NEET-PG 2015 - Biochemistry Practice Questions

Q: Which of the following processes primarily utilizes lactate produced anaerobically?

Cori cycle. ***Cori cycle*** - The **Cori cycle** (lactic acid cycle) involves the transport of **lactate** produced during anaerobic metabolism in muscles to the liver. - In the **liver**, this lactate is then converted back to **glucose** via gluconeogenesis, which can be returned to the muscles. *Gluconeogenesis* - **Gluconeogenesis** is the synthesis of glucose from non-carbohydrate precursors, one of which is lactate. - While it uses lactate, it is only one component of the broader **Cori cycle**, which describes the inter-organ cooperation. *Glycolysis* - **Glycolysis** is the metabolic pathway that breaks down glucose into pyruvate, which can then be converted to lactate under anaerobic conditions. - This process *produces* lactate but does not *utilize* it, acting upstream of lactate production. *TCA cycle* - The **TCA cycle** (Krebs cycle) is a central part of aerobic respiration that oxidizes acetyl-CoA to produce ATP, NADH, and FADH2. - It does not directly utilize lactate; instead, lactate is typically converted to pyruvate before potentially entering the TCA cycle under aerobic conditions.

Q: Citrate synthase is inhibited by -

ATP. ***ATP*** - **Citrate synthase**, a key enzyme in the Krebs cycle, is inhibited by **high levels of ATP**, indicating a high energy state in the cell. - This allosteric inhibition helps regulate the metabolic flux through the cycle, slowing it down when energy is abundant. *ADP* - **ADP** typically signifies a low energy state and would generally act as an **activator** rather than an inhibitor for metabolic pathways that produce ATP. - In this context, ADP would promote the activity of enzymes involved in energy generation, including those in the Krebs cycle. *Insulin* - **Insulin** is a hormone that promotes fuel storage and utilization, generally **activating** metabolic pathways rather than directly inhibiting enzymes like citrate synthase. - Its primary role is to regulate blood glucose levels and promote glucose uptake and utilization. *Glucagon* - **Glucagon** is a hormone that mobilizes fuel from storage and is typically associated with **catabolic processes**, often increasing metabolic activity in response to low blood glucose. - It does not directly inhibit citrate synthase; its main actions are on glucoregulation.

Q: Fumarate is formed from which amino acid?

Tyrosine. ***Tyrosine*** - **Tyrosine** is a **glucogenic and ketogenic amino acid** that is catabolized to acetoacetate and fumarate. - **Fumarate** then enters the **citric acid cycle (Krebs cycle)**, whereas acetoacetate is a ketone body. *Methionine* - **Methionine** is an **essential amino acid** and a precursor for **S-adenosylmethionine (SAM)**, a methyl donor in many reactions. - Its catabolism produces **succinyl CoA**, not fumarate, through a series of steps via propionyl CoA. *Valine* - **Valine** is a **branched-chain amino acid (BCAA)** that is exclusively **glucogenic**. - Its catabolism ultimately leads to the formation of **succinyl CoA**, which can enter the citric acid cycle. *Histidine* - **Histidine** is an **essential amino acid** that is catabolized to **formiminoglutamate (FIGLU)**. - FIGLU is then converted to **glutamate**, which can eventually be deaminated to α-ketoglutarate, a citric acid cycle intermediate, but not directly fumarate.

Q: Enzyme deficient in Hers disease -

Liver phosphorylase. ***Liver phosphorylase*** - Hers disease, also known as Glycogen Storage Disease Type VI, is specifically caused by a deficiency of **liver phosphorylase**. - This enzyme is crucial for the breakdown of **glycogen in the liver**, leading to an inability to release glucose into the bloodstream during fasting. *Muscle phosphorylase* - Deficiency of **muscle phosphorylase** (myophosphorylase) causes **McArdle disease** (Glycogen Storage Disease Type V), which primarily affects muscle energy. - Patients typically present with exercise intolerance, muscle pain, and cramps, not the hepatic symptoms seen in Hers disease. *Acid maltase* - Deficiency of **acid maltase** (also known as alpha-glucosidase) is responsible for **Pompe disease** (Glycogen Storage Disease Type II), a lysosomal storage disorder. - This enzyme deficiency leads to glycogen accumulation in lysosomes in various tissues, including muscle, liver, and heart, causing muscle weakness and cardiomyopathy. *Debranching enzyme* - A deficiency in the **debranching enzyme** (amylo-1,6-glucosidase) causes **Cori disease** or **Forbes disease** (Glycogen Storage Disease Type III). - This results in the accumulation of abnormally structured glycogen with short outer branches in the liver, muscle, and heart.

Q: Which of the following statements about the enzymes involved in the conversion of glucose to glucose-6-phosphate in glycolysis is true?

Glucokinase is induced by insulin.. ***Glucokinase is induced by insulin.*** - **Insulin** promotes glucose uptake and utilization in the liver and pancreatic beta cells, where glucokinase is primarily expressed. - Induction of **glucokinase** by insulin ensures that glucose is efficiently phosphorylated and trapped within hepatocytes when blood glucose levels are high. - This is a key mechanism for postprandial glucose homeostasis. *Incorrect: Hexokinase is specific for glucose.* - **Hexokinase** is NOT specific for glucose; it can phosphorylate various hexoses including **fructose**, **mannose**, and **galactose**. - Its broad substrate specificity distinguishes it from glucokinase, which has greater specificity for glucose. *Incorrect: Glucokinase is inhibited by glucose-6-phosphate.* - Unlike **hexokinase**, which is subject to product inhibition by glucose-6-phosphate, **glucokinase is NOT inhibited** by its product. - This lack of feedback inhibition allows glucokinase to continue phosphorylating glucose even when glucose-6-phosphate levels are elevated, which is appropriate for its role as a glucose sensor in liver and pancreatic beta cells. *Incorrect: Hexokinase has a high Km for glucose.* - **Hexokinase** has a **low Km** (~0.1 mM) for glucose, meaning it has high affinity and is saturated at normal blood glucose levels. - In contrast, **glucokinase** has a high Km (~10 mM), allowing it to respond proportionally to changes in blood glucose concentration.

Q: Most important carbohydrate store for maintaining blood glucose homeostasis -

Hepatic glycogen. ***Hepatic glycogen*** - The liver contains **100-120g of glycogen**, which is the most crucial carbohydrate store for **maintaining blood glucose homeostasis**. - **Hepatic glycogen** can be mobilized and released as glucose into the bloodstream to supply all body tissues, especially during fasting. - Although muscle glycogen is quantitatively larger (~400-500g), it cannot contribute to blood glucose as muscle lacks glucose-6-phosphatase. - The liver's unique ability to release free glucose makes hepatic glycogen the **most metabolically important** carbohydrate store. *Blood glucose* - **Blood glucose** (~5g total in circulation) represents carbohydrates available for immediate energy, not a storage form. - This is far too small to be considered a major carbohydrate reserve. *Glycogen in adipose tissue* - **Adipose tissue** primarily stores **fat (triglycerides)**, with negligible glycogen content. - Adipose tissue plays virtually no role in carbohydrate storage. *None of the options* - This is incorrect because **hepatic glycogen** is indeed the most important carbohydrate store for glucose homeostasis.

Q: Most abundant source of fuel in starvation -

Adipose tissue. ***Adipose tissue*** - **Adipose tissue** stores **triglycerides**, which are hydrolyzed into fatty acids and glycerol to serve as the body's primary energy source during prolonged starvation. - The energy reserve in adipose tissue is significantly larger than glycogen stores, providing **sustained fuel** for days or weeks. *Liver glycogen* - **Liver glycogen** is a readily available source of glucose but is rapidly depleted within **12-24 hours** during starvation. - Its primary role is to maintain **blood glucose levels** for glucose-dependent tissues like the brain. *Muscle glycogen* - **Muscle glycogen** is used primarily for **muscle contraction** and cannot be directly released into the bloodstream to maintain blood glucose levels. - While it's a significant energy reserve for working muscles, it does not contribute to systemic fuel needs during starvation. *Blood glucose* - **Blood glucose** is the immediate circulating fuel, but it is tightly regulated and its levels decrease during starvation as glycogen stores are depleted. - It is not an abundant stored source of fuel but rather a transport form of energy.

Q: Bile acids are synthesized from ?

Cholesterol. ***Cholesterol*** - **Bile acids** are derivatives of **cholesterol**, synthesized in the liver through a multi-step enzymatic pathway. - The conversion of cholesterol to bile acids is a primary mechanism for the excretion and transport of cholesterol from the body. *Heme* - **Heme** is a component of hemoglobin and myoglobin, primarily involved in oxygen transport and storage. - Its degradation product is **bilirubin**, which forms part of bile but is distinct from bile acids. *Ribulose* - **Ribulose** is a 5-carbon sugar, playing a key role in the **pentose phosphate pathway** and the **Calvin cycle** in photosynthesis. - It is not a precursor for bile acid synthesis. *Arachidonic acid* - **Arachidonic acid** is a polyunsaturated fatty acid that serves as a precursor for **eicosanoids** (prostaglandins, thromboxanes, and leukotrienes). - These molecules are involved in inflammation and immune responses but are unrelated to bile acid synthesis.

Q: Which method is used to separate a mixture of lipids?

Chromatography. ***Chromatography*** - **Chromatography** (e.g., thin-layer chromatography, gas chromatography, high-performance liquid chromatography) is widely used to separate lipids based on differences in their **polarity**, **molecular weight**, or **solubility** in various solvents. - This method allows for the isolation and identification of different lipid classes and individual lipid species from a complex mixture. *Electrophoresis* - **Electrophoresis** separates molecules based on their **charge** and **size** in an electric field, making it more commonly used for proteins and nucleic acids. - Lipids are generally **uncharged** or have very low charge, which makes them poorly suited for separation by standard electrophoretic methods without modification. *Isoelectric focusing* - **Isoelectric focusing** is a type of electrophoresis that separates molecules based on their **isoelectric point (pI)**, which is the pH at which a molecule has no net charge. - This technique is primarily used for **proteins** and **peptides**, as lipids typically lack ionizable groups necessary for establishing a distinct pI. *PAGE* - **PAGE** (Polyacrylamide Gel Electrophoresis) is a common method used to separate **proteins** and **nucleic acids** based on their size and charge. - Lipids are **hydrophobic** and do not readily migrate through an aqueous polyacrylamide gel matrix, making PAGE unsuitable for their direct separation.

Q: Which of the following is an example of an exopeptidase?

Carboxypeptidases. ***Carboxypeptidases*** - **Carboxypeptidases** are enzymes that cleave the **C-terminal** (carboxyl end) amino acid from a polypeptide chain, making them a type of exopeptidase. - They are crucial in protein digestion, releasing individual amino acids from the end of protein chains. *Trypsin* - **Trypsin** is an **endopeptidase** that cleaves peptide bonds within protein chains, specifically at the carboxyl side of **lysine** or **arginine** residues. - It does not cleave amino acids from the ends of polypeptide chains. *Chymotrypsin* - **Chymotrypsin** is an **endopeptidase** that cleaves peptide bonds within a polypeptide chain, primarily at the carboxyl side of **tyrosine**, **tryptophan**, or **phenylalanine**. - Its action is internal to the protein sequence, not at the termini. *Elastase* - **Elastase** is also an **endopeptidase** that cleaves peptide bonds internally, specifically targeting small, uncharged amino acid residues like **alanine**, **glycine**, and **valine**. - Its primary role is to break down elastin, an elastic protein in connective tissues, but it does so by internal cleavage.

Question 1

Which of the following processes primarily utilizes lactate produced anaerobically?

Accepted Answer

Cori cycle

Answer

Gluconeogenesis

Answer

TCA cycle

Answer

Glycolysis

Question 2

Citrate synthase is inhibited by -

Accepted Answer

ATP

Answer

Insulin

Answer

Glucagon

Answer

ADP

Question 3

Fumarate is formed from which amino acid?

Accepted Answer

Tyrosine

Answer

Methionine

Answer

Valine

Answer

Histidine

Question 4

Enzyme deficient in Hers disease -

Accepted Answer

Liver phosphorylase

Answer

Muscle phosphorylase

Answer

Acid maltase

Answer

Debranching enzyme

Question 5

Which of the following statements about the enzymes involved in the conversion of glucose to glucose-6-phosphate in glycolysis is true?

Accepted Answer

Glucokinase is induced by insulin.

Answer

Hexokinase is specific for glucose.

Answer

Glucokinase is inhibited by glucose-6-phosphate.

Answer

Hexokinase has a high Km for glucose.

Question 6

Most important carbohydrate store for maintaining blood glucose homeostasis -

Accepted Answer

Hepatic glycogen

Answer

Blood glucose

Answer

Glycogen in adipose tissue

Answer

None of the options

Question 7

Most abundant source of fuel in starvation -

Accepted Answer

Adipose tissue

Answer

Liver glycogen

Answer

Muscle glycogen

Answer

Blood glucose

Question 8

Bile acids are synthesized from ?

Accepted Answer

Cholesterol

Answer

Heme

Answer

Ribulose

Answer

Arachidonic acid

Question 9

Which method is used to separate a mixture of lipids?

Accepted Answer

Chromatography

Answer

Electrophoresis

Answer

Isoelectric focusing

Answer

PAGE

Question 10

Which of the following is an example of an exopeptidase?

Accepted Answer

Carboxypeptidases

Answer

Trypsin

Answer

Chymotrypsin

Answer

Elastase

NEET-PG 2015 — Biochemistry