NEET-PG 2013 - Biochemistry Practice Questions

Q: Which of the following is an aldose?

Glucose. ***Glucose*** - An **aldose** is a monosaccharide containing an **aldehyde group** (—CHO) in its open-chain form. - **Glucose** possesses an aldehyde group at carbon-1 and is therefore classified as an aldose. *Fructose* - **Fructose** is a **ketose**, meaning it contains a **ketone group** (C=O) in its open-chain structure, typically at carbon-2. - While it is a monosaccharide, its functional group differentiates it from aldoses. *Erythrulose* - **Erythrulose** is a **ketotetrose**, meaning it is a four-carbon sugar with a **ketone group**. - Unlike aldoses, which have an aldehyde group, erythrulose's defining characteristic is its ketone functional group. *None of the options* - This option is incorrect because **Glucose** is indeed an aldose, fitting the definition of having an aldehyde functional group. - Therefore, there is a correct option provided among the choices.

Q: Phosphofructokinase-1 occupies a key position in regulating glycolysis and is also subjected to feedback control. Which among the following are the allosteric activators of phosphofructokinase-1?

Fructose 2,6-bisphosphate. ***Fructose 2,6-bisphosphate*** - **Fructose 2,6-bisphosphate** is a potent **allosteric activator** of **phosphofructokinase-1 (PFK-1)**, increasing its affinity for fructose 6-phosphate and overcoming ATP inhibition. - Its synthesis is regulated by **insulin** (stimulating) and **glucagon** (inhibiting), linking glucose availability to glycolytic flux. *2,3-Bisphosphoglycerate (2,3-BPG)* - **2,3-BPG** is an important regulator of **hemoglobin oxygen affinity** in red blood cells. - It is not an allosteric activator of **PFK-1**; its primary role is in oxygen delivery. *Glucokinase* - **Glucokinase** is an **enzyme** in glycolysis, specifically catalyzing the phosphorylation of glucose to glucose 6-phosphate in the liver and pancreatic beta cells. - It is not an allosteric activator of **PFK-1** but rather an upstream enzyme in the pathway. *Phosphoenolpyruvate (PEP)* - **PEP** is an intermediate in glycolysis, formed from 2-phosphoglycerate and converted to pyruvate by pyruvate kinase. - It acts as an **allosteric inhibitor** of phosphofructokinase-1, signaling high energy status and slowing down glycolysis.

Q: Hexokinase is inhibited by?

Glucose-6-phosphate (G6P). ***Glucose-6-phosphate (G6P)*** - Hexokinase is subject to **feedback inhibition** by its product, **glucose-6-phosphate**, preventing the accumulation of high levels of G6P inside the cell. - This regulatory mechanism ensures that glycolysis does not proceed unchecked when energy needs are met or when G6P levels are already sufficient. *Glucagon* - **Glucagon** is a hormone that generally promotes **glucose production** and release, primarily by stimulating gluconeogenesis and glycogenolysis, rather than directly inhibiting hexokinase. - Its effects on glucose metabolism are more about increasing blood glucose levels than directly regulating the initial step of glycolysis in most tissues. *Glucose* - **Glucose** is the **substrate** for hexokinase, meaning it is the molecule that hexokinase acts upon to convert it into glucose-6-phosphate. - Therefore, glucose does not inhibit hexokinase; instead, its presence is necessary for the enzyme's activity. *Insulin* - **Insulin** is a hormone that promotes **glucose uptake** and utilization by cells, often by increasing the number of glucose transporters on cell surfaces. - While insulin can indirectly influence glycolysis by increasing glucose availability, it does not directly inhibit hexokinase; rather, it generally supports cellular glucose metabolism.

Q: Gluconeogenesis occurs in all except:

Muscle. ***Muscle*** - **Muscle tissue** lacks the enzyme **glucose-6-phosphatase**, which is essential for releasing free glucose into the bloodstream during gluconeogenesis. - While muscle can store glycogen, it primarily uses glucose for its own energy needs and does not contribute significantly to systemic glucose homeostasis through gluconeogenesis. *Liver* - The **liver** is the primary site of **gluconeogenesis**, producing glucose to maintain blood glucose levels during fasting and starvation. - It contains all the necessary enzymes, including **glucose-6-phosphatase**, to convert precursors like lactate, amino acids, and glycerol into glucose. *Kidney* - The **kidney** becomes a significant site of **gluconeogenesis** during prolonged fasting, contributing up to 10-20% of the body's glucose production. - Renal gluconeogenesis primarily utilizes **lactate** and **glutamine** as substrates. *Gut* - The **small intestine (gut)** has been identified as a site of **gluconeogenesis**, particularly following a meal rich in protein. - Its contribution is relatively smaller compared to the liver but plays a role in **postprandial glucose homeostasis**.

Q: Which of the following statements about gluconeogenesis is correct?

Occurs mainly in the liver. ***Occurs mainly in the liver*** - The **liver** is the primary site for **gluconeogenesis**, responsible for maintaining blood glucose levels during fasting. - The kidneys also contribute, especially during prolonged fasting, but to a lesser extent. *It uses exactly the same enzymes as glycolysis in reverse* - While gluconeogenesis shares some enzymes with glycolysis, there are **three irreversible steps in glycolysis** that require different enzymes in gluconeogenesis to bypass them. - Key bypass enzymes include **pyruvate carboxylase**, **phosphoenolpyruvate carboxykinase (PEPCK)**, **fructose-1,6-bisphosphatase**, and **glucose-6-phosphatase**. *It only occurs during fed state when insulin levels are high* - **Gluconeogenesis is activated during fasting or starvation** when blood glucose levels are low, and it is largely **inhibited by high insulin levels**. - Its purpose is to produce new glucose to prevent hypoglycemia, not to store excess glucose. *Fatty acids are the primary substrate for gluconeogenesis* - **Fatty acids cannot be directly converted to glucose** in significant amounts in humans because they are broken down into acetyl-CoA, which cannot be used for net glucose synthesis. - Primary substrates include **lactate**, **amino acids** (from protein breakdown), and **glycerol** (from triglyceride breakdown).

Q: Which of the following is NOT required for gluconeogenesis from lactate?

Transamination of pyruvate to alanine. ***Transamination of pyruvate to alanine*** - While **alanine** can be a substrate for gluconeogenesis, **lactate** is directly converted to pyruvate, which then enters the gluconeogenesis pathway. **Transamination to alanine** is not a required intermediate step for lactate-derived glucose production. - The direct conversion of **lactate to pyruvate** by **lactate dehydrogenase** is the key initial step, not its conversion to alanine. *Transport of lactate from muscle to liver* - **Lactate** produced in muscles (e.g., during intense exercise) must be transported to the **liver** via the bloodstream to be used for **gluconeogenesis** in the **Cori cycle**. - This transport is essential for clearing lactate from the periphery and supplying the liver with a gluconeogenic precursor. *Conversion of lactate to pyruvate* - **Lactate dehydrogenase** catalyzes the reversible conversion of **lactate to pyruvate**, which is the critical first step in converting lactate into a gluconeogenic substrate. - This reaction regenerates **NAD+** (not NADH), which is necessary for glycolysis to continue in muscle tissue. *None of the above* - This option is incorrect because there IS a step listed above that is not required: **transamination of pyruvate to alanine** is indeed not necessary for gluconeogenesis from lactate, making Option A the correct answer to this "NOT required" question.

Q: Which of the following tissues relies EXCLUSIVELY on anaerobic glycolysis for ATP production?

Mature RBCs (exclusively anaerobic). ***Mature RBCs (exclusively anaerobic)*** - **Mature red blood cells** lack mitochondria, making them incapable of **oxidative phosphorylation** and thus relying entirely on **anaerobic glycolysis** for ATP. - This pathway produces **2 net ATP** molecules per glucose molecule, which is sufficient for their metabolic needs like maintaining ion gradients. *Skeletal muscle during exercise (anaerobic)* - While skeletal muscle can perform **anaerobic glycolysis** during intense exercise when oxygen supply is limited, it is not an exclusive reliance. - Skeletal muscle also utilizes **aerobic respiration** and **creatine phosphate** for ATP production depending on activity level and oxygen availability. *Cardiac muscle (primarily aerobic)* - **Cardiac muscle** has a very high metabolic demand and is rich in **mitochondria**, relying almost exclusively on **aerobic respiration** for ATP production. - It uses fatty acids, glucose, and lactate as fuel sources, producing a large amount of ATP efficiently with oxygen. *Liver hepatocytes (primarily aerobic)* - **Liver hepatocytes** are highly metabolically active and primarily rely on **aerobic respiration** for ATP production, performing diverse functions such as gluconeogenesis, glycogenolysis, and detoxification. - Although the liver can perform some anaerobic glycolysis under hypoxic conditions, it is not its exclusive or primary mode of ATP synthesis.

Q: The energy for glycogenesis is provided by -

UTP. ***UTP*** - **Uridine triphosphate (UTP)** is essential for **glycogenesis** as it activates glucose by forming **UDP-glucose** from glucose-1-phosphate. - The reaction (Glucose-1-P + UTP → UDP-glucose + PPi) creates a **high-energy intermediate** that drives glycogen synthesis. - The subsequent hydrolysis of pyrophosphate (PPi) makes this activation step **irreversible**, and the energy stored in UDP-glucose is used for **glycosidic bond formation** when glucose is added to the growing glycogen chain. *GTP* - **Guanosine triphosphate (GTP)** is primarily involved in **protein synthesis**, G-protein signaling, and the citric acid cycle. - It is not used for glucose activation in glycogenesis; that role is specific to **UTP**. *GDP* - **Guanosine diphosphate (GDP)** is a product of GTP hydrolysis and functions in regulatory processes. - It does not serve as an energy donor for glycogen synthesis. *AMP* - **Adenosine monophosphate (AMP)** is a low-energy signal molecule that indicates cellular energy depletion. - High AMP levels **inhibit glycogenesis** and activate glycogenolysis through allosteric regulation of key enzymes. - It does not provide energy for anabolic pathways like glycogen synthesis.

Q: Which of the following tests is most commonly used to detect glucose in urine?

c) Glucose-oxidase test. ***Glucose-oxidase test*** - The **glucose-oxidase test** is a highly specific and sensitive enzymatic test used to detect **glucose** in urine. - It uses the enzyme glucose oxidase which specifically catalyzes the oxidation of glucose to gluconic acid and hydrogen peroxide, which then produces a color change. - This is the **most commonly used method** in modern clinical practice for detecting glucosuria due to its **high specificity for glucose** and ease of use (dipstick method). - It is the preferred test for **monitoring diabetes** and screening for hyperglycemia. *Benedict's test* - **Benedict's test** is a general chemical test for **all reducing sugars** (glucose, fructose, galactose, lactose, maltose), not specifically glucose. - It works by reducing copper sulfate (Cu²⁺) to copper oxide (Cu⁺) in an alkaline solution, forming a colored precipitate (green, yellow, orange, or brick-red depending on sugar concentration). - While it can detect glucose, it **lacks specificity** and can give false positives with other reducing substances (vitamin C, certain drugs), making it less suitable for routine clinical testing. *Fehling's solution* - **Fehling's solution** is also a general chemical test for **reducing sugars** based on copper reduction, similar to Benedict's test. - It consists of two solutions mixed before use and detects various reducing sugars, not just glucose. - It is **not commonly used in clinical urine analysis** due to lack of specificity and the need for heating and mixing two solutions, making it impractical compared to the simple glucose-oxidase dipstick. *None of the above* - This option is incorrect because the **glucose-oxidase test** is indeed the most commonly used test for detecting glucose in urine in modern clinical practice.

Q: What is the typical Q10 value for enzymatic reactions?

2. ***2*** - The **Q10 value** represents the factor by which the rate of a reaction increases for every 10°C rise in temperature. - For most enzymatic and biological reactions, the **Q10 value** is typically around **2 to 3**. *3* - While **3** is within the typical range for some biological reactions, **2** is often considered the most common or average value cited for enzymatic reactions. - A **Q10 of 3** means the reaction rate triples with a 10°C increase, which is observed in certain cases but is not the most general "typical" value. *4* - A **Q10 value of 4** indicates a significantly higher temperature sensitivity than what is commonly observed for most enzymatic reactions. - Such a high Q10 would imply that the reaction rate quadruples for every 10°C increase, which is less typical. *5* - A **Q10 value of 5** is exceptionally high and rarely observed for common enzymatic reactions under physiological conditions. - This would suggest an extreme sensitivity to temperature changes, which is not characteristic of most enzyme kinetics.

Question 1

Which of the following is an aldose?

Accepted Answer

Glucose

Answer

Fructose

Answer

Erythrulose

Answer

None of the options

Question 2

Phosphofructokinase-1 occupies a key position in regulating glycolysis and is also subjected to feedback control. Which among the following are the allosteric activators of phosphofructokinase-1?

Accepted Answer

Fructose 2,6-bisphosphate

Answer

2,3-Bisphosphoglycerate (2,3-BPG)

Answer

Glucokinase

Answer

Phosphoenolpyruvate (PEP)

Question 3

Hexokinase is inhibited by?

Accepted Answer

Glucose-6-phosphate (G6P)

Answer

Glucose

Answer

Insulin

Answer

Glucagon

Question 4

Gluconeogenesis occurs in all except:

Accepted Answer

Muscle

Answer

Kidney

Answer

Gut

Answer

Liver

Question 5

Which of the following statements about gluconeogenesis is correct?

Accepted Answer

Occurs mainly in the liver

Answer

It uses exactly the same enzymes as glycolysis in reverse

Answer

It only occurs during fed state when insulin levels are high

Answer

Fatty acids are the primary substrate for gluconeogenesis

Question 6

Which of the following is NOT required for gluconeogenesis from lactate?

Accepted Answer

Transamination of pyruvate to alanine

Answer

Transport of lactate from muscle to liver

Answer

Conversion of lactate to pyruvate

Answer

None of the above

Question 7

Which of the following tissues relies EXCLUSIVELY on anaerobic glycolysis for ATP production?

Accepted Answer

Mature RBCs (exclusively anaerobic)

Answer

Skeletal muscle during exercise (anaerobic)

Answer

Liver hepatocytes (primarily aerobic)

Answer

Cardiac muscle (primarily aerobic)

Question 8

The energy for glycogenesis is provided by -

Accepted Answer

UTP

Answer

GTP

Answer

GDP

Answer

AMP

Question 9

Which of the following tests is most commonly used to detect glucose in urine?

Accepted Answer

c) Glucose-oxidase test

Answer

a) Benedicts test

Answer

b) Fehling solution

Answer

d) None of the above

Question 10

What is the typical Q10 value for enzymatic reactions?

Accepted Answer

2

Answer

3

Answer

4

Answer

5

NEET-PG 2013 — Biochemistry