Which of the following substances does not inhibit glycolysis?
Ochronosis is due to accumulation of ?
Mutation in GLUT-2 causes which syndrome?
Which of the following is monoenoic acid ?
Which of the following fatty acids has the maximum number of carbon atoms?
Chylomicron remnants are associated with ?
Which of the following statements is true regarding medium chain fatty acids?
In which condition does serum appear milky white?
Which of the following is a non-essential amino acid?
Glucose is converted to glucuronate by which process?
NEET-PG 2012 - Biochemistry NEET-PG Practice Questions and MCQs
Question 31: Which of the following substances does not inhibit glycolysis?
- A. Fluoride
- B. Arsenite
- C. Iodoacetate
- D. Fluoroacetate (Correct Answer)
Explanation: ***Fluoroacetate*** - **Fluoroacetate** is not a direct inhibitor of glycolysis. Instead, it is metabolized to **fluorocitrate**, which then acts as an inhibitor of **aconitase** in the **Krebs cycle (TCA cycle)**, thereby affecting cellular respiration at a later stage. - Its primary role in metabolic inhibition is within the **mitochondria**, impacting energy production via the TCA cycle rather than the glycolytic pathway. *Fluoride* - **Fluoride** is a known inhibitor of **enolase**, an enzyme in the penultimate step of glycolysis. - It forms a complex with **magnesium** and **phosphate** to block the active site of enolase, preventing the conversion of 2-phosphoglycerate to phosphoenolpyruvate. *Arsenite* - **Arsenite** inhibits glycolysis by targeting enzymes containing **sulfhydryl (–SH) groups**, particularly **glyceraldehyde-3-phosphate dehydrogenase (GAPDH)**, a critical enzyme in the glycolytic pathway. - It also inhibits the **pyruvate dehydrogenase complex** (linking glycolysis to the TCA cycle) and TCA cycle enzymes like **α-ketoglutarate dehydrogenase**, thereby affecting multiple stages of cellular respiration. *Iodoacetate* - **Iodoacetate** is a potent inhibitor of the enzyme **glyceraldehyde-3-phosphate dehydrogenase (GAPDH)**. - It specifically alkylates the **cysteine residue** at the active site of GAPDH, preventing the conversion of glyceraldehyde-3-phosphate to 1,3-bisphosphoglycerate, thereby blocking glycolysis.
Question 32: Ochronosis is due to accumulation of ?
- A. Homogentisic acid (Correct Answer)
- B. Phenylpyruvate
- C. Xanthurenate
- D. Glyoxylate
Explanation: ***Homogentisic acid*** - Ochronosis is a condition caused by the excessive accumulation of **homogentisic acid** in connective tissues. - This accumulation occurs due to a deficiency of the enzyme **homogentisate 1,2-dioxygenase** (HGD), which is responsible for breaking down homogentisic acid in the tyrosine degradation pathway. *Phenylpyruvate* - **Phenylpyruvate** accumulates in **phenylketonuria (PKU)**, a different metabolic disorder where there's a deficiency in phenylalanine hydroxylase. - PKU leads to intellectual disability and other neurological problems, not the characteristic pigmentation and arthritis of ochronosis. *Xanthurenate* - **Xanthurenate** is an abnormal metabolite of tryptophan metabolism that can accumulate in **vitamin B6 deficiency**. - Its accumulation is associated with increased urinary excretion and is not directly implicated in the pathology of ochronosis. *Glyoxylate* - **Glyoxylate** is involved in various metabolic pathways, and its accumulation (or accumulation of its metabolic products like oxalate) is associated with **primary hyperoxaluria**. - Primary hyperoxaluria leads to kidney stones and kidney failure, a condition distinct from ochronosis.
Question 33: Mutation in GLUT-2 causes which syndrome?
- A. Dandy walker syndrome
- B. Beckwith-Wiedemann syndrome
- C. Menke's disease
- D. Fanconi-Bickel syndrome (Correct Answer)
Explanation: ***Fanconi-Bickel syndrome*** - This syndrome is caused by a **mutation in the GLUT-2 gene**, leading to dysfunctional glucose transport in the liver, kidneys, and intestines. - Key features include **hepatorenal glycogen accumulation**, **renal tubulopathy** (Fanconi syndrome), and **impaired glucose and galactose utilization**. *Dandy-Walker syndrome* - This is a **congenital brain malformation** involving the cerebellum and fourth ventricle. - It is often associated with hydrocephalus, but not directly linked to glucose transporter defects. *Beckwith-Wiedemann syndrome* - This is an **overgrowth disorder** characterized by a high risk of childhood cancer and congenital anomalies. - It is primarily caused by genetic abnormalities on **chromosome 11p15.5** and is unrelated to GLUT-2 mutations. *Menke's disease* - This is a rare X-linked recessive disorder of **copper metabolism**, leading to severe neurological degeneration. - It results from mutations in the **ATP7A gene**, which encodes a copper-transporting ATPase.
Question 34: Which of the following is monoenoic acid ?
- A. Linoleic acid
- B. Oleic acid (Correct Answer)
- C. Linolenic acid
- D. Arachidonic acid
Explanation: ***Oleic acid*** - **Oleic acid** is a **monounsaturated fatty acid** (MUFA), meaning it has **one double bond** in its hydrocarbon chain. - Its presence in many natural fats and oils makes it a significant component of the human diet. *Arachidonic acid* - **Arachidonic acid** is a **polyunsaturated fatty acid** (PUFA) containing **four double bonds**. - It is a precursor for **eicosanoids**, including prostaglandins and leukotrienes, involved in inflammation and other physiological processes. *Linoleic acid* - **Linoleic acid** is an **essential omega-6 polyunsaturated fatty acid** with **two double bonds**. - It is crucial for human health and serves as a precursor for other fatty acids like arachidonic acid. *Linolenic acid* - **Linolenic acid** refers to two essential fatty acids: **alpha-linolenic acid (ALA)**, an omega-3 fatty acid with **three double bonds**, and **gamma-linolenic acid (GLA)**, an omega-6 fatty acid also with three double bonds. - Both are **polyunsaturated fatty acids** with multiple double bonds.
Question 35: Which of the following fatty acids has the maximum number of carbon atoms?
- A. Oleic acid
- B. Linolenic acid
- C. Arachidonic acid
- D. Cervonic acid (Correct Answer)
Explanation: **Cervonic acid** - **Cervonic acid**, also known as **docosahexaenoic acid (DHA)**, is a long-chain omega-3 fatty acid with **22 carbon atoms** and 6 double bonds (22:6). - It is a primary structural component of the brain and retina and is the longest fatty acid among the options provided. *Oleic acid* - **Oleic acid** is a monounsaturated fatty acid with **18 carbon atoms** and one double bond (18:1). - It is a common fatty acid found in many animal fats and vegetable oils, but it has fewer carbon atoms than cervonic acid. *Linolenic acid* - **Linolenic acid** refers to two essential fatty acids: alpha-linolenic acid (ALA) and gamma-linolenic acid (GLA). Both have **18 carbon atoms**. - Alpha-linolenic acid (ALA) is an omega-3 fatty acid with 3 double bonds (18:3), while gamma-linolenic acid (GLA) is an omega-6 fatty acid with 3 double bonds (18:3), neither of which has more carbon atoms than cervonic acid. *Arachidonic acid* - **Arachidonic acid** is an omega-6 fatty acid with **20 carbon atoms** and four double bonds (20:4). - It is a precursor to eicosanoids and is longer than oleic and linolenic acids but shorter than cervonic acid.
Question 36: Chylomicron remnants are associated with ?
- A. Apo-C
- B. Apo-A
- C. Apo-E (Correct Answer)
- D. Apo-B100
Explanation: ***Apo-E*** - **Apolipoprotein E (Apo-E)** is a crucial apolipoprotein on the surface of chylomicron remnants, acting as a **ligand for the LDL receptor-related protein 1 (LRP1)** in the liver. - This binding facilitates the **hepatic uptake and clearance** of chylomicron remnants from circulation. *Apo-A* - **Apo-AI** is the primary apolipoprotein of **HDL** and plays a key role in reverse cholesterol transport by activating **lecithin-cholesterol acyltransferase (LCAT)**. - While chylomicrons *acquire* some Apo-AI from HDL, it is not the primary apolipoprotein defining their remnants' hepatic clearance. *Apo-C* - **Apo-CII** is a vital activator of **lipoprotein lipase (LPL)**, which metabolizes triglycerides in chylomicrons and VLDL. - **Apo-CIII** inhibits LPL and hinders receptor-mediated uptake, but **Apo-E** is the key for remnant recognition and uptake, not Apo-C in general. *Apo-B100* - **Apo-B100** is the main structural apolipoprotein of **LDL** and **VLDL**, serving as the ligand for the LDL receptor, mediating their hepatic uptake. - While chylomicrons have **Apo-B48**, which is a truncated form of Apo-B100, Apo-B100 itself is not found on chylomicron remnants.
Question 37: Which of the following statements is true regarding medium chain fatty acids?
- A. All of the options are true (Correct Answer)
- B. Do not require pancreatic lipase for digestion
- C. Absorb directly into portal circulation
- D. Are less likely to be deposited in adipose tissue compared to long-chain fatty acids
Explanation: ***All of the options are true*** - This option is correct because medium-chain fatty acids (MCFAs) possess unique metabolic properties that differentiate them from long-chain fatty acids (LCFAs), making all listed statements accurate. - Their shorter chain length allows for distinct digestion, absorption, and metabolic fates, which are beneficial in various clinical contexts. *Do not require pancreatic lipase for digestion* - MCFAs have **shorter carbon chains** (typically 6-12 carbons) and are more hydrophilic than LCFAs. - This property allows them to be digested by **lingual and gastric lipases** to a greater extent, reducing the reliance on pancreatic lipase. *Absorb directly into portal circulation* - Unlike LCFAs, which are re-esterified into triglycerides, packaged into **chylomicrons**, and absorbed into the lymphatic system, MCFAs are absorbed directly into the **portal vein**. - This bypasses the lymphatic system and directly transports them to the liver, making them a rapid energy source. *Are less likely to be deposited in adipose tissue compared to long-chain fatty acids* - MCFAs are **rapidly oxidized** in the liver for energy via beta-oxidation and are less likely to be stored as triglycerides in adipose tissue. - They are also not efficiently utilized for the synthesis of complex lipids or stored fat due to their unique metabolic pathway and preference for oxidation.
Question 38: In which condition does serum appear milky white?
- A. Increased LDL
- B. Increased HDL
- C. Increased VLDL
- D. Increased Chylomicrons (Correct Answer)
Explanation: ***Increased Chylomicrons*** - **Chylomicrons** are the largest lipoprotein particles (75-1200 nm) with the highest **triglyceride content (85-95%)**, giving serum a characteristic **milky white** or "creamy" appearance - This intense milky appearance occurs after **fatty meals** (postprandial lipemia) or in **Type I and V hyperlipidemias** (familial chylomicronemia syndrome) - The **light scattering** by these large particles makes the serum completely opaque, distinguishing it from other lipid abnormalities - Classic clinical finding: **"cream layer" forms on top** when lipemic serum stands overnight in refrigerator *Increased LDL* - Elevated **Low-Density Lipoprotein (LDL)** produces **clear to slightly hazy** serum, never milky white - LDL particles are much smaller (18-25 nm) than chylomicrons and contain primarily **cholesterol**, not triglycerides - High LDL is a cardiovascular risk factor but does not cause visible lipemia *Increased HDL* - **High-Density Lipoprotein (HDL)** elevation results in **clear serum** - HDL particles are the smallest (5-12 nm) and densest lipoproteins - High HDL is protective and causes no turbidity *Increased VLDL* - **Very Low-Density Lipoprotein (VLDL)** elevation can cause **turbid or hazy** serum in severe hypertriglyceridemia, but typically less intensely milky than chylomicrons - VLDL particles are smaller (30-80 nm) than chylomicrons with lower triglyceride content (50-65%) - In Type IV hyperlipidemia (isolated VLDL elevation), serum appears uniformly turbid without cream layer formation - The most dramatic "milky white" appearance is specifically associated with **chylomicronemia**
Question 39: Which of the following is a non-essential amino acid?
- A. Tyrosine (Correct Answer)
- B. Phenylalanine
- C. Lysine
- D. Threonine
Explanation: ***Tyrosine*** - **Tyrosine** is considered a **non-essential amino acid** because the human body can synthesize it from the essential amino acid **phenylalanine**. - This synthesis occurs via the enzyme **phenylalanine hydroxylase**, making its dietary intake not strictly necessary if phenylalanine is available. *Phenylalanine* - **Phenylalanine** is an **essential amino acid**, meaning the human body **cannot synthesize it** and it must be obtained through the diet. - It serves as a precursor for various important molecules, including tyrosine, contributing to neurotransmitter synthesis. *Lysine* - **Lysine** is an **essential amino acid** that the human body **cannot synthesize** and must be acquired from dietary sources. - It plays a crucial role in **protein synthesis**, calcium absorption, and the production of hormones and enzymes. *Threonine* - **Threonine** is another example of an **essential amino acid** that the human body is **unable to produce** on its own. - It is important for the formation of **collagen** and elastin, and contributes to immune function.
Question 40: Glucose is converted to glucuronate by which process?
- A. Oxidation of aldehyde group
- B. Oxidation of both
- C. Oxidation of the terminal alcohol group (Correct Answer)
- D. None of the options
Explanation: ***Oxidation of the terminal alcohol group*** - **Glucuronate** is formed by the **oxidation of the C-6 carbon** (the terminal primary alcohol group) of glucose. - This process is crucial for the detoxification of various substances in the body, as glucuronate is a key component in **glucuronidation reactions**. *Oxidation of aldehyde group* - Oxidation of the **aldehyde group (C-1)** of glucose typically forms **gluconic acid**, not glucuronate. - Gluconate is derived from the oxidation of the first carbon, while glucuronate is derived from the oxidation of the last carbon. *Oxidation of both* - If both the aldehyde group (C-1) and the terminal alcohol group (C-6) of glucose were oxidized, it would result in the formation of **glucaric acid** (saccharic acid), not glucuronate. - Glucaric acid has two carboxyl groups, one at each end of the molecule. *None of the options* - This option is incorrect because the specific biochemical pathway for glucuronate formation involves the oxidation of the terminal alcohol group. - The conversion of glucose to glucuronate is a well-established metabolic process.