Carbohydrate Metabolism Practice Questions

Q: A patient with hypoglycemia and hepatomegaly is found to have a defect in the enzyme glucose-6-phosphatase. What is the metabolic impact of this deficiency?

Impaired gluconeogenesis and glycogenolysis. ***Impaired gluconeogenesis and glycogenolysis*** - Glucose-6-phosphatase is essential for the final step in both **gluconeogenesis** and **glycogenolysis**, converting glucose-6-phosphate to free glucose for release into the bloodstream. - A deficiency in this enzyme, characteristic of **Von Gierke disease (Type I glycogen storage disease)**, prevents the liver from producing and releasing sufficient glucose, leading to **hypoglycemia** and **hepatomegaly** due to accumulated glycogen. *Increased glycogen synthesis* - While glycogen accumulates in the liver due to the inability to break it down, the primary defect isn't an *increase* in synthesis but rather a block in the **breakdown and release** of glucose. - Glycogen synthase activity might even be indirectly affected by the buildup of glucose-6-phosphate, but the core metabolic impact is impaired release. *Enhanced glycolysis* - Glycolysis is the breakdown of glucose, and while some extra glucose-6-phosphate might be shunted towards glycolysis, the overall metabolic picture is dominated by the inability to *produce* glucose from stores or other precursors. - The liver's main role in maintaining blood glucose means impaired glucose release has a far greater systemic impact. *Decreased lipid metabolism* - This deficiency actually leads to **increased lipid synthesis** and **hyperlipidemia**, not decreased lipid metabolism. - The accumulation of glucose-6-phosphate promotes divergent pathways like the **pentose phosphate pathway** and subsequent increase in acetyl-CoA, which serves as a precursor for fatty acid synthesis.

Q: Galactosemia is due to deficiency of which enzyme?

Galactose-1-phosphate uridyltransferase. ***Galactose-1-phosphate uridyltransferase*** - Deficiency of **galactose-1-phosphate uridyltransferase (GALT)** leads to the most severe form, **classic galactosemia**. - This enzyme is crucial for converting **galactose-1-phosphate** to **glucose-1-phosphate** in the Leloir pathway. *HGPRT* - **HGPRT** (hypoxanthine-guanine phosphoribosyltransferase) deficiency causes **Lesch-Nyhan syndrome**, a distinct metabolic disorder. - Lesch-Nyhan syndrome is characterized by **hyperuricemia**, neurological dysfunction, and self-mutilation, unrelated to galactose metabolism. *Galactokinase* - Deficiency of **galactokinase** causes Type II galactosemia, a milder form than classic galactosemia. - This defect primarily leads to **cataracts** due to galactitol accumulation but does not result in the severe systemic issues seen in classic galactosemia. *Epimerase* - Deficiency of **UDP-galactose-4'-epimerase** (GALE) causes Type III galactosemia, which has a variable clinical presentation from mild to severe. - While involved in galactose metabolism, it's not the primary enzyme deficient in the most common and severe form of **galactosemia**.

Q: A 3 months old child was started on supplemental foods along with breastmilk. The child was fed with fruit pulp and sweetened cereals. Soon the child developed bloating of abdomen, vomiting, lethargy, and irritability. On investigation, there was hyperbilirubinemia and elevated transaminase levels. The child is suffering from which of the following enzyme deficiencies?

Aldolase B. ***Aldolase B*** - This presentation is characteristic of **hereditary fructose intolerance**, an autosomal recessive disorder caused by a deficiency of **aldolase B**. - Infants typically appear normal until fructose or sucrose (hydrolyzed to glucose and fructose) is introduced into their diet, leading to symptoms like **vomiting**, **bloating**, **lethargy**, and **liver and kidney dysfunction** (hyperbilirubinemia, elevated transaminases). ***Fructokinase*** - Deficiency in fructokinase causes **essential fructosuria**, a benign condition where fructose accumulates in the blood and urine. - It does not lead to the severe gastrointestinal or hepatic symptoms described, as fructose metabolism is not completely blocked. ***Galactokinase*** - Deficiency of galactokinase results in **Type II galactosemia**, primarily causing **cataracts** due to galactitol accumulation. - While galactosemia can present with liver dysfunction, it typically involves lactose intolerance from breastmilk or formula and doesn't align with the introduction of fruit pulp and sweetened cereals as the trigger. ***Galactose-1-phosphate uridyl transferase*** - Deficiency of this enzyme causes **Classic Galactosemia (Type I)**, a severe genetic disorder often diagnosed early due to intolerance to lactose in breast milk or formula. - Symptoms include **vomiting**, **jaundice**, **hepatomegaly**, and **failure to thrive**, with potential for severe complications if untreated. However, the trigger of fruit pulp and sweetened cereals (sources of fructose/sucrose) more strongly points away from galactosemia and towards fructose intolerance.

Q: Which enzyme deficiency is associated with increased susceptibility to infections, including E. coli?

Glucose-6-phosphate dehydrogenase (G6PD) deficiency. ***Glucose-6-phosphate dehydrogenase (G6PD) deficiency*** - G6PD deficiency impairs the **pentose phosphate pathway**, reducing the production of **NADPH** in phagocytes. - Reduced **NADPH** leads to an inability to generate a robust **respiratory burst** required to kill pathogens like *E. coli*, thereby increasing susceptibility to infections. *Lactase deficiency* - This deficiency affects the digestion of **lactose**, leading to gastrointestinal symptoms such as bloating and diarrhea. - It does not directly impact the **immune system's ability to combat bacterial infections**. *Pepsin deficiency* - Pepsin is a gastric enzyme essential for **protein digestion** in the stomach. - While it can lead to impaired protein breakdown, it is not directly linked to an increased susceptibility to **bacterial infections** like *E. coli*. *Trypsin deficiency* - Trypsin is a pancreatic enzyme crucial for **protein digestion** in the small intestine. - Its deficiency affects nutrient absorption but does not directly compromise the **innate immune response** against bacteria.

Q: Neonatal hypoglycemia that does not respond to treatment with counter-regulatory hormones is diagnostic of which condition?

Von Gierke's disease. ***Von Gierke's disease*** - This condition (Glycogen Storage Disease Type I) results from a **deficiency of glucose-6-phosphatase**, essential for releasing glucose from the liver. - The inability to produce free glucose from glycogen or gluconeogenesis leads to severe hypoglycemia that **does not respond to counter-regulatory hormones** like glucagon, as the enzyme needed for glucose release is non-functional. *Hereditary fructose intolerance* - This condition involves a deficiency in **aldolase B**, leading to the accumulation of fructose-1-phosphate after fructose ingestion. - While it can cause hypoglycemia, it generally occurs after **fructose exposure** and is not characterized by hypoglycemia refractory to counter-regulatory hormones in the neonatal period without such exposure. *Cori's disease (Glycogen storage disease type III)* - Caused by a deficiency in the **glycogen debranching enzyme**, leading to the accumulation of abnormal glycogen. - Patients can present with hypoglycemia, but often respond to glucagon administration, as the remaining glycogen structure can still be partially broken down, unlike in Von Gierke's. *Anderson's disease (Glycogen storage disease type IV)* - Result of a deficiency in the **glycogen branching enzyme**, leading to the formation of abnormally structured glycogen with long, unbranched chains. - This disease primarily affects the liver and muscles, causing **cirrhosis** and muscle weakness, and typically does not present with severe, refractory neonatal hypoglycemia as the primary or most characteristic symptom.

Q: In glycogen synthesis the active form of glucose used is-

UDP glucose. **UDP glucose** - **UDP-glucose** (uridine diphosphate glucose) is the activated form of glucose that donates glucose units for the elongation of the **glycogen chain** during glycogen synthesis. - The formation of UDP-glucose from **glucose-1-phosphate** and **UTP** (uridine triphosphate) is catalyzed by UDP-glucose pyrophosphorylase, making glucose-1-phosphate a precursor, not the active form. *Glucose 6 phosphate* - **Glucose 6-phosphate** is an important intermediate in glycolysis and gluconeogenesis, and it can be isomerized to glucose 1-phosphate, but it is not the direct substrate for glycogen synthase. - Its formation is the first committed step in glucose metabolism within the cell, trapping glucose inside. *Glucose I phosphate* - **Glucose 1-phosphate** is a precursor to UDP-glucose, formed from glucose 6-phosphate by **phosphoglucomutase**. - While essential for glycogen synthesis, it is not the directly active form that donates glucose to the glycogen chain itself. *GTP glucose* - **GTP glucose** is not a known active form of glucose involved in glycogen synthesis. - **GTP** (guanosine triphosphate) is primarily involved in other metabolic processes, such as protein synthesis and signal transduction.

Q: Which of the following is NOT a key enzyme of gluconeogenesis?

Pyruvate kinase. ***Pyruvate kinase*** - **Pyruvate kinase** is a key regulatory enzyme in **glycolysis**, catalyzing the irreversible conversion of phosphoenolpyruvate (PEP) to pyruvate. - Since gluconeogenesis is essentially the reversal of glycolysis, pyruvate kinase is **NOT involved in gluconeogenesis**. Instead, this glycolytic step must be bypassed by different enzymes. - This is the correct answer as it is NOT a key enzyme of gluconeogenesis. *Pyruvate carboxylase* - This **IS a key enzyme of gluconeogenesis**, converting **pyruvate to oxaloacetate** in the mitochondria, thereby bypassing the pyruvate kinase step of glycolysis. - It uses **biotin** as a coenzyme and requires ATP. - This is one of the four key regulatory enzymes unique to gluconeogenesis. *PEP carboxykinase* - This **IS a key enzyme of gluconeogenesis**, converting **oxaloacetate to phosphoenolpyruvate (PEP)**, bypassing the irreversible pyruvate kinase step of glycolysis. - This enzyme is located in both the cytoplasm and mitochondria, depending on the species (cytoplasmic in humans). - This is one of the four key regulatory enzymes unique to gluconeogenesis. *Glucose-6-phosphatase* - This **IS a key enzyme of gluconeogenesis**, catalyzing the final step by dephosphorylating **glucose-6-phosphate to free glucose**, enabling its release from the liver into the bloodstream. - This enzyme bypasses the irreversible hexokinase/glucokinase step of glycolysis. - Located in the endoplasmic reticulum, it is exclusively found in gluconeogenic tissues like the liver and kidney.

Q: Which among the following glucose transporters is present in beta cells?

GLUT2. ***GLUT2*** - **GLUT2** is a **low-affinity** glucose transporter predominantly found in pancreatic **beta cells**, liver, kidneys, and intestines. - Its low affinity allows beta cells to accurately sense high blood glucose levels, triggering **insulin release**. *GLUT1* - **GLUT1** is a widely distributed glucose transporter found in nearly all mammalian cells, including **red blood cells** and cells of the **blood-brain barrier**. - It exhibits **high affinity** for glucose, responsible for basal glucose uptake. *GLUT3* - **GLUT3** is a high-affinity glucose transporter primarily found in **neurons** and the **placenta**. - Its high affinity ensures a constant glucose supply to these metabolically demanding tissues, even at low blood glucose concentrations. *GLUT4* - **GLUT4** is an **insulin-sensitive** glucose transporter found in **adipose tissue** and **striated muscle** (skeletal and cardiac). - Its translocation to the cell surface from intracellular vesicles is stimulated by insulin, promoting glucose uptake into these tissues.

Q: All of the following are true about lactate utilization in the liver except -

Total net number of ATP formed because of Cori's cycle is 4. ***Total net number of ATP formed because of cori's cycle is 4*** - This statement is incorrect. The **Cori cycle (lactic acid cycle)** is an energy-consuming process overall, as **6 ATP** molecules are consumed in the liver for gluconeogenesis to resynthesize glucose from lactate, while only a total of **2 ATP** are gained from glycolysis in the muscle. - The primary purpose of the Cori cycle is not net ATP production, but rather to shift the metabolic burden and regenerate glucose for tissues that rely on glycolysis (e.g., muscle, red blood cells). *Cori's cycle shifts the metabolic burden from muscle to liver* - This is true because **lactate produced in muscle** (during anaerobic conditions) is transported to the liver, where it is converted back to glucose. - The liver then bears the metabolic cost of **gluconeogenesis**, allowing the muscle to continue glycolysis and ATP production. *Cori's cycle can not be sustained indefinitely because it is energetically unfavourable* - This is true because the cycle involves a net consumption of ATP. **Six ATP equivalents** are used in gluconeogenesis in the liver to convert two molecules of lactate to one molecule of glucose. - In contrast, the glycolysis that produces the two lactate molecules in muscle yields only **two net ATP**. This energy deficit makes prolonged reliance on the Cori cycle unsustainable. *Cori's cycle is linked to glycogen synthesis in muscle* - This is true because the **glucose produced by the liver** via gluconeogenesis (from lactate) is released into the bloodstream. - This glucose can then be taken up by muscles and other tissues to **replenish glycogen stores** or be used for energy.

Q: Gas released from oligosaccharide metabolism by intestinal bacteria is

Carbon dioxide. ***Carbon dioxide*** - **Carbon dioxide (CO₂)** is the **most universally produced gas** from oligosaccharide and carbohydrate fermentation by intestinal bacteria in the colon. - Nearly all colonic bacteria produce CO₂ during the fermentation of undigested carbohydrates and oligosaccharides. - Along with **hydrogen (H₂)**, CO₂ forms the bulk of intestinal gas from bacterial metabolism. - This is part of normal gut flora activity contributing to **flatulence**. *Methane* - While **methane (CH₄)** is produced during oligosaccharide fermentation, it is only generated by individuals harboring **methanogenic archaea** (approximately 30-50% of the population). - Methane production is not universal, unlike CO₂, making it less representative as "THE gas" from oligosaccharide metabolism. - Methanogens use H₂ and CO₂ to produce methane as a secondary process. *Sulfur dioxide* - **Sulfur dioxide (SO₂)** is primarily associated with industrial pollution and is not a product of normal intestinal bacterial metabolism. - Hydrogen sulfide (H₂S) may be produced from sulfur-containing compounds, but not sulfur dioxide. *Nitric oxide* - **Nitric oxide (NO)** is a signaling molecule involved in vasodilation and immune responses. - It is not a major gas produced from bacterial fermentation of oligosaccharides in the intestines.

Question 1

A patient with hypoglycemia and hepatomegaly is found to have a defect in the enzyme glucose-6-phosphatase. What is the metabolic impact of this deficiency?

Accepted Answer

Impaired gluconeogenesis and glycogenolysis

Answer

Increased glycogen synthesis

Answer

Enhanced glycolysis

Answer

Decreased lipid metabolism

Question 2

Galactosemia is due to deficiency of which enzyme?

Accepted Answer

Galactose-1-phosphate uridyltransferase

Answer

HGPRT

Answer

Galactokinase

Answer

Epimerase

Question 3

A 3 months old child was started on supplemental foods along with breastmilk. The child was fed with fruit pulp and sweetened cereals. Soon the child developed bloating of abdomen, vomiting, lethargy, and irritability. On investigation, there was hyperbilirubinemia and elevated transaminase levels. The child is suffering from which of the following enzyme deficiencies?

Accepted Answer

Aldolase B

Answer

Fructokinase

Answer

Galactose - 1 - phosphate uridyl transferase

Answer

Galactokinase

Question 4

Which enzyme deficiency is associated with increased susceptibility to infections, including E. coli?

Accepted Answer

Glucose-6-phosphate dehydrogenase (G6PD) deficiency

Answer

Lactase deficiency

Answer

Pepsin deficiency

Answer

Trypsin deficiency

Question 5

Neonatal hypoglycemia that does not respond to treatment with counter-regulatory hormones is diagnostic of which condition?

Accepted Answer

Von Gierke's disease

Answer

Hereditary fructose intolerance

Answer

Cori's disease (Glycogen storage disease type III)

Answer

Anderson's disease (Glycogen storage disease type IV)

Question 6

In glycogen synthesis the active form of glucose used is-

Accepted Answer

UDP glucose

Answer

Glucose 6 phosphate

Answer

GTP glucose

Answer

Glucose 1 phosphate

Question 7

Which of the following is NOT a key enzyme of gluconeogenesis?

Accepted Answer

Pyruvate kinase

Answer

Pyruvate carboxylase

Answer

PEP carboxykinase

Answer

Glucose-6-phosphatase

Question 8

Which among the following glucose transporters is present in beta cells?

Accepted Answer

GLUT2

Answer

GLUT1

Answer

GLUT3

Answer

GLUT4

Question 9

All of the following are true about lactate utilization in the liver except -

Accepted Answer

Total net number of ATP formed because of Cori's cycle is 4

Answer

Cori's cycle shifts the metabolic burden from muscle to liver

Answer

Cori's cycle can not be sustained indefinitely because it is energetically unfavourable

Answer

Cori's cycle is linked to glycogen synthesis in muscle

Question 10

Gas released from oligosaccharide metabolism by intestinal bacteria is

Accepted Answer

Carbon dioxide

Answer

Sulfur dioxide

Answer

Nitric oxide

Answer

Methane

Carbohydrate Metabolism — MCQs

Carbohydrate Metabolism — MCQs

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