Biochemistry
9 questionsWhat is the primary action of metalloproteinases in the extracellular matrix?
In which of the following conditions is protein catabolism MOST increased?
Which porphyrin forms the organic component of heme?
Hay's sulfur test is used to detect which of the following?
Which of the following statements about hemoglobin is true?
Which of the following statements about chaperones is false?
Which of the following is the major glycosaminoglycan of synovial fluid?
What is the mechanism by which mercury causes damage?
Which of the following statements regarding collagen synthesis is incorrect?
NEET-PG 2013 - Biochemistry NEET-PG Practice Questions and MCQs
Question 311: What is the primary action of metalloproteinases in the extracellular matrix?
- A. Modification of collagen structure
- B. Degradation of extracellular matrix components, including collagen (Correct Answer)
- C. Formation of collagen
- D. Activation of collagen synthesis
Explanation: ***Degradation of extracellular matrix components, including collagen*** - **Metalloproteinases (MMPs)** are a family of zinc-dependent endopeptidases that are crucial for breaking down various components of the **extracellular matrix (ECM)**. - This degradation is essential for processes like **tissue remodeling**, development, wound healing, and also plays a role in disease pathogenesis such as metastasis and inflammation. *Formation of collagen* - The formation of collagen is primarily mediated by **fibroblasts** and involves a complex process of synthesis, hydroxylation, glycosylation, and assembly of procollagen molecules, not MMPs. - MMPs act to break down existing collagen, not to create new collagen fibers. *Modification of collagen structure* - While collagen undergoes post-translational modifications (e.g., hydroxylation, glycosylation) within cells, MMPs are involved in cleaving the peptide bonds, leading to **degradation**, rather than structural modification of intact collagen. - Enzymes like **lysyl hydroxylase** and **prolyl hydroxylase** are responsible for modifying collagen structure. *Activation of collagen synthesis* - Collagen synthesis is primarily regulated by various **growth factors (e.g., TGF-β)** and hormones that stimulate fibroblasts to produce collagen. - MMPs are involved in the breakdown of collagen, which is the opposite of activating its synthesis.
Question 312: In which of the following conditions is protein catabolism MOST increased?
- A. Burns (Correct Answer)
- B. Surgery
- C. Starvation
- D. Fever
Explanation: ***Burns*** - Severe burns lead to a profound **hypermetabolic state** with the highest increase in **protein catabolism** among all the options listed. - The extensive tissue damage triggers massive breakdown of muscle protein to provide amino acids for **wound healing**, **acute phase protein synthesis**, and **immune response**. - Burns can increase metabolic rate by **100-200%**, with protein catabolism far exceeding that of other stress conditions. *Starvation* - While starvation initially increases protein catabolism, the body adapts within days by shifting towards **ketone body utilization** to spare protein. - After adaptation, protein breakdown decreases to **20-30 grams per day** to preserve lean body mass. - The goal is survival through metabolic adaptation, not tissue repair. *Surgery* - Major surgery induces a **stress response** that increases protein catabolism, but it is typically less severe and shorter-lived than burns. - The degree of catabolism is proportional to the **magnitude of surgical trauma** and usually resolves within days. - Protein catabolism increases by **50-75%** in major surgery compared to **100-200%** in severe burns. *Fever* - Fever increases basal metabolic rate by approximately **13% per degree Celsius** rise in body temperature. - While metabolism is elevated, protein catabolism is **modest** compared to the massive tissue destruction and repair demands of severe burns. - The increase is primarily in energy expenditure, not protein breakdown.
Question 313: Which porphyrin forms the organic component of heme?
- A. Uroporphyrin
- B. Coproporphyrin
- C. Deuteroporphyrin
- D. Protoporphyrin IX (Correct Answer)
Explanation: ***Protoporphyrin IX*** - **Heme** is formed by the insertion of an **iron atom (Fe2+)** into the center of **protoporphyrin IX**. - **Protoporphyrin IX** is the immediate precursor to heme in the **heme synthesis pathway**. *Uroporphyrin* - **Uroporphyrin** is an earlier precursor in the **heme synthesis pathway** and is much more hydrophilic than protoporphyrin. - It accumulates in diseases like **congenital erythropoietic porphyria (CEP)**, leading to photosensitivity. *Coproporphyrin* - **Coproporphyrin** is an intermediate in the **heme synthesis pathway**, formed after uroporphyrinogen. - It is also more water-soluble than protoporphyrin and its accumulation can be seen in various porphyrias. *Deuteroporphyrin* - **Deuteroporphyrin** is a synthetic porphyrin or a less common natural porphyrin that is not directly involved as the organic component of heme in mammals. - While it is structurally similar to protoporphyrin, it does not serve as the direct precursor for heme formation in the human body.
Question 314: Hay's sulfur test is used to detect which of the following?
- A. Bile salts in urine (Correct Answer)
- B. Reducing sugar in urine
- C. Ketone bodies in urine
- D. Urobilinogen in urine
Explanation: ***Bile salts in urine*** - Hay's sulfur test is a classic qualitative test used to detect the presence of **bile salts** in a urine sample. - Bile salts reduce the **surface tension** of urine, causing sulfur powder to sink when sprinkled on the surface. *Reducing sugar in urine* - Reducing sugars (like glucose) are typically detected using tests such as **Benedict's test** or glucose oxidase strips, not Hay's sulfur test. - These tests rely on color changes due to the **reduction of copper ions** or enzymatic reactions, respectively. *Ketone bodies in urine* - Ketone bodies (acetoacetate, beta-hydroxybutyrate, acetone) are detected using tests like the **Rothera's test** or dipsticks, which react with acetoacetate. - These reactions produce color changes in the presence of ketones, unrelated to surface tension. *Urobilinogen in urine* - Urobilinogen in urine is commonly detected using **Ehrlich's reagent** (e.g., in a dipstick test) which forms a red color. - Elevated urobilinogen indicates issues with liver function or hemolysis, and its detection does not involve surface tension.
Question 315: Which of the following statements about hemoglobin is true?
- A. Each hemoglobin molecule can bind up to six O2 molecules.
- B. Each hemoglobin subunit contains two heme groups, which bind oxygen.
- C. Hemoglobin consists of two alpha and two beta subunits, each capable of binding one O2 molecule. (Correct Answer)
- D. Each hemoglobin molecule is made of 6 polypeptide chains.
Explanation: ***Hemoglobin consists of two alpha and two beta subunits, each capable of binding one O2 molecule.*** - A **hemoglobin molecule is a tetramer**, meaning it is composed of four protein subunits: two alpha (α) chains and two beta (β) chains. - Each of these four subunits contains one **heme group**, which is an iron-containing porphyrin complex that can reversibly bind one molecule of **oxygen (O2)**. *Each hemoglobin molecule can bind up to six O2 molecules.* - A single hemoglobin molecule, with its **four heme groups**, can bind a maximum of **four O2 molecules**, not six. - The capacity for oxygen binding is directly proportional to the number of heme groups present in the hemoglobin molecule. *Each hemoglobin subunit contains two heme groups, which bind oxygen.* - Each individual **hemoglobin subunit (alpha or beta)** contains **only one heme group**, not two. - Therefore, a complete hemoglobin molecule (with four subunits) contains a total of four heme groups. *Each hemoglobin molecule is made of 6 polypeptides, one for each subunit.* - A hemoglobin molecule is composed of **four polypeptide chains** (two alpha and two beta), not six. - This tetrameric structure is crucial for its function and **cooperative oxygen binding**.
Question 316: Which of the following statements about chaperones is false?
- A. Are lipid in nature (Correct Answer)
- B. Cause folding of proteins
- C. Include heat shock proteins
- D. May have ATPase activity
Explanation: ***Are lipid in nature*** - Chaperones are **proteins** (typically **heat shock proteins** or **chaperonins**), not lipids. - Their function involves assisting in the proper **folding and assembly of other proteins**, and they are composed of amino acids. *Cause folding of proteins* - Chaperones **do not cause** proteins to fold; rather, they **assist in proper folding** and refolding by preventing aggregation or misfolding. - They bind to nascent or partially unfolded proteins to guide them towards their correct three-dimensional structure. *May have ATPase activity* - Many chaperones, especially **Hsp70** and **chaperonins** like GroEL/GroES, utilize **ATP hydrolysis** for their function. - This **ATPase activity** drives conformational changes essential for binding, release, and refolding of their client proteins. *Include heat shock proteins* - The **heat shock protein (Hsp)** families (e.g., Hsp70, Hsp90, Hsp60) are a major class of chaperones. - Hsps are upregulated in response to stress (like heat) to help refold damaged proteins and prevent aggregation.
Question 317: Which of the following is the major glycosaminoglycan of synovial fluid?
- A. Chondroitin sulfate
- B. Dermatan sulfate
- C. Heparan sulfate
- D. Hyaluronic acid (Correct Answer)
Explanation: ***Hyaluronic acid*** - **Hyaluronic acid** is the primary glycosaminoglycan in **synovial fluid**, providing its characteristic **viscosity** and **lubricating properties**. - It plays a crucial role in maintaining **joint health** by reducing friction and acting as a shock absorber. *Chondroitin sulfate* - **Chondroitin sulfate** is abundant in **cartilage**, contributing to its **compressive strength**. - While present in connective tissues, it is not the major glycosaminoglycan of synovial fluid. *Dermatan sulfate* - **Dermatan sulfate** is primarily found in **skin**, **blood vessels**, and **heart valves**. - Its main roles involve tissue structure and repair, not lubrication of synovial fluid. *Heparan sulfate* - **Heparan sulfate** is found on **cell surfaces** and in the **extracellular matrix**, especially in the **basement membranes**. - It regulates cell growth, adhesion, and signaling, and is not a major component of synovial fluid viscosity.
Question 318: What is the mechanism by which mercury causes damage?
- A. Causes toxicity through various mechanisms
- B. Binds to -SH groups of enzymes (Correct Answer)
- C. Inhibits electron transport chain
- D. Inhibits protein synthesis
Explanation: ***Binds to -SH groups of enzymes*** - Mercury, particularly its inorganic and organic forms, has a high affinity for **sulfhydryl (-SH) groups** found in **cysteine residues** of proteins and enzymes. - This binding disrupts the **tertiary structure** and **catalytic activity** of vital enzymes, leading to widespread cellular dysfunction and toxicity. *Causes toxicity through various mechanisms (not specific to -SH binding)* - While mercury can indeed cause toxicity through various mechanisms, the **most prominent and fundamental mechanism** underpins many of these downstream effects. - This option is too general and does not pinpoint the primary molecular interaction responsible for mercury's widespread cellular damage. *Indirectly inhibits the electron transport chain (ETC) by enzyme disruption* - This statement is partially true in that mercury's enzyme disruption can affect the ETC, but it's an **indirect consequence** rather than the primary mechanism itself. - The direct mechanism involves the initial binding to -SH groups, which then leads to the dysfunction of enzymes, including those involved in the ETC. *Indirectly inhibits protein synthesis by disrupting enzyme function* - Similar to ETC inhibition, mercury's disruption of enzyme function can ultimately impair protein synthesis, but this is an **effect down the causal chain**. - The initial and direct molecular interaction is the binding to sulfhydryl groups of key enzymes involved in various cellular processes, including protein synthesis.
Question 319: Which of the following statements regarding collagen synthesis is incorrect?
- A. Hydroxylation of lysine occurs in ER
- B. Synthesized in ribosomes as preprocollagen
- C. Triple helix assembly occurs in ER
- D. Hydroxylation of proline occurs in Golgi apparatus (Correct Answer)
Explanation: ***Hydroxylation of proline occurs in Golgi apparatus*** - This statement is incorrect because the **hydroxylation of proline** residues occurs in the **endoplasmic reticulum** (ER), not the Golgi apparatus. - This step is critical for forming stable **triple helix** structures of collagen and requires **vitamin C**. *Synthesized in ribosomes as preprocollagen* - This statement is correct. Collagen synthesis begins in the cytoplasm, where mRNA is translated by **ribosomes** into **preprocollagen**, which contains a signal peptide. - The signal peptide directs the nascent polypeptide chain into the lumen of the **endoplasmic reticulum**. *Hydroxylation of lysine occurs in ER* - This statement is correct. Following entry into the ER, specific **lysine** residues are hydroxylated by **lysyl hydroxylase** to form hydroxylysine. - This hydroxylation, along with that of proline, is crucial for **cross-linking** and stability of the collagen molecule. *Triple helix assembly occurs in ER* - This statement is correct. After hydroxylation and glycosylation of some residues, three procollagen alpha chains self-assemble to form a **triple helix** within the **endoplasmic reticulum**. - This assembly is stabilized by **disulfide bonds** at the C-terminal ends and molecular chaperones.
Internal Medicine
1 questionsWhich of the following statements about hypercalcemia in sarcoidosis is false?
NEET-PG 2013 - Internal Medicine NEET-PG Practice Questions and MCQs
Question 311: Which of the following statements about hypercalcemia in sarcoidosis is false?
- A. PTHrP level is increased
- B. Parathormone level is increased (Correct Answer)
- C. Oral steroids are useful
- D. Calcitriol level is increased
Explanation: ***Parathormone level is increased*** - In **sarcoidosis-associated hypercalcemia**, the parathormone (PTH) level is typically **low or suppressed**. [1] - This is because the hypercalcemia is due to **extra-renal 1-$\alpha$ hydroxylation** of 25-hydroxyvitamin D to 1,25-dihydroxyvitamin D (calcitriol) by macrophages in granulomas, not primary hyperparathyroidism. [1] *PTHrP level is increased* - This statement is **false** for sarcoidosis. Elevated **parathyroid hormone-related peptide (PTHrP)** is a common cause of hypercalcemia in **malignancy**, particularly squamous cell carcinomas, but not in sarcoidosis. - Hypercalcemia in sarcoidosis is **PTH-independent** and not mediated by PTHrP. [1] *Oral steroids are useful* - This statement is **true**. **Corticosteroids** (like oral prednisone) are effective in treating hypercalcemia in sarcoidosis. - They work by **inhibiting the activity of 1-$\alpha$ hydroxylase** in alveolar macrophages and reducing intestinal calcium absorption. *Calcitriol level is increased* - This statement is **true**. In sarcoidosis, activated **macrophages within granulomas** aberrantly express **1-$\alpha$ hydroxylase**. [1] - This leads to the **extra-renal synthesis of calcitriol** (1,25-dihydroxyvitamin D), which increases intestinal calcium absorption and bone resorption, causing hypercalcemia. [1]