Carbohydrate Metabolism Practice Questions

Q: Glucagon acts on liver to cause:

Glycogenolysis. ***Glycogenolysis*** * **Glucagon's primary role** is to increase blood glucose levels during fasting states * Glucagon binds to **glucagon receptors on hepatocytes** (liver cells) * This activates **adenylyl cyclase → cAMP → protein kinase A** cascade * PKA phosphorylates and activates **phosphorylase kinase**, which activates **glycogen phosphorylase** * Result: **Glycogen breakdown to glucose-1-phosphate → glucose-6-phosphate → free glucose** (via glucose-6-phosphatase in liver) * The free glucose is released into bloodstream to maintain blood glucose levels *Kreb's cycle* * The **Kreb's cycle (citric acid cycle)** oxidizes acetyl-CoA to produce ATP, NADH, and FADH2 * It is not directly stimulated by glucagon; its activity is regulated by **substrate availability** and **energy demands** (ATP/ADP ratio, NADH/NAD+ ratio) * Glucagon's effects are primarily on glucose homeostasis, not directly on oxidative metabolism *Glycolysis* * **Glycolysis** breaks down glucose into pyruvate, generating ATP * Glucagon **inhibits glycolysis** in the liver by decreasing fructose-2,6-bisphosphate levels * This inhibition makes sense: glucagon's role is to **increase** blood glucose, not consume it * Glucagon activates phosphodiesterase which reduces cAMP levels needed for PFK-2 activity *Gluconeogenesis* * While glucagon does **stimulate gluconeogenesis** in the liver (glucose synthesis from non-carbohydrate sources) * The question asks about the direct action, and **glycogenolysis is the primary and immediate response** to glucagon * Gluconeogenesis is a slower process that becomes more important during prolonged fasting

Q: Which of the following is active in dephosphorylated state?

Glycogen Synthase. ***Glycogen Synthase*** - **Glycogen synthase** is primarily active in its **dephosphorylated state**, which is promoted by insulin and signals glycogen synthesis. - Dephosphorylation relieves the inhibitory effect of phosphorylation, allowing the enzyme to efficiently add glucose units to a **growing glycogen chain**. *PEPCK* - **Phosphoenolpyruvate carboxykinase (PEPCK)** activity is primarily regulated at the transcriptional level, not typically by phosphorylation state for activation. - Its expression is induced by **glucagon** and **cortisol** during gluconeogenesis. *Pyruvate Carboxylase* - **Pyruvate carboxylase** is allosterically activated by **acetyl-CoA** and its activity is not directly regulated by phosphorylation/dephosphorylation in the same manner as glycogen synthase. - This enzyme plays a key role in **gluconeogenesis** by converting pyruvate to oxaloacetate. *Glycogen Phosphorylase* - **Glycogen phosphorylase** is active in its **phosphorylated state**, particularly the 'a' form, which is promoted by glucagon and adrenaline for glycogen breakdown. - Phosphorylation activates the enzyme, leading to the **breakdown of glycogen** into glucose-1-phosphate.

Q: Which of the following is the PRIMARY physiological action of insulin in the fed state?

Activation of key enzymes of glycolysis. ***Activation of key enzymes of glycolysis*** - In the fed state, **insulin's primary and immediate action** is to promote **glucose utilization** through activation of glycolytic enzymes including **hexokinase, phosphofructokinase (PFK-1), and pyruvate kinase**. - This represents the **most direct and immediate metabolic response** to elevated blood glucose after a meal, allowing cells to use glucose for **immediate energy production**. - Insulin also promotes **GLUT4 translocation** to cell membranes in muscle and adipose tissue, enhancing glucose uptake, which directly feeds into **glycolysis**. - **Glycogen synthesis** (glycogenesis) occurs simultaneously as another primary action for glucose storage in liver and muscle. *Increased lipogenesis* - While insulin does stimulate **lipogenesis** (fatty acid and triglyceride synthesis) in the fed state, this is a **secondary action** that becomes significant primarily when **glycogen stores are replete**. - Lipogenesis represents **long-term energy storage**, but the primary immediate concern in the fed state is handling the glucose load through **direct utilization and glycogen storage**. - Only the excess glucose beyond immediate energy needs and glycogen storage capacity is converted to fat via lipogenesis. *Stimulation of gluconeogenesis* - This is **incorrect** - insulin **inhibits gluconeogenesis** in the fed state by suppressing key enzymes like **PEPCK and glucose-6-phosphatase**. - Stimulating gluconeogenesis would counteract insulin's primary function of **lowering blood glucose levels**. *Increased amino acid entry in the cell* - Insulin does promote **amino acid uptake** and **protein synthesis**, which is an important anabolic action. - However, in the context of the fed state's primary metabolic challenge (handling elevated blood glucose), **glucose utilization through glycolysis** takes precedence as the primary action.

Q: The enzyme deficient in Galactosemia is:-

Galactose-1-phosphate uridyltransferase (GALT). ***Galactose-1-phosphate uridyltransferase (GALT)*** - Deficiency of **GALT** leads to the accumulation of **galactose-1-phosphate** in tissues, which is toxic and causes the symptoms of **classical galactosemia**. - This enzyme is crucial for the second step in the Leloir pathway, converting **galactose-1-phosphate** and UDP-glucose into UDP-galactose and glucose-1-phosphate. *Sphingomyelinase (associated with Niemann-Pick disease)* - Deficiency in **sphingomyelinase** causes the accumulation of **sphingomyelin**, leading to **Niemann-Pick disease**, characterized by hepatosplenomegaly and neurological deterioration. - This enzyme is involved in the catabolism of lipids, distinct from carbohydrate metabolism. *Glucocerebrosidase (associated with Gaucher's disease)* - Deficiency of **glucocerebrosidase** results in the accumulation of **glucocerebroside**, leading to **Gaucher's disease**, which affects the spleen, liver, bone marrow, and sometimes the brain. - This is a lysosomal storage disorder involving glycosphingolipids, not galactose metabolism. *Hexosaminidase (associated with Tay-Sachs disease)* - Deficiency of **hexosaminidase A** causes the accumulation of **GM2 ganglioside**, resulting in **Tay-Sachs disease**, a severe neurodegenerative disorder. - This enzyme primarily functions in the breakdown of gangliosides, which are complex lipids, not galactose.

Q: Enzyme activated by decrease in Insulin: glucagon ratio:

Glucose 6 phosphatase. ***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.

Q: Which absorbs least water?

Cellulose. ***Cellulose*** - **Cellulose** is a **polysaccharide** with strong **intermolecular hydrogen bonding** between its linear chains. - These strong bonds form a highly ordered, crystalline structure that makes it **insoluble in water** and resistant to water absorption. *Mucilage* - **Mucilage** consists of **polysaccharides** that have a high capacity to absorb water, forming a slimy, gelatinous mass. - This property is due to its highly branched structure and abundance of **hydroxyl groups**, which readily form hydrogen bonds with water. *Pectin* - **Pectin** is a complex **polysaccharide** found in plant cell walls, known for its ability to absorb significant amounts of water. - It forms **gels** with water, a property widely utilized in food production. *Gums* - **Gums** are a diverse group of **polysaccharides** that are highly soluble in water and have an excellent capacity for water absorption. - They tend to form **viscous solutions** or gels when mixed with water.

Q: All of the following should be avoided by a patient with lactose intolerance, EXCEPT:

Yoghurt. ***Yoghurt*** - Yoghurt contains **lactic acid bacteria**, such as *Lactobacillus bulgaricus* and *Streptococcus thermophilus*, which produce **lactase** and break down the lactose into simpler sugars. - This enzymatic action makes yoghurt generally **better tolerated** by individuals with lactose intolerance compared to other dairy products [1]. *Skimmed milk* - **Skimmed milk** is still a dairy product and contains **lactose**, similar to whole milk, just with less fat [1], [2]. - Therefore, it will likely cause **digestive symptoms** in individuals with lactose intolerance due to the lack of lactase to break down the lactose [2], [3]. *Ice-cream* - **Ice cream** is rich in milk and cream, both of which are high in **lactose**. - Its high lactose content makes it a common trigger for **gastrointestinal discomfort** in lactose-intolerant individuals [1]. *Condensed milk* - **Condensed milk** is milk from which water has been removed, resulting in a concentrated product, often sweetened. - It contains a **high concentration of lactose**, making it unsuitable for individuals with lactose intolerance.

Q: A child presented with abdominal distension, hepatomegaly, doll like facies and recurrent episodes of hypoglycemia. Which of the following is the most likely diagnosis?

Von Gierke disease. ***Von Gierke disease*** - **Von Gierke disease** (Glycogen Storage Disease Type I) is characterized by **hepatomegaly**, **abdominal distension**, **severe hypoglycemia**, and a distinct "doll-like" facies due to fat deposits. - This condition results from a deficiency in **glucose-6-phosphatase**, impairing the liver's ability to release glucose, leading to glycogen accumulation and the described symptoms. *Down syndrome* - **Down syndrome** (Trisomy 21) is a chromosomal disorder associated with intellectual disability, characteristic facial features, and congenital heart defects, but not typically with these metabolic symptoms. - While children with Down syndrome may have various health issues, recurrent severe hypoglycemia and hepatomegaly to this extent are not characteristic features of the syndrome itself. *Lesch-Nyhan syndrome* - **Lesch-Nyhan syndrome** is an X-linked recessive disorder of purine metabolism, leading to overproduction of uric acid, severe neurological dysfunction, and self-mutilation. - It does not present with hepatomegaly, abdominal distension, or the recurrent episodes of hypoglycemia seen in this case. *Menkes disease* - **Menkes disease** is an X-linked recessive disorder of copper transport, leading to copper deficiency and affecting various organs, including the brain, hair, and connective tissue. - Key features include sparse, brittle, "steely" hair (`pili torti`), neurodegeneration, and developmental delay, which are distinct from the metabolic presentation described.

Q: A 25-year-old male presents with jaundice, fatigue, and dark urine after taking antimalarial drugs. Labs show elevated bilirubin, reticulocytosis, and splenomegaly. Peripheral smear shows bite cells and Heinz bodies. Which enzyme deficiency is most likely?

Glucose-6-phosphate dehydrogenase. ***Glucose-6-phosphate dehydrogenase*** - The combination of **jaundice**, **fatigue**, **dark urine** after antimalarial drug exposure, along with **reticulocytosis**, **splenomegaly**, **bite cells**, and **Heinz bodies**, is highly indicative of **G6PD deficiency** [1], [4]. - **G6PD deficiency** impairs the hexose monophosphate shunt, leading to reduced NADPH production and increased oxidative stress in red blood cells when exposed to certain drugs or stressors, resulting in **hemolytic anemia** [2], [3]. *Glucose-6-phosphatase* - Deficiency in **glucose-6-phosphatase** is associated with **Von Gierke disease** (Type I glycogen storage disease). - This condition primarily presents with **hypoglycemia**, **lactic acidosis**, and **hepatomegaly**, not hemolytic anemia with bite cells. *Pyruvate kinase* - **Pyruvate kinase deficiency** also causes **hemolytic anemia**, but it typically presents with **echinocytes** (burr cells) on peripheral smear, rather than bite cells and Heinz bodies. - The hemolysis is ongoing and not acutely precipitated by oxidative stressors in the same manner as G6PD deficiency. *Aldolase A* - **Aldolase A deficiency** is a rare cause of **hemolytic anemia** with associated **myopathy**. - It would not typically present with **bite cells** and **Heinz bodies** in response to oxidative stress from antimalarial drugs.

Q: Which is not a feature of G6PD deficiency?

Males and females are equally affected. ***Males and females are equally affected*** - G6PD deficiency is an **X-linked recessive disorder**, meaning males are predominantly and more severely affected because they have only one X chromosome [2]. - Females are typically carriers and are less commonly affected, or may experience milder symptoms, due to **X-chromosome inactivation** (Lyonization). *Presence of Heinz bodies* - **Heinz bodies** are formed from denatured hemoglobin precipitates within red blood cells, a characteristic feature of **oxidative stress** in G6PD deficiency [2]. - These bodies are removed by the spleen, contributing to **hemolytic anemia**. *Absence of NADPH* - G6PD is the rate-limiting enzyme in the **pentose phosphate pathway**, which generates **NADPH** [1], [2]. - Without sufficient G6PD, the production of **NADPH** is severely impaired, leading to a deficiency in this critical reducing agent. *Oxidative stress* - **NADPH** is crucial for reducing **glutathione**, which in turn detoxifies reactive oxygen species [2]. - The lack of NADPH makes red blood cells vulnerable to **oxidative damage**, manifesting as hemolytic anemia upon exposure to oxidative agents [3].

Question 1

Glucagon acts on liver to cause:

Accepted Answer

Glycogenolysis

Answer

Kreb's cycle

Answer

Glycolysis

Answer

Gluconeogenesis

Question 2

Which of the following is active in dephosphorylated state?

Accepted Answer

Glycogen Synthase

Answer

PEPCK

Answer

Pyruvate Carboxylase

Answer

Glycogen Phosphorylase

Question 3

Which of the following is the PRIMARY physiological action of insulin in the fed state?

Accepted Answer

Activation of key enzymes of glycolysis

Answer

Stimulation of gluconeogenesis

Answer

Increased amino acid entry in the cell

Answer

Increased lipogenesis

Question 4

The enzyme deficient in Galactosemia is:-

Accepted Answer

Galactose-1-phosphate uridyltransferase (GALT)

Answer

Sphingomyelinase

Answer

Glucocerebrosidase

Answer

Hexosaminidase

Question 5

Enzyme activated by decrease in Insulin: glucagon ratio:

Accepted Answer

Glucose 6 phosphatase

Answer

PFK

Answer

Glucokinase

Answer

Hexokinase

Question 6

Which absorbs least water?

Accepted Answer

Cellulose

Answer

Mucilage

Answer

Pectin

Answer

Gums

Question 7

All of the following should be avoided by a patient with lactose intolerance, EXCEPT:

Accepted Answer

Yoghurt

Answer

Skimmed milk

Answer

Ice-cream

Answer

Condensed milk

Question 8

A child presented with abdominal distension, hepatomegaly, doll like facies and recurrent episodes of hypoglycemia. Which of the following is the most likely diagnosis?

Accepted Answer

Von Gierke disease

Answer

Down syndrome

Answer

Lesch-Nyhan syndrome

Answer

Menkes disease

Question 9

A 25-year-old male presents with jaundice, fatigue, and dark urine after taking antimalarial drugs. Labs show elevated bilirubin, reticulocytosis, and splenomegaly. Peripheral smear shows bite cells and Heinz bodies. Which enzyme deficiency is most likely?

Accepted Answer

Glucose-6-phosphate dehydrogenase

Answer

Glucose-6-phosphatase

Answer

Pyruvate kinase

Answer

Aldolase A

Question 10

Which is not a feature of G6PD deficiency?

Accepted Answer

Males and females are equally affected

Answer

Presence of Heinz bodies

Answer

Absence of NADPH

Answer

Oxidative stress

Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism — MCQs

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