Which of the following statements about the enzymes involved in the conversion of glucose to glucose-6-phosphate in glycolysis is true?
Most important carbohydrate store for maintaining blood glucose homeostasis -
How many isoenzymes does lactate dehydrogenase (LDH) have?
Bile acids are synthesized from ?
Which method is used to separate a mixture of lipids?
Transport of lipids from the intestine to other tissues is by -
What primarily forms the core of chylomicrons?
How many molecules of Acetyl CoA are produced from β-oxidation of palmitic acid?
Which of the following statements about Niemann-Pick disease is false?
Which protein is responsible for conserving iron in the body?
NEET-PG 2015 - Biochemistry NEET-PG Practice Questions and MCQs
Question 21: Which of the following statements about the enzymes involved in the conversion of glucose to glucose-6-phosphate in glycolysis is true?
- A. Glucokinase is induced by insulin. (Correct Answer)
- B. Hexokinase is specific for glucose.
- C. Glucokinase is inhibited by glucose-6-phosphate.
- D. Hexokinase has a high Km for glucose.
Explanation: ***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.
Question 22: Most important carbohydrate store for maintaining blood glucose homeostasis -
- A. Blood glucose
- B. Glycogen in adipose tissue
- C. Hepatic glycogen (Correct Answer)
- D. None of the options
Explanation: ***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.
Question 23: How many isoenzymes does lactate dehydrogenase (LDH) have?
- A. 5, based on H and M polypeptide subunits (Correct Answer)
- B. 7, based on H and M polypeptide subunits
- C. 9, based on H and M polypeptide subunits
- D. 3, based on H and M polypeptide subunits
Explanation: **5, based on H and M polypeptide subunits** - **Lactate dehydrogenase (LDH)** is a tetrameric enzyme, meaning it is composed of four polypeptide subunits. - These subunits can be either **H (heart)** type or **M (muscle)** type, leading to five distinct isoenzymes (**LDH-1, LDH-2, LDH-3, LDH-4, LDH-5**) based on their combinations (HHHH, HHHM, HHMM, HMMM, MMMM). *7, based on H and M polypeptide subunits* - While LDH is composed of two types of subunits, H and M, the possible combinations of these four subunits result in **five distinct isoenzymes**, not seven. - Seven isoenzymes are not a recognized number for LDH. *9, based on H and M polypeptide subunits* - The combination of two types of subunits in a tetrameric structure cannot yield nine unique isoenzymes. - This number is incorrect and not supported by the biochemistry of LDH. *3, based on H and M polypeptide subunits* - Three isoenzymes would imply either fewer than four subunits or a more restricted combination, which is not the case for LDH's tetrameric structure with H and M subunits. - This number is insufficient to account for all possible combinations.
Question 24: Bile acids are synthesized from ?
- A. Heme
- B. Ribulose
- C. Arachidonic acid
- D. Cholesterol (Correct Answer)
Explanation: ***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.
Question 25: Which method is used to separate a mixture of lipids?
- A. Electrophoresis
- B. Chromatography (Correct Answer)
- C. Isoelectric focusing
- D. PAGE
Explanation: ***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.
Question 26: Transport of lipids from the intestine to other tissues is by -
- A. Chylomicrons (Correct Answer)
- B. LDL
- C. HDL
- D. VLDL
Explanation: ***Chylomicrons*** - **Chylomicrons** are the **largest lipoprotein particles** that transport **dietary (exogenous) lipids** from the **intestine** to peripheral tissues - They are synthesized in **intestinal enterocytes** after fat absorption and enter the bloodstream via the **lymphatic system (thoracic duct)** - They carry **triglycerides (85-95%), cholesterol, phospholipids, and fat-soluble vitamins** (A, D, E, K) - **Apolipoprotein B-48** is the characteristic structural protein of chylomicrons - After delivering triglycerides to tissues (via lipoprotein lipase), chylomicron remnants are taken up by the **liver** *LDL (Low-Density Lipoprotein)* - LDL transports **cholesterol from the liver to peripheral tissues** (not from intestine) - It carries **endogenous cholesterol**, not dietary lipids from the intestine - Often called "**bad cholesterol**" due to its role in atherosclerosis - Contains **Apolipoprotein B-100** *HDL (High-Density Lipoprotein)* - HDL performs **reverse cholesterol transport** - moving excess cholesterol from peripheral tissues **back to the liver** - It does **not transport lipids from the intestine** to tissues - Called "**good cholesterol**" for its protective cardiovascular role - Contains **Apolipoprotein A-I and A-II** *VLDL (Very-Low-Density Lipoprotein)* - VLDL is synthesized in the **liver** (not intestine) and transports **endogenous triglycerides** to peripheral tissues - It carries lipids **from the liver**, not from the intestine - VLDL is converted to IDL and then LDL after losing triglycerides - Contains **Apolipoprotein B-100**
Question 27: What primarily forms the core of chylomicrons?
- A. Triglycerides and Cholesterol together
- B. Triglycerides (Correct Answer)
- C. Free fatty acids
- D. Triglyceride, Cholesterol and Phospholipids
Explanation: ***Triglycerides*** - Chylomicrons are primarily responsible for transporting **dietary triglycerides** from the intestines to other tissues. - Their large core, composed mainly of **triglycerides**, allows efficient transport of these hydrophobic molecules. *Triglycerides and Cholesterol together* - While **cholesterol** is present in chylomicrons, it is less abundant than **triglycerides** and primarily exists as **cholesterol esters** in the core. - The core is not an equal mixture; **triglycerides** overwhelmingly dominate the volume. *Free fatty acids* - **Free fatty acids** are transported in the blood primarily bound to **albumin**, not within the core of chylomicrons. - Chylomicrons typically carry **esterified fatty acids** as part of triglycerides. *Triglyceride, Cholesterol and Phospholipids* - **Phospholipids** form the outer monolayer of the chylomicron, along with apoproteins, making them **amphipathic**. - They do not constitute a core component but rather the **surface interface** with the aqueous environment.
Question 28: How many molecules of Acetyl CoA are produced from β-oxidation of palmitic acid?
- A. 3 acetyl CoA
- B. 16 Acetyl CoA
- C. 6 acetyl CoA
- D. 8 acetyl CoA (Correct Answer)
Explanation: ***8 acetyl CoA*** - Palmitic acid is a **16-carbon saturated fatty acid (C16:0)**. During β-oxidation, each cycle cleaves two carbons as **acetyl CoA**. - The formula for acetyl CoA produced is **n/2**, where n = number of carbons. For palmitic acid: 16/2 = **8 acetyl CoA molecules**. - Alternatively: Palmitic acid undergoes **7 cycles of β-oxidation** [(n/2) - 1 = 7], each producing 1 acetyl CoA (7 total), plus the final 2-carbon fragment forming the 8th acetyl CoA. *3 acetyl CoA* - This number is too low for a 16-carbon fatty acid. **Short-chain fatty acids** would produce fewer acetyl CoA molecules. - This value corresponds to β-oxidation of a **6-carbon fatty acid** (hexanoic acid), not palmitic acid. *6 acetyl CoA* - This number is also too low for a 16-carbon fatty acid. - This quantity would be produced from a **12-carbon fatty acid** (lauric acid), not palmitic acid. *16 Acetyl CoA* - This number is too high and would incorrectly imply that each carbon forms an acetyl CoA independently. - Sixteen acetyl CoA molecules would be produced from a **32-carbon fatty acid**, which is extremely rare in biological systems.
Question 29: Which of the following statements about Niemann-Pick disease is false?
- A. Due to deficiency of sphingomyelinase.
- B. CNS symptoms are present in type A.
- C. Type B Niemann-Pick disease is characterized by severe neurological symptoms. (Correct Answer)
- D. Histiocytes show PAS positive inclusions, and Type A is more severe.
Explanation: ***Type B Niemann-Pick disease is characterized by severe neurological symptoms.*** - This statement is **false** because **Type B Niemann-Pick disease** generally presents with **visceral involvement** (e.g., hepatosplenomegaly, lung disease) with **minimal to no neurological symptoms**. - **Severe neurological symptoms** are characteristic of **Type A Niemann-Pick disease**, which involves widespread CNS degeneration and a more rapidly progressive course. *Due to deficiency of sphingomyelinase.* - This statement is **true**. - Niemann-Pick disease (Types A and B) is caused by a deficiency of the enzyme **acid sphingomyelinase**, leading to the accumulation of sphingomyelin within lysosomes, particularly in macrophages. *CNS symptoms are present in type A.* - This statement is **true**. - **Type A Niemann-Pick disease** is the most severe form and is characterized by significant **neurodegeneration** in addition to visceral involvement. - Patients typically present with **developmental regression**, **ataxia**, and **spasticity** due to extensive sphingomyelin deposition in the central nervous system. *Histiocytes show PAS positive inclusions, and Type A is more severe.* - This statement is **true**. - The characteristic "foam cells" (lipid-laden macrophages/histiocytes) found in tissues of Niemann-Pick patients stain positive with **periodic acid–Schiff (PAS)** due to accumulated sphingomyelin. - **Type A Niemann-Pick disease** is indeed the most severe form, with a rapidly progressive course and early fatality, usually by early childhood.
Question 30: Which protein is responsible for conserving iron in the body?
- A. Ferritin (Correct Answer)
- B. Hepcidin
- C. Hemopexin
- D. Transferrin
Explanation: ***Ferritin*** - **Ferritin** is the primary intracellular protein that **conserves iron** in the body by storing it in a safe, non-toxic, and bioavailable form - It is found mainly in the liver, spleen, and bone marrow, serving as the body's **iron reserve** - When iron is abundant, ferritin stores it; when iron is needed, ferritin releases it, thus **conserving iron for future use** - Serum ferritin levels directly reflect total body iron stores *Hepcidin* - **Hepcidin** is a regulatory peptide hormone that controls iron homeostasis by inhibiting **ferroportin**, the iron export channel - It reduces iron absorption from the intestine and iron release from macrophages during inflammation or iron overload - While it regulates iron distribution, it is a hormone, not a storage protein, and does not directly conserve iron within cells *Hemopexin* - **Hemopexin** binds free **heme** in plasma, preventing oxidative damage and delivering it to the liver for catabolism - It helps recover iron from heme but does not store or conserve iron in the body *Transferrin* - **Transferrin** is a plasma protein that **transports iron** from absorption sites (intestine) and storage sites (liver, spleen) to tissues that need it - Its role is iron delivery, not conservation or storage