Biochemistry
7 questionsWhich of the following statements is true regarding the functions of cAMP and cGMP?
Abnormal proteins which are bound to ubiquitin are degraded in -
Which of the following proteins is primarily responsible for marking other proteins for degradation?
GlcNAc-P-P-oligosaccharide is -
Prolyl hydroxylase requires which cofactor?
What type of bond is involved in the side chain linkage of proteoglycans?
Which of the following is not a free radical?
NEET-PG 2015 - Biochemistry NEET-PG Practice Questions and MCQs
Question 171: Which of the following statements is true regarding the functions of cAMP and cGMP?
- A. All of the above statements are true
- B. They act on membrane receptors.
- C. They are both second messengers. (Correct Answer)
- D. They act by post-translational modification.
Explanation: ***They are both second messengers.*** - **cAMP (cyclic adenosine monophosphate)** and **cGMP (cyclic guanosine monophosphate)** are crucial intracellular signaling molecules. - They relay signals from **first messengers** (like hormones or neurotransmitters) received at the cell surface to intracellular targets, thus acting as second messengers. *They act on membrane receptors.* - **cAMP** and **cGMP** are *produced* in response to activation of **membrane receptors** by first messengers, but they themselves do not act directly on these receptors. - Their action is primarily *intracellular*, binding to and activating various enzymes and proteins like **protein kinases**. *All of the above statements are true* - This statement is incorrect because the claim that they act on membrane receptors is false. - Only one of the statements provided is accurate regarding the functions of cAMP and cGMP. *They act by post-translational modification.* - While cAMP and cGMP can lead to **post-translational modification** (e.g., phosphorylation by protein kinases A and G), they are not themselves the direct modifiers. - They act as **allosteric regulators** of enzymes, which then catalyze the modifications.
Question 172: Abnormal proteins which are bound to ubiquitin are degraded in -
- A. Proteasomes (Correct Answer)
- B. Golgi apparatus
- C. Smooth ER
- D. Lysosomes
Explanation: ***Proteasomes*** - **Proteasomes** are multi-subunit protein complexes responsible for degrading **ubiquitin-tagged proteins**. - This degradation is a tightly regulated process essential for cell cycle control, gene expression, and immune response. *Golgi apparatus* - The **Golgi apparatus** primarily functions in modifying, sorting, and packaging proteins and lipids synthesized in the Endoplasmic Reticulum. - It does not directly participate in the degradation of **ubiquitin-bound proteins**. *Smooth ER* - The **smooth endoplasmic reticulum (SER)** is involved in lipid synthesis, detoxification of drugs and poisons, and storage of calcium ions. - It lacks ribosomes and is not directly implicated in the degradation of misfolded proteins tagged with ubiquitin. *Lysosomes* - **Lysosomes** are organelles containing various hydrolytic enzymes that break down waste materials and cellular debris, as well as foreign invaders like bacteria. - While they degrade proteins, they primarily target **extracellular proteins** taken up by endocytosis or cellular components via **autophagy**, not specifically ubiquitin-bound proteins.
Question 173: Which of the following proteins is primarily responsible for marking other proteins for degradation?
- A. Ubiquitin (Correct Answer)
- B. RNAse
- C. Zymase
- D. Chaperone
Explanation: **Ubiquitin** - **Ubiquitin** is a small regulatory protein that marks proteins for degradation by targeting them to the **proteasome**. - The ubiquitination process involves a cascade of enzymes (E1, E2, E3) that sequentially attach ubiquitin to the target protein, forming a **polyubiquitin chain**. *RNAse* - **RNAse** (Ribonuclease) is an enzyme that catalyzes the degradation of **RNA into smaller components**. - Its primary function is in **RNA processing** and turnover, not protein degradation. *Zymase* - **Zymase** is a complex of enzymes that catalyzes the **fermentation of sugar into ethanol and carbon dioxide**. - It is commonly found in yeast and is essential for **alcoholic fermentation**, with no role in protein degradation. *Chaperone* - **Chaperone proteins** assist in the **folding of newly synthesized proteins** and the refolding of misfolded or denatured proteins. - Their role is to ensure proper protein structure and function, preventing aggregation, rather than marking proteins for destruction.
Question 174: GlcNAc-P-P-oligosaccharide is -
- A. Proteoglycan
- B. Glycoprotein (Correct Answer)
- C. Collagen
- D. Phospholipid
Explanation: ***Glycoprotein*** - **GlcNAc-P-P-oligosaccharide** refers to the **N-linked oligosaccharide precursor** that is synthesized on a **dolichol pyrophosphate** carrier (`-P-P`). This complex is characteristic of the initial stages of **N-linked glycosylation**, a process that forms glycoproteins. - **N-acetylglucosamine (GlcNAc)** is a crucial sugar residue found at the reducing end of this precursor, linking it to the dolichol carrier. *Proteoglycan* - Proteoglycans consist of a **core protein** covalently attached to long, unbranched **glycosaminoglycan (GAG)** chains, such as chondroitin sulfate or heparin. - While they contain sugar units, their structure and synthesis pathway are distinct from the GlcNAc-P-P-oligosaccharide described, which is specific to N-linked glycoprotein synthesis. *Collagen* - **Collagen** is a fibrous protein, primarily composed of a triple helix of polypeptide chains rich in **glycine, proline, and hydroxyproline**. - Although collagen undergoes some post-translational modifications like **glycosylation**, it does not involve the GlcNAc-P-P-oligosaccharide precursor in its typical synthesis. *Phospholipid* - **Phospholipids** are a major component of cell membranes, composed of a **hydrophilic head** (containing a phosphate group) and two **hydrophobic fatty acid tails**. - They are lipids and do not contain carbohydrate structures like GlcNAc-P-P-oligosaccharide.
Question 175: Prolyl hydroxylase requires which cofactor?
- A. Vitamin C (Correct Answer)
- B. Iron (Fe²⁺)
- C. Molybdenum
- D. Vitamin K1
Explanation: ***Vitamin C*** - **Prolyl hydroxylase** is an enzyme critical for the hydroxylation of proline residues during **collagen synthesis**. - **Vitamin C** (ascorbic acid) acts as an essential **cofactor**, reducing the ferric iron of the enzyme back to its ferrous state after each catalytic cycle, enabling continued activity. - The enzyme requires both **iron (Fe²⁺)** as a metal cofactor and **vitamin C** to maintain the iron in its reduced state. *Iron (Fe²⁺)* - While **iron** is indeed required by prolyl hydroxylase as a **metal cofactor**, the question asks for the cofactor, which specifically refers to **vitamin C**. - Iron functions as part of the enzyme's active site, but vitamin C is the reducing agent that keeps iron functional. - Vitamin C deficiency (scurvy) leads to defective collagen synthesis despite adequate iron. *Molybdenum* - **Molybdenum** is a cofactor for several human enzymes, including **xanthine oxidase** and **sulfite oxidase**. - However, it plays no direct role in the activity of prolyl hydroxylase. *Vitamin K1* - **Vitamin K1** is a crucial cofactor for **gamma-glutamyl carboxylase**, an enzyme involved in the carboxylation of glutamic acid residues in clotting factors. - It is not involved in the hydroxylation of proline by prolyl hydroxylase.
Question 176: What type of bond is involved in the side chain linkage of proteoglycans?
- A. Covalent (Correct Answer)
- B. Hydrogen bond
- C. Electrostatic bond
- D. Van-der Waal's force
Explanation: ***Covalent*** - Proteoglycans are formed by **glycosaminoglycan (GAG)** chains that are covalently linked to a protein core. - Specifically, an **O-glycosidic bond** forms between a xylose residue on the GAG chain and a serine residue on the core protein. *Hydrogen bond* - **Hydrogen bonds** are weaker intermolecular forces that stabilize protein secondary structures and interactions between water molecules. - They are not strong enough to form the primary structural linkage between the GAG chains and the core protein in proteoglycans. *Electrostatic bond* - **Electrostatic bonds**, or ionic bonds, involve attraction between oppositely charged ions. While proteoglycans have many charged groups, these bonds are not the primary linkage connecting the GAG chains to the protein core. - They contribute to the overall structure and interactions of proteoglycans with other molecules but do not form the main side chain linkage. *Van-der Waal's force* - **Van der Waals forces** are weak, short-range intermolecular forces that arise from temporary fluctuations in electron distribution. - These forces play a role in tertiary and quaternary protein structure and molecular packing, but they are far too weak to establish the covalent attachments of GAG chains to the proteoglycan core protein.
Question 177: Which of the following is not a free radical?
- A. Superoxide anion
- B. Hydrogen peroxide (H2O2) (Correct Answer)
- C. Nitric oxide (NO·)
- D. Hydroxyl radical (.OH)
Explanation: ***Hydrogen peroxide (H₂O₂)*** - **Hydrogen peroxide** is a **reactive oxygen species (ROS)** but is not a free radical because it has **no unpaired electrons** in its outermost shell. - While it can be converted into the highly reactive hydroxyl radical via the **Fenton reaction**, it is stable enough to be transported across membranes. *Superoxide anion (O₂⁻)* - The **superoxide anion (O₂⁻)** is a free radical because it has an **unpaired electron** in its outer shell. - It is one of the primary **reactive oxygen species** formed during cellular metabolism and can damage cellular components. *Nitric oxide (NO·)* - **Nitric oxide** is an important **free radical** with a single **unpaired electron** in its molecular structure. - It functions as a vital signaling molecule in vascular biology, regulating blood pressure and neurotransmission, despite being a free radical. *Hydroxyl radical (·OH)* - The **hydroxyl radical (·OH)** is one of the most reactive and damaging **free radicals** in biological systems. - It has a single **unpaired electron**, making it highly unstable and able to react indiscriminately with virtually all types of biomolecules.
Pathology
3 questionsHyaline degeneration is found in -
Which molecule is primarily responsible for nuclear fragmentation during apoptosis?
During cell death, myelin figures are derived from which of the following?
NEET-PG 2015 - Pathology NEET-PG Practice Questions and MCQs
Question 171: Hyaline degeneration is found in -
- A. Alzheimer's disease
- B. Alcoholic liver disease (Correct Answer)
- C. Acute myocardial infarction
- D. Acute appendicitis
Explanation: ***Alcoholic liver disease*** - **Mallory bodies**, a form of hyaline degeneration, are characteristic histologic findings in hepatocytes in alcoholic liver disease. - They represent aggregates of **intermediate filaments** and other proteins, indicating severe hepatocellular damage. *Acute myocardial infarction* - Characterized by **coagulative necrosis** of cardiac myocytes due to ischemia, not hyaline degeneration. - Inflammation and subsequent repair with **fibrosis** are key features. *Alzheimer's disease* - Defined by the presence of **senile plaques** (amyloid-beta deposits) and **neurofibrillary tangles** (hyperphosphorylated tau protein). - These are specific protein aggregates, distinct from hyaline degeneration of cellular components. *Acute appendicitis* - Involves acute inflammation of the appendix, leading to **neutrophilic infiltration** and often **fibrinopurulent exudate**. - There is no characteristic hyaline degeneration associated with this inflammatory process.
Question 172: Which molecule is primarily responsible for nuclear fragmentation during apoptosis?
- A. Caspases (Correct Answer)
- B. Apaf - 1
- C. Oxygen free radicals
- D. Endonuclease G
Explanation: ***Caspases*** - **Caspases** are a family of proteases that play a central role in the execution phase of apoptosis, including the **cleavage of nuclear proteins** and DNA fragmentation [1]. - Specifically, **executioner caspases** (e.g., caspase-3, -6, -7) activate **CAD (caspase-activated DNase)** by cleaving its inhibitor ICAD, leading to **nuclear fragmentation** and DNA laddering [1]. - This is the **primary mechanism** of nuclear breakdown in apoptosis. *Apaf-1* - **Apaf-1 (apoptotic protease activating factor 1)** is an adaptor protein that, upon activation by cytochrome c, forms the **apoptosome** [1]. - While essential for **caspase activation** (specifically caspase-9), Apaf-1 does not directly cleave nuclear components or cause fragmentation itself [1]. *Oxygen free radicals* - **Oxygen free radicals** (reactive oxygen species) can induce cellular damage and stress, and in high concentrations, can trigger apoptosis [2]. - However, they are generally upstream initiators of apoptosis pathways and do not directly mediate nuclear fragmentation; this process is carried out by **caspases**. *Endonuclease G* - **Endonuclease G** is a mitochondrial nuclease released during apoptosis that can contribute to DNA degradation. - However, it plays a **secondary role** and acts in a caspase-independent manner, whereas **caspases** remain the primary executors of nuclear fragmentation in apoptosis. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Cellular Responses to Stress and Toxic Insults: Adaptation, Injury, and Death, pp. 64-67. [2] Cross SS. Underwood's Pathology: A Clinical Approach. 6th ed. (Basic Pathology) introduces the student to key general principles of pathology, both as a medical science and as a clinical activity with a vital role in patient care. Part 2 (Disease Mechanisms) provides fundamental knowledge about the cellular and molecular processes involved in diseases, providing the rationale for their treatment. Part 3 (Systematic Pathology) deals in detail with specific diseases, with emphasis on the clinically important aspects., pp. 100-101.
Question 173: During cell death, myelin figures are derived from which of the following?
- A. Cell membrane (lipid bilayer) (Correct Answer)
- B. Cytoplasmic components
- C. Mitochondrial structures
- D. Nuclear membrane
Explanation: ***Cell membrane (lipid bilayer)*** - **Myelin figures** are whorled phospholipid masses formed during cell injury and death from the breakdown of **cellular membranes**, particularly the plasma membrane and **endoplasmic reticulum**. - These structures represent damaged membrane lipids (phospholipids) that undergo structural rearrangement into concentric lamellar (layered) configurations resembling myelin. - The term "cell membrane" encompasses both the plasma membrane and lipid-rich intracellular membranes, making this the most accurate answer among the options provided. - They are a characteristic morphologic feature of **irreversible cell injury** and can be seen with electron microscopy. *Cytoplasmic components* - While cytoplasmic proteins and organelles do degrade during cell death, they do not form the organized **phospholipid structures** characteristic of myelin figures. - Cytoplasmic breakdown produces different morphologic changes such as cytoplasmic eosinophilia and loss of ribosomes. *Mitochondrial structures* - Mitochondria have their own membranes that are damaged during cell death (leading to release of cytochrome c and other apoptotic factors). - However, mitochondrial membranes are not the primary source of **myelin figures**, which predominantly arise from ER and plasma membranes. *Nuclear membrane* - The nuclear envelope does fragment during cell death, contributing to nuclear changes like **karyopyknosis, karyorrhexis, and karyolysis**. - While technically a membrane structure, the nuclear envelope is not the primary source of myelin figures, which are mainly derived from the more abundant plasma and ER membranes.