Structure-Function Relationships — MCQs
All are true about prions EXCEPT:
A 78-year-old woman presents with a progressive decline in daily activity. She gives a history of convulsions and visual hallucinations. She does not talk to anyone and keeps looking at the sky. Pathological examination shows the presence of Lewy bodies within the neurons. What is the most probable diagnosis?
Which of the following methods cannot be used to precipitate proteins?
What is the primary physiological effect of increased 2,3-DPG on hemoglobin?
Size of fibrillary proteins in amyloidosis is:
Which of the following statements regarding prion diseases is NOT true?
Which one of the following shows allosteric inhibition of glycolysis?
What do chaperones assist in?
A pregnant woman is able to transfer oxygen to her fetus because fetal hemoglobin has a greater affinity for oxygen than does adult hemoglobin. Why is the affinity of fetal hemoglobin for oxygen higher?
Which of the following statements about protein structures is most accurate?
Structure-Function Relationships Indian Medical PG Practice Questions and MCQs
Question 1: All are true about prions EXCEPT:
- A. Contain nucleic acid (Correct Answer)
- B. Protease resistant
- C. Not affected by radiation
- D. Cause spongiform changes
Explanation: ***Contain nucleic acid*** - This statement is **FALSE** - prions do NOT contain nucleic acid, making this the correct answer for an EXCEPT question. - Prions are unique infectious agents composed solely of **abnormally folded proteins (PrPSc)**, completely lacking any genetic material such as **DNA or RNA**. - This fundamental characteristic differentiates them from all conventional pathogens including **viruses, bacteria, fungi, and parasites**. *Protease resistant* - This statement is TRUE about prions. - The **abnormal folding** of prion proteins (β-pleated sheet conformation) renders them highly **resistant to degradation** by proteases. - This resistance contributes to their **accumulation in neurological tissue** and the progressive pathogenesis of transmissible spongiform encephalopathies. *Not affected by radiation* - This statement is TRUE about prions. - Because prions lack nucleic acid, they are **resistant to inactivation** by treatments like **ultraviolet (UV) radiation** and **ionizing radiation**. - These forms of radiation primarily damage genetic material (DNA/RNA), which prions completely lack. - Prions require **autoclaving at 134°C for extended periods** or treatment with strong alkalis for effective inactivation. *Cause spongiform changes* - This statement is TRUE about prions. - Prion diseases are characterized by **vacuolation** of brain tissue, giving it a distinctive **spongy appearance** on microscopic examination. - These spongiform changes are pathognomonic features of prion diseases such as **Creutzfeldt-Jakob disease (CJD)**, **kuru**, **Gerstmann-Sträussler-Scheinker syndrome**, and **bovine spongiform encephalopathy (BSE)**.
Question 2: A 78-year-old woman presents with a progressive decline in daily activity. She gives a history of convulsions and visual hallucinations. She does not talk to anyone and keeps looking at the sky. Pathological examination shows the presence of Lewy bodies within the neurons. What is the most probable diagnosis?
- A. Prion disease
- B. Huntington's disease
- C. Lewy body dementia (Correct Answer)
- D. Alzheimer's disease
Explanation: ***Lewy body dementia*** - The presence of **progressive cognitive decline**, **visual hallucinations**, and **convulsions** in an elderly patient is highly indicative of Lewy body dementia. - The definitive pathological finding of **Lewy bodies** within neurons confirms the diagnosis. *Prion disease* - Characterized by rapidly progressive dementia, **myoclonus**, and cerebellar ataxia, without typical visual hallucinations or convulsions. - Pathological examination typically shows **spongiform changes** and accumulation of abnormal prion protein, not Lewy bodies. *Huntington's disease* - Presents with a classic triad of **motor dysfunction** (chorea), psychiatric symptoms, and cognitive decline, typically with an earlier onset (30-50 years). - It is an inherited neurodegenerative disorder, and its pathology involves neuronal loss in the striatum, without Lewy bodies. *Alzheimer's disease* - The most common cause of dementia, characterized by **memory impairment** as an early and prominent feature. - Pathological findings include **amyloid plaques** and **neurofibrillary tangles**, not Lewy bodies, and visual hallucinations are less common or occur later in the disease.
Question 3: Which of the following methods cannot be used to precipitate proteins?
- A. Add alcohol and acetone
- B. Using heavy metal ions
- C. Adding trichloroacetic acid
- D. Moving pH away from isoelectric pH (Correct Answer)
Explanation: ***Moving pH away from isoelectric pH*** - Proteins are **least soluble** at their **isoelectric point (pI)**, where their net charge is zero, causing them to aggregate and precipitate. - Moving the pH **away from the isoelectric point** increases the net charge on the protein, enhancing its solubility and preventing precipitation. *Add alcohol and acetone* - **Organic solvents** like alcohol and acetone reduce the dielectric constant of water, weakening the **hydrophobic interactions** that maintain protein solubility. - This leads to increased protein-protein interactions and **precipitation** as the protein unfolds or aggregates. *Using heavy metal ions* - **Heavy metal ions** (e.g., lead, mercury) are positively charged and bind strongly to the negatively charged groups on proteins, such as **carboxylates** and **sulfhydryl groups**. - This binding can disrupt protein structure, lead to aggregation, and cause **precipitation**. *Adding trichloroacetic acid* - **Trichloroacetic acid (TCA)** is a strong acid that significantly lowers the pH of the solution, causing proteins to become **protonated**. - This change in charge and the disruption of **salt bridges** and hydrogen bonds lead to protein denaturation and **precipitation**.
Question 4: What is the primary physiological effect of increased 2,3-DPG on hemoglobin?
- A. Increased affinity of hemoglobin to oxygen
- B. Decreased affinity of hemoglobin to oxygen (Correct Answer)
- C. Left shift of oxygen-hemoglobin dissociation curve
- D. Right shift of oxygen-hemoglobin dissociation curve
Explanation: ***Decreased affinity of hemoglobin to oxygen*** - **2,3-Diphosphoglycerate (2,3-DPG)** binds to the beta subunits of deoxyhemoglobin, stabilizing the **deoxygenated state** and thus **reducing hemoglobin's affinity for oxygen**. - This is the **primary molecular mechanism** by which 2,3-DPG exerts its effect, facilitating **oxygen unloading** in peripheral tissues. - This decreased affinity manifests graphically as a **right shift** in the oxygen-hemoglobin dissociation curve. *Increased affinity of hemoglobin to oxygen* - This is incorrect because 2,3-DPG specifically works to **decrease hemoglobin's affinity** for oxygen, promoting oxygen release. - Increased affinity would mean oxygen is held more tightly, which is counterproductive for **oxygen delivery** to tissues. *Left shift of oxygen-hemoglobin dissociation curve* - A **left shift** indicates **increased affinity** of hemoglobin for oxygen, meaning oxygen is held more tightly. - Since 2,3-DPG decreases affinity, it causes a **right shift**, not a left shift. *Right shift of oxygen-hemoglobin dissociation curve* - While this is the **graphical representation** of 2,3-DPG's effect, it is a **consequence** of the primary molecular mechanism (decreased affinity). - A right shift signifies that for any given partial pressure of oxygen, hemoglobin is **less saturated** with oxygen, reflecting the decreased affinity caused by 2,3-DPG binding.
Question 5: Size of fibrillary proteins in amyloidosis is:
- A. 0-5 nm
- B. 7.5-10 nm (Correct Answer)
- C. 12-17 nm
- D. 18-20 nm
Explanation: ***7.5-10 nm*** - Amyloid fibrils are characteristically **non-branching**, **insoluble protein fibrils** that range in diameter from **7.5 to 10 nm**. - This specific size and morphology are crucial for their identification via **electron microscopy**, which is a key diagnostic tool for amyloidosis. *0-5 nm* - This range is generally too small for the characteristic amyloid fibrils and would likely represent **monomeric proteins** or very small aggregates. - Fibrillary structures typically need to be larger to achieve the stable, ordered beta-pleated sheet conformation seen in amyloid. *12-17 nm* - This diameter is typically **too large** for classic amyloid fibrils, which are known for their consistent size. - Fibrils in this range might suggest different types of protein aggregates or other pathological structures not characteristic of amyloid. *18-20 nm* - Fibrils of this diameter are significantly **larger than the typical amyloid fibrils** and would not be consistent with the ultrastructural definition of amyloid. - This size might be indicative of bundled fibrils or other forms of protein deposits.
Question 6: Which of the following statements regarding prion diseases is NOT true?
- A. Myoclonus found only in 10% cases (Correct Answer)
- B. They are caused by infectious proteins
- C. Brain biopsy is diagnostic
- D. Neurodegenerative disease
Explanation: ***Myoclonus found only in 10% cases*** - This statement is incorrect because **myoclonus (sudden, involuntary muscle jerks)** is a very common and prominent feature of **Creutzfeldt-Jakob disease (CJD)** and other prion diseases, occurring in more than 90% of cases as the disease progresses. - The prevalence of myoclonus is significantly higher than 10%, making this an inaccurate representation of the clinical presentation. *They are caused by infectious proteins* - This statement is true; prion diseases are unique neurodegenerative disorders caused by the misfolding and aggregation of a host-encoded protein called the **prion protein (PrP)**. - These rogue proteins, known as **prions (PrPSc)**, can induce normal PrP molecules to misfold, leading to a chain reaction of protein aggregation and neuronal damage [1]. *Brain biopsy is diagnostic* - This statement is true; **brain biopsy** and post-mortem examination are considered definitive diagnostic methods for prion diseases like CJD. - These procedures allow for the detection of **spongiform changes**, **neuronal loss**, and the presence of **PrPSc aggregates** in brain tissue [1]. *Neurodegenerative disease* - This statement is true; prion diseases are characterized by progressive **neurodegeneration**, meaning they cause irreversible damage and loss of neurons in the brain. - This leads to a decline in cognitive function, motor control, and ultimately, death.
Question 7: Which one of the following shows allosteric inhibition of glycolysis?
- A. Amino acid alanine
- B. 2,3 BPG
- C. Citrate (Correct Answer)
- D. Malonic acid
Explanation: ***Citrate*** - **Citrate** is a classic **allosteric inhibitor** of **phosphofructokinase-1 (PFK-1)**, the key regulatory enzyme in glycolysis - It binds to an allosteric site (distinct from the active site), reducing PFK-1's affinity for **fructose-6-phosphate** - This is a **negative feedback mechanism** - when citrate accumulates (indicating sufficient ATP production via the citric acid cycle), glycolysis slows down *Malonic acid* - **Malonic acid** is a **competitive inhibitor** (NOT allosteric) of succinate dehydrogenase in the citric acid cycle - It structurally resembles succinate and competes for the active site directly *2,3-BPG* - **2,3-Bisphosphoglycerate (2,3-BPG)** is an **allosteric effector** of hemoglobin (decreases oxygen affinity), not an enzyme inhibitor in glycolysis - It binds to hemoglobin, not to glycolytic enzymes *Amino acid alanine* - **Alanine** is an allosteric inhibitor of **pyruvate kinase** (not a glycolytic regulator in this context) - While it does show allosteric inhibition, it acts on gluconeogenesis regulation in the liver, not as a direct glycolytic inhibitor
Question 8: What do chaperones assist in?
- A. Protein Cleavage
- B. Protein Degradation
- C. Protein Modification
- D. Protein Folding (Correct Answer)
Explanation: ***Protein Folding*** - **Chaperone proteins** bind to newly synthesized polypeptide chains and unfolded proteins, helping them achieve their **correct three-dimensional structure**. - They also prevent **misfolding** and **aggregation** of proteins, which can be detrimental to cellular function. *Protein Cleavage* - **Protein cleavage** involves the enzymatic hydrolysis of peptide bonds, often performed by **proteases**. - This process is not directly facilitated by chaperones; chaperones primarily function in structural maturation, not degradation or processing. *Protein Degradation* - **Protein degradation** is carried out by systems like the **ubiquitin-proteasome pathway** or lysosomes, which break down damaged or unwanted proteins. - While chaperones can triage misfolded proteins for degradation, they do not directly perform the degradation themselves. *Protein Modification* - **Protein modification** involves the covalent attachment of chemical groups (e.g., phosphorylation, glycosylation) or other molecules to a protein. - This process is performed by specific enzymes like kinases or glycosyltransferases; chaperones’ role is more structural than enzymatic modification.
Question 9: A pregnant woman is able to transfer oxygen to her fetus because fetal hemoglobin has a greater affinity for oxygen than does adult hemoglobin. Why is the affinity of fetal hemoglobin for oxygen higher?
- A. There is less 2,3-BPG in the fetal circulation as compared to maternal circulation
- B. Fetal hemoglobin binds 2,3-BPG with fewer ionic bonds than the adult form. (Correct Answer)
- C. The tense form of hemoglobin is more prevalent in the circulation of the fetus
- D. The oxygen-binding curve of fetal hemoglobin is shifted to the right.
Explanation: ***Fetal hemoglobin binds 2,3-BPG with fewer ionic bonds than the adult form.*** * **Fetal hemoglobin (HbF)**, composed of two alpha and two gamma subunits, interacts less effectively with **2,3-bisphosphoglycerate (2,3-BPG)** due to a difference in its gamma subunits compared to the beta subunits of **adult hemoglobin (HbA)**. * The reduced binding of 2,3-BPG to HbF stabilizes its **R (relaxed) state**, which has a higher oxygen affinity, facilitating oxygen transfer from the mother to the fetus. *There is less 2,3-BPG in the fetal circulation as compared to maternal circulation* * While 2,3-BPG plays a crucial role in regulating oxygen affinity, the primary reason for **fetal hemoglobin's higher oxygen affinity** is its inherent structural difference that leads to weaker binding of 2,3-BPG, not necessarily the concentration of 2,3-BPG in the fetal circulation. * The **concentration of 2,3-BPG is typically similar or even slightly higher in fetal blood** to enhance oxygen unloading at the tissues, but its effect on HbF is diminished. *The tense form of hemoglobin is more prevalent in the circulation of the fetus* * The **tense form (T-state)** of hemoglobin has a **lower affinity for oxygen**, and its prevalence would lead to reduced oxygen binding, which is contrary to the physiological need of the fetus to extract oxygen from the maternal blood. * **Fetal hemoglobin's higher oxygen affinity** means it spends more time in the **relaxed form (R-state)**, which is responsible for tighter oxygen binding. *The oxygen-binding curve of fetal hemoglobin is shifted to the right.* * An **oxygen-binding curve shifted to the right** indicates a **decreased affinity for oxygen** and would facilitate oxygen unloading, not oxygen loading. * For fetal hemoglobin to effectively extract oxygen from maternal blood, its **oxygen-binding curve must be shifted to the left**, signifying a higher oxygen affinity.
Question 10: Which of the following statements about protein structures is most accurate?
- A. Secondary structure is stabilized by hydrogen bonds.
- B. Denaturation primarily affects secondary and tertiary structures, leaving the primary structure intact.
- C. The sequence of amino acids determines the secondary and tertiary structures of proteins. (Correct Answer)
- D. The three-dimensional structure of a protein is referred to as its tertiary structure.
Explanation: ***The sequence of amino acids determines the secondary and tertiary structures of proteins.*** - This represents **Anfinsen's principle**, the most fundamental concept in protein folding: the **primary structure (amino acid sequence) contains all the information necessary** to determine the final three-dimensional structure of a protein. - This was demonstrated by **Nobel Prize-winning experiments** showing that denatured proteins can spontaneously refold into their native structure based solely on their amino acid sequence. - This is the **foundational principle** from which all other structural concepts derive - the sequence dictates everything else about protein structure. *Secondary structure is stabilized by hydrogen bonds.* - While this statement is **factually correct**, it describes a *mechanism* of structural stabilization rather than the fundamental principle of protein structure determination. - Hydrogen bonds are **one type of interaction** that stabilizes already-formed secondary structures, but the formation pattern itself is determined by the amino acid sequence. *Denaturation primarily affects secondary and tertiary structures, leaving the primary structure intact.* - This statement is also **factually correct** and describes what happens during denaturation (loss of 3D structure while peptide bonds remain intact). - However, it describes a *consequence* or phenomenon rather than the fundamental organizing principle of protein structure. *The three-dimensional structure of a protein is referred to as its tertiary structure.* - This is a **correct definition** but merely terminology rather than a principle. - It defines what tertiary structure means but doesn't explain the underlying mechanism of how protein structures are determined.
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