Metabolic Integration Practice Questions

Q: During starvation, which of the following hormones is primarily produced to maintain blood glucose levels?

Glucagon. ***Glucagon*** - Glucagon is the **primary counter-regulatory hormone** secreted by the pancreatic **alpha cells** in response to hypoglycemia during starvation. - It acts mainly on the liver to stimulate rapid glucose release through **glycogenolysis** and sustain long-term glucose production via **gluconeogenesis**. - Glucagon levels rise significantly within hours of fasting and remain elevated throughout prolonged starvation. *Insulin* - Insulin is an **anabolic hormone** secreted in response to high blood glucose (hyperglycemia) to promote storage and glucose uptake, thus lowering blood glucose levels. - During starvation, insulin secretion is characteristically **suppressed** to minimize glucose uptake by peripheral tissues and conserve it for the brain. *Cortisol* - Cortisol is a **glucocorticoid stress hormone** that does increase during prolonged starvation and contributes to gluconeogenesis and protein catabolism. - However, **glucagon is the primary and most rapid responder** to falling blood glucose levels, making it the correct answer to this question. *Somatostatin* - Somatostatin is a **paracrine inhibitor** secreted by pancreatic delta cells that locally suppresses the release of both insulin and glucagon. - While it modulates islet function, it is not the primary hormone responsible for mobilizing stored fuels and **raising blood glucose** during periods of fasting.

Q: A man is trapped in a tunnel for 5 consecutive days without access to food but survives. What is the primary source of energy for his brain during this period?

Ketone bodies. ***Ketone bodies*** - During prolonged fasting (beyond 48-72 hours, such as 5 days), the liver generates **ketone bodies** (β-hydroxybutyrate and acetoacetate) from fatty acid oxidation via **ketogenesis**. - These **ketone bodies** efficiently cross the **blood-brain barrier** and replace up to **60-70%** of the brain's energy needs during prolonged starvation, thus conserving essential muscle protein. - This metabolic adaptation (ketosis) is crucial for survival during extended fasting periods. *Gluconeogenesis* - **Gluconeogenesis** (synthesis of new glucose from non-carbohydrate precursors) remains active during starvation to provide glucose for obligate glucose users like **RBCs** and the renal medulla. - However, the brain minimizes its dependence on glucose and shifts primarily to ketone body utilization by day 5. - Primary substrates for gluconeogenesis are **amino acids** (from muscle protein) and **glycerol** (from lipolysis). *Glycogenolysis* - **Glycogenolysis** (breakdown of liver glycogen to glucose) is the first-line response during early fasting, typically lasting only **12-24 hours**. - By 5 consecutive days, liver glycogen stores are **completely depleted**, making this pathway inactive and unable to fuel the brain. *Lipolysis* - **Lipolysis** (breakdown of adipose triglycerides) releases **fatty acids** and **glycerol** into circulation. - Fatty acids fuel peripheral tissues (skeletal muscle, heart) but **cannot cross the blood-brain barrier** efficiently. - Lipolysis provides substrates for hepatic ketogenesis, but is not the direct energy source for the brain itself.

Q: Most common progesterone metabolite in urine is:

Pregnanediol. ***Pregnanediol*** - **Pregnanediol** is the primary urinary metabolite of **progesterone** and is formed by the reduction of progesterone in the liver. - Its measurement in urine can be used as an indicator of **progesterone production** in the body, reflecting ovarian or placental function. *Pregnenolone* - **Pregnenolone** is a precursor to progesterone and other steroid hormones, not a direct metabolite excreted in the urine. - It is mainly metabolized within steroid-producing tissues rather than being excreted unchanged. *17 hydroxy pregnenolone* - **17-hydroxypregnenolone** is an intermediate in the synthesis of androgens and estrogens, originating from pregnenolone. - It is not a direct or "most common" urinary metabolite of progesterone itself. *Pregnanetriol* - **Pregnanetriol** is a urinary metabolite of **17-hydroxyprogesterone**, a downstream steroid from progesterone in the metabolic pathway. - Its presence is commonly used as a marker for **congenital adrenal hyperplasia (CAH)** due to 21-hydroxylase deficiency, not as the primary metabolite of progesterone.

Q: Which of these is an example of anaplerotic reaction?

Pyruvate to oxaloacetate. ***Pyruvate to oxaloacetate*** - This reaction, catalyzed by **pyruvate carboxylase**, replenishes intermediates of the **TCA cycle (Krebs cycle)**. - **Oxaloacetate** is a key intermediate that combines with acetyl-CoA to initiate the TCA cycle, thus anaplerotic reactions ensure the cycle can continue. *Pyruvate to acetaldehyde* - This conversion occurs in alcoholic fermentation, primarily in yeast, and is not an anaplerotic reaction in human metabolism. - It involves the enzyme **pyruvate decarboxylase** and produces **carbon dioxide** as a byproduct. *Pyruvate to lactic acid* - This is an anaerobic pathway for pyruvate metabolism, catalyzed by **lactate dehydrogenase**, which regenerates NAD+ for glycolysis. - It does not directly replenish intermediates of the **TCA cycle**. *Pyruvate to acetyl-CoA* - This reaction, catalyzed by the **pyruvate dehydrogenase complex**, links glycolysis to the TCA cycle by producing acetyl-CoA. - However, it consumes pyruvate and forms an entry point for the cycle, rather than replenishing existing intermediates.

Q: Regarding glutathione all of the following are true EXCEPT

It inactivates some enzymes. ***It inactivates some enzymes*** - Glutathione is a crucial **antioxidant** that plays a vital role in protecting enzymes and other cellular components from oxidative damage by **reducing disulfide bonds**, thereby preserving or restoring enzyme activity rather than inactivating them. - Its primary function is to **detoxify harmful compounds** and maintain the cellular redox state, which requires enzymes to be active. *It helps in membrane transport* - Glutathione is involved in the active transport of certain substances across cell membranes, particularly in the **GSH-dependent transport systems** in the kidney and liver. - It forms conjugates with toxins, which are then transported out of the cell or body, a process often referred to as **efflux pumps**. *It helps in conjugation reactions* - Glutathione is a key substrate for **glutathione S-transferases (GSTs)**, enzymes that catalyze the conjugation of glutathione with various electrophilic compounds. - This process is vital for the **detoxification** and elimination of xenobiotics and endogenous toxic metabolites. *It helps in absorption of certain amino acids* - The **gamma-glutamyl cycle** (also known as the Meister cycle) in the kidneys utilizes glutathione for the transport of amino acids across cell membranes. - In this cycle, glutathione is broken down to release an amino acid, which is then transported into the cell, thus facilitating **amino acid uptake**.

Q: Sirtuins are associated with

Metabolism. ***Metabolism*** - **Sirtuins** are a family of NAD+-dependent protein deacetylases that play a **primary and crucial role** in regulating cellular **metabolism**. - They are involved in various metabolic processes including **glucose metabolism**, **fatty acid oxidation**, **mitochondrial biogenesis**, and energy homeostasis, often in response to cellular NAD+/NADH ratios. - This is their **most well-established and widely recognized function** in biochemistry and cellular biology. *Vision* - **Vision** is primarily mediated by photoreceptor cells in the retina and relies on proteins like **rhodopsin** and photopsins. - While sirtuins may influence retinal cell health indirectly, they have **no direct primary role** in the visual transduction cascade. *Olfaction* - **Olfaction (sense of smell)** involves **olfactory receptors** in the nasal epithelium that bind specific odor molecules, initiating a signal cascade. - Sirtuins do **not have a primary role** in the molecular mechanisms of odorant binding or signal transduction in the olfactory system. *Memory* - While emerging research shows sirtuins (particularly **SIRT1**) can influence synaptic plasticity, neuronal health, and cognitive function, this is **not their primary or defining association**. - Their role in memory is **secondary and indirect** compared to their fundamental metabolic functions, making metabolism the most appropriate answer in a biochemistry context.

Q: The first step in alcohol metabolism by the liver is the formation of acetaldehyde from alcohol, a chemical reaction catalyzed by

Alcohol dehydrogenase. ***Alcohol dehydrogenase*** - **Alcohol dehydrogenase (ADH)** is the primary enzyme responsible for the first step of alcohol metabolism, converting **ethanol** to **acetaldehyde**. - This reaction occurs predominantly in the **cytosol of hepatocytes**. *Cytochrome P450* - While certain **cytochrome P450 enzymes** (specifically CYP2E1, part of the **microsomal ethanol oxidizing system or MEOS**) can metabolize alcohol, it is a secondary pathway, especially at higher alcohol concentrations. - Its primary role in alcohol metabolism is less significant than **ADH** under normal consumption. *Catalase* - **Catalase** can metabolize a small amount of alcohol in the **peroxisomes**, particularly at very high alcohol concentrations. - However, its contribution to overall alcohol metabolism is minimal compared to **alcohol dehydrogenase**. *NADPH-cytochrome reductase P450* - This enzyme is a component of the **microsomal electron transport chain** and is involved in the function of cytochrome P450 enzymes. - It does not directly catalyze the oxidation of **alcohol to acetaldehyde** but rather facilitates the function of the enzymes that do.

Q: Which of the following is FALSE about insulin action?

Insulin promotes ketogenesis. ***Insulin promotes ketogenesis*** - Insulin is an **anabolic hormone** that works to prevent excessive **fat breakdown** and the formation of **ketone bodies**. - High insulin levels actively **inhibit** enzymes involved in ketogenesis, such as **carnitine palmitoyltransferase-1 (CPT1)**, thereby reducing the transport of fatty acids into mitochondria for oxidation. *Insulin promotes glycolysis* - Insulin stimulates **glycolysis**, particularly in the liver and muscle, by increasing the activity of key enzymes like **glucokinase** and **phosphofructokinase-1**. - This promotes the breakdown of glucose for **energy production** and provides substrates for fat synthesis. *Insulin promotes glycogen synthesis* - Insulin is a primary regulator of **glycogen synthesis** in the liver and muscles. - It activates **glycogen synthase** and inhibits glycogen phosphorylase, thereby shunting glucose towards storage as **glycogen**. *Insulin promotes lipogenesis* - Insulin promotes **lipogenesis** (fat synthesis) in adipose tissue and liver. - It increases glucose uptake into adipocytes and stimulates enzymes like **acetyl-CoA carboxylase** and **fatty acid synthase**, converting excess carbohydrates into fatty acids and subsequently **triglycerides**.

Question 1

During prolonged fasting (7 days), what is the primary source of energy for the body?

Accepted Answer

Acetone

Answer

Acetoacetate

Answer

Glucose

Answer

Alanine

Question 2

Which of the following statements is not true regarding acute starvation?

Accepted Answer

Only ketone bodies are utilized by the brain for energy

Answer

Increased lipolysis

Answer

Increased gluconeogenesis in liver

Answer

Increased glycogenolysis in liver

Question 3

During starvation, which of the following hormones is primarily produced to maintain blood glucose levels?

Accepted Answer

Glucagon

Answer

Insulin

Answer

Cortisol

Answer

Somatostatin

Question 4

A man is trapped in a tunnel for 5 consecutive days without access to food but survives. What is the primary source of energy for his brain during this period?

Accepted Answer

Ketone bodies

Answer

Glycogenolysis

Answer

Lipolysis

Answer

Gluconeogenesis

Question 5

Most common progesterone metabolite in urine is:

Accepted Answer

Pregnanediol

Answer

Pregnenolone

Answer

17 hydroxy pregnenolone

Answer

Pregnanetriol

Question 6

Which of these is an example of anaplerotic reaction?

Accepted Answer

Pyruvate to oxaloacetate

Answer

Pyruvate to acetaldehyde

Answer

Pyruvate to lactic acid

Answer

Pyruvate to acetyl-CoA

Question 7

Regarding glutathione all of the following are true EXCEPT

Accepted Answer

It inactivates some enzymes

Answer

It helps in membrane transport

Answer

It helps in conjugation reactions

Answer

It helps in absorption of certain amino acids

Question 8

Sirtuins are associated with

Accepted Answer

Metabolism

Answer

Vision

Answer

Olfaction

Answer

Memory

Question 9

The first step in alcohol metabolism by the liver is the formation of acetaldehyde from alcohol, a chemical reaction catalyzed by

Accepted Answer

Alcohol dehydrogenase

Answer

Cytochrome P450

Answer

Catalase

Answer

NADPH-cytochrome reductase P450

Question 10

Which of the following is FALSE about insulin action?

Accepted Answer

Insulin promotes ketogenesis

Answer

Insulin promotes glycolysis

Answer

Insulin promotes glycogen synthesis

Answer

Insulin promotes lipogenesis

Metabolic Integration — MCQs

Metabolic Integration — MCQs

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