities.

Q: Pyruvate can be a substrate of all except

Aspartate transaminase. ***Aspartate transaminase*** - **Aspartate transaminase (AST)** catalyzes the reversible transfer of an amino group from **aspartate** to **α-ketoglutarate**, producing **oxaloacetate** and **glutamate**. - **Pyruvate** is not a substrate for AST; its primary substrates are aspartate and α-ketoglutarate. *Lactate Dehydrogenase* - **Lactate dehydrogenase (LDH)** catalyzes the reversible conversion of **pyruvate** to **lactate** during anaerobic metabolism. - This reaction regenerates **NAD+** from **NADH**, crucial for continuing glycolysis in the absence of oxygen. - **Pyruvate** serves as a substrate for LDH. *Malic enzyme* - **Malic enzyme** catalyzes the oxidative decarboxylation of **malate** to **pyruvate**, generating **NADPH**. - In this reaction, **malate** (not pyruvate) is the substrate, and **pyruvate** is the product. - This is an essentially irreversible reaction under physiological conditions. *Alanine transaminase* - **Alanine transaminase (ALT)** catalyzes the reversible transfer of an amino group from **alanine** to **α-ketoglutarate**, producing **pyruvate** and **glutamate**. - In the reverse reaction, **pyruvate** serves as a substrate, accepting an amino group to form **alanine**.

Q: Autophagic vacuoles fuse with ?

Lysosomes. ***Lysosomes*** - **Autophagic vacuoles (autophagosomes)** are double-membraned vesicles that sequester cellular components destined for degradation. - The fusion of these autophagosomes with **lysosomes**, which contain hydrolytic enzymes, forms **autolysosomes**, where degradation occurs. *Golgi complex* - The **Golgi complex** is involved in modifying, sorting, and packaging proteins and lipids. - It does not directly participate in the final degradative step of autophagy. *ER* - The **endoplasmic reticulum (ER)** is involved in protein synthesis and lipid metabolism. - While it can be a source of membranes for autophagosome formation, it is not the destination for their fusion. *Mitochondria* - **Mitochondria** are primarily involved in energy production through cellular respiration. - They are often targets for **autophagy** (a process called mitophagy), but they do not fuse with autophagic vacuoles for degradation.

Q: In starvation, earliest to become depleted -

Carbohydrates. ***Carbohydrates*** - **Glycogen stores** (primarily liver and muscle glycogen) are the body's most readily accessible energy source and are depleted within hours of starvation. - The liver initially maintains blood glucose levels by breaking down **glycogen** before resorting to gluconeogenesis. *Proteins* - **Proteins** are conserved as much as possible during early starvation to preserve vital body functions. - Significant **protein breakdown** for energy (gluconeogenesis) typically occurs in later stages of prolonged starvation, after carbohydrate and most fat reserves are diminished. *Fats* - **Fats** (in the form of triglycerides stored in adipose tissue) become the primary energy source after glycogen stores are depleted. - While they provide a large energy reserve, their mobilization and utilization as fuel take longer than glycogen, and they are not the **earliest to be depleted**. *None of the options* - This option is incorrect because **carbohydrates** are indeed the earliest to be depleted during starvation.

Q: Sirtuins are associated with?

Longevity of life span. ***Longevity of life span*** - Sirtuins are a family of **NAD+-dependent deacetylases** that play a crucial role in regulating cellular processes related to **aging** and metabolism. - Activation of sirtuins, particularly **SIRT1**, has been shown in various model organisms like yeast, worms, and flies to extend their lifespan. *Antioxidant mechanism in body* - While sirtuins can indirectly influence antioxidant defenses by regulating enzymes like **superoxide dismutase** (SOD), their primary association is not directly with the antioxidant mechanism itself but with broader cellular stress responses. - Antioxidant mechanisms typically involve direct scavenging of **reactive oxygen species** (ROS) by specific enzymes or molecules. *Regeneration of liver after partial resection* - Sirtuins do have roles in **metabolism** and cellular repair, but their direct and primary association is not specifically with liver regeneration after partial resection. - Liver regeneration involves complex pathways including growth factors and cytokine signaling. *Carcinogenesis in human* - Sirtuins have a complex and dual role in cancer, acting as both **tumor suppressors** and **oncogenes** depending on the specific sirtuin isoform, cancer type, and cellular context. - Their association with carcinogenesis is multifaceted, not a singular, consistent mechanism like their link to longevity.

Q: In Krebs cycle and Urea cycle the linking amino acid is

Aspartate. ***Aspartate*** - **Aspartate** acts as the crucial amino acid link between the two cycles - In the urea cycle, aspartate condenses with citrulline to form **argininosuccinate** (via argininosuccinate synthetase) - When argininosuccinate is cleaved, it produces **fumarate**, which enters the Krebs cycle - In the Krebs cycle, fumarate is converted to malate, then to **oxaloacetate**, which can be transaminated back to aspartate - This creates the **aspartate-argininosuccinate shunt**, linking both cycles through nitrogen metabolism *Fumarate* - While **fumarate** is a key metabolic intermediate connecting both cycles, it is **not an amino acid** (it's a dicarboxylic acid) - It is produced in the urea cycle from argininosuccinate cleavage and feeds into the Krebs cycle - This is a common distractor since fumarate does link the cycles, but the question specifically asks for an amino acid *Alanine* - **Alanine** participates in the glucose-alanine cycle for nitrogen transport from muscle to liver - It does not directly link the Krebs cycle and urea cycle in the same manner as aspartate *Arginine* - **Arginine** is a urea cycle intermediate that is cleaved by arginase to produce urea and ornithine - While it's an amino acid in the urea cycle, it does not serve as the linking amino acid between the Krebs cycle and urea cycle

Q: Which of the following enzyme activity decreases in fasting?

Phosphofructokinase I. ***Phosphofructokinase I*** - **Phosphofructokinase I (PFK-1)** activity **decreases** during fasting due to **decreased insulin-to-glucagon ratio**, which reduces **fructose-2,6-bisphosphate (F-2,6-BP)** levels, a powerful allosteric activator of PFK-1. - This reduction in activity slows down **glycolysis**, conserving glucose for critical tissues like the brain and redirecting metabolism toward **gluconeogenesis**. - **PFK-1 is the rate-limiting enzyme of glycolysis**, making its regulation particularly significant in the fasted state. *Hormone sensitive lipase* - **Hormone sensitive lipase (HSL)** activity **increases** during fasting due to elevated **glucagon** and **epinephrine** levels, which stimulate its phosphorylation via **protein kinase A (PKA)**. - This increased activity promotes the breakdown of stored **triglycerides** in adipose tissue, releasing **fatty acids** for β-oxidation and energy production. *Glycogen phosphorylase* - **Glycogen phosphorylase** activity **increases** during fasting, primarily stimulated by **glucagon** and **epinephrine**, leading to the breakdown of **glycogen** stores. - This enzyme is crucial for **glycogenolysis**, providing glucose to maintain blood sugar levels when dietary intake is absent. *Acetyl CoA Carboxylase* - **Acetyl CoA Carboxylase (ACC)** activity also **decreases** during fasting, as it is inhibited by **phosphorylation** mediated by **AMP-activated protein kinase (AMPK)** and **protein kinase A (PKA)**. - This reduction in activity inhibits **fatty acid synthesis**, shifting metabolism towards fatty acid **oxidation** for energy production. - **Note:** While ACC activity does decrease during fasting, **PFK-1** is considered the primary answer as it represents the key regulatory point for **glucose metabolism** (glycolysis vs. gluconeogenesis), which is the central metabolic shift during fasting.

Q: Energy source used by brain in later days of starvation is

Ketone bodies. ***Ketone bodies*** - During **prolonged starvation**, the liver produces **ketone bodies** (acetoacetate and β-hydroxybutyrate) from fatty acid breakdown. - The brain adapts to utilize these ketone bodies as a primary energy source, reducing its reliance on **glucose**. *Glucose* - While **glucose** is the primary energy source for the brain under normal conditions, its availability diminishes significantly during prolonged starvation. - The brain attempts to conserve glucose for essential functions by switching to alternative fuels. *Glycogen* - The brain stores very limited amounts of **glycogen**, which are rapidly depleted within minutes of glucose deprivation. - It is not a sustainable or significant energy source during extended periods of starvation. *Fatty acids* - **Fatty acids** cannot directly cross the **blood-brain barrier** to a significant extent, thus they are not a direct fuel source for brain cells. - They are, however, used by the liver to synthesize ketone bodies, which then serve as brain fuel.

Q: Which metabolic pathway is least active during 12 days of fasting?

Glycogenolysis. ***Correct: Glycogenolysis*** - **Glycogenolysis**, the breakdown of glycogen stores, is very active during the **initial hours of fasting** (first 24-48 hours) to maintain blood glucose levels. - However, after **12 days of fasting**, liver and muscle **glycogen stores are completely depleted**, making this pathway **essentially inactive** or the least active of all the metabolic pathways. - Once glycogen is exhausted, this pathway cannot contribute further to energy metabolism. *Incorrect: Gluconeogenesis* - This pathway becomes **increasingly active** during prolonged fasting to **synthesize new glucose** from non-carbohydrate precursors (amino acids, lactate, glycerol). - Essential for maintaining blood glucose for **glucose-dependent tissues** like red blood cells and parts of the brain that haven't fully adapted to ketones. - Remains a **crucial and active pathway** throughout prolonged fasting. *Incorrect: Ketogenesis* - **Ketogenesis** is **highly active** during prolonged fasting, producing **ketone bodies** (acetoacetate, β-hydroxybutyrate) from fatty acids in the liver. - Provides the **primary alternative fuel** for the brain (up to 70% of brain energy needs) and other tissues. - This is a **key metabolic adaptation** to preserve protein and glucose during starvation. *Incorrect: Lipolysis* - **Lipolysis** (breakdown of triglycerides into fatty acids and glycerol) is **highly active** during fasting to mobilize stored energy. - Provides **fatty acids** for direct oxidation by most tissues and **glycerol** as a gluconeogenic substrate. - A **fundamental process** for energy supply during nutrient deprivation.

Q: Which of the following organs does not primarily utilize fatty acids for energy?

Brain. ***Brain*** - The **brain primarily uses glucose** as its main energy source because fatty acids cannot efficiently cross the **blood-brain barrier**. - During prolonged starvation, the brain can adapt to use **ketone bodies**, which are derived from fatty acid breakdown in the liver. *Muscle* - **Skeletal muscle** can utilize both **glucose and fatty acids** for energy, with fatty acids becoming a more prominent fuel source during prolonged exercise and at rest. - **Cardiac muscle** (heart) heavily relies on **fatty acid oxidation** as its primary energy substrate, especially during basal conditions. *Liver* - The **liver is highly metabolically flexible** and readily oxidizes fatty acids for its own energy needs, particularly during fasting states. - It also plays a key role in **fatty acid metabolism**, including synthesis, breakdown, and conversion into ketone bodies. *Kidney* - The **renal cortex** is rich in mitochondria and has a high metabolic rate, primarily utilizing **fatty acid oxidation** to meet its significant energy demands for filtration and reabsorption. - While the renal medulla can use glucose, the cortex's reliance on fatty acids makes it a significant consumer.

Question 1

What happens to excess proteins in the body?

Accepted Answer

Converted to energy or stored as fat

Answer

Used to build structural proteins

Answer

Used to synthesize functional proteins

Answer

Stored as amino acids in tissues

Question 2

Pyruvate can be a substrate of all except

Accepted Answer

Aspartate transaminase

Answer

Lactate Dehydrogenase

Answer

Malic enzyme

Answer

Alanine transaminase

Question 3

Autophagic vacuoles fuse with ?

Accepted Answer

Lysosomes

Answer

Golgi complex

Answer

ER

Answer

Mitochondria

Question 4

In starvation, earliest to become depleted -

Accepted Answer

Carbohydrates

Answer

Proteins

Answer

Fats

Answer

None of the options

Question 5

Sirtuins are associated with?

Accepted Answer

Longevity of life span

Answer

Antioxidant mechanism in body

Answer

Regeneration of liver after partial resection

Answer

Carcinogenesis in human

Question 6

In Krebs cycle and Urea cycle the linking amino acid is

Accepted Answer

Aspartate

Answer

Fumarate

Answer

Alanine

Answer

Arginine

Question 7

Which of the following enzyme activity decreases in fasting?

Accepted Answer

Phosphofructokinase I

Answer

Hormone sensitive lipase

Answer

Glycogen phosphorylase

Answer

Acetyl CoA Carboxylase

Question 8

Energy source used by brain in later days of starvation is

Accepted Answer

Ketone bodies

Answer

Glucose

Answer

Glycogen

Answer

Fatty acids

Question 9

Which metabolic pathway is least active during 12 days of fasting?

Accepted Answer

Glycogenolysis

Answer

Gluconeogenesis

Answer

Ketogenesis

Answer

Lipolysis

Question 10

Which of the following organs does not primarily utilize fatty acids for energy?

Accepted Answer

Brain

Answer

Muscle

Answer

Liver

Answer

Kidney

Metabolic Integration — MCQs

Metabolic Integration — MCQs

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