Protein Structure and Function — MCQs

Protein Structure and Function Indian Medical PG Practice Questions and MCQs

Question 491: What is the biological value of egg protein?

A. 90
B. 100 (Correct Answer)
C. 120
D. 80

Explanation: ***Correct Answer: 100*** - The **biological value (BV)** of a protein measures how efficiently the body utilizes the protein consumed for growth and maintenance, with 100 representing the highest possible value. - **Egg protein** is often used as a reference point for protein quality because it contains all **essential amino acids** in proportions that closely match human requirements, making its BV approximately **100**. - Egg protein serves as the **gold standard** for biological value assessment. *Option: 80* - A biological value of **80** indicates a good quality protein, but it suggests that the protein is not as efficiently utilized as egg protein. - Proteins with a BV of 80 might be deficient in one or more essential amino acids compared to a perfect profile. - Examples include beef and some legumes which have BV around 80. *Option: 90* - A biological value of **90** signifies excellent protein quality, indicating that it is highly utilized by the body. - While very high, it is still slightly lower than the perfect biological value assigned to egg protein. - Some fish proteins approach this value but do not reach 100. *Option: 120* - A biological value of **120** is theoretically **not possible** for food proteins because BV is measured against how much nitrogen (from protein) is retained by the body compared to how much is absorbed. - The maximum BV is **100**, meaning all absorbed protein is retained; values above 100 would imply the body synthesizes protein from non-protein sources, which is not what BV measures. - This tests understanding that BV has an upper limit of 100.

Question 492: What does the 'C' in CRP (C-reactive protein) stand for?

A. Concanavalin A
B. Chondroitin sulfate
C. C-polysaccharide of pneumococcus (Correct Answer)
D. Cellular

Explanation: ***Capsular polysaccharide of pneumococcus*** - The "C" in CRP refers to the **C-reactive protein**, which was named after its interaction with the **capsular polysaccharide** of *Streptococcus pneumoniae*. - CRP is an acute phase reactant that increases in response to **inflammation and infection**, particularly in infections caused by encapsulated bacteria [1]. *Concanavalin A* - This is a **lectin derived from the Canavalia ensiformis** plant and is not related to CRP or its function. - It is mostly used in **immunological studies** and does not play a role in the acute phase response. *Cellular* - This term is too vague and does not accurately represent any component of CRP. - CRP is primarily a **protein** and not related to any specific cellular structure or classification. *Chondrotin sulfate in series with ARP, BRP* - Chondroitin sulfate refers to components involved in **cartilage structure**, not related to CRP. - ARP and BRP are not standard abbreviations associated with CRP or its function. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Diseases of Infancy and Childhood, pp. 501-502.

Question 493: Which of the following hormones is not a protein derivative?

A. Growth hormone
B. Insulin
C. Glucagon
D. Thyroid hormone (Correct Answer)

Explanation: ***Thyroid hormone*** - **Thyroid hormone** is a **derivative of the amino acid tyrosine** and contains iodine, classifying it as an **amine hormone**, not a protein. - Its synthesis involves the iodination of tyrosine residues on thyroglobulin, followed by coupling reactions. *Growth hormone* - **Growth hormone (GH)** is a **polypeptide hormone** synthesized and secreted by the somatotroph cells of the anterior pituitary gland. - It consists of 191 amino acid residues and is therefore a protein derivative. *Insulin* - **Insulin** is a **peptide hormone** composed of two polypeptide chains linked by disulfide bonds, making it a protein derivative. - It is synthesized in the beta cells of the pancreatic islets and plays a crucial role in glucose metabolism. *Glucagon* - **Glucagon** is a **peptide hormone** secreted by the alpha cells of the pancreatic islets, and is thus a protein derivative. - It consists of 29 amino acids and has an antagonistic effect to insulin, raising blood glucose levels.

Question 494: Diphtheria toxin acts by:

A. inhibiting acetylcholine release
B. inhibiting protein synthesis (Correct Answer)
C. inhibiting glucose transport
D. increasing levels of cyclic AMP

Explanation: ***inhibiting protein synthesis*** - Diphtheria toxin is an **exotoxin** produced by *Corynebacterium diphtheriae* that enters host cells. - It catalyzes the **ADP-ribosylation of elongation factor 2 (EF-2)**, which is crucial for protein synthesis, rendering it inactive and ultimately leading to cell death. *inhibiting acetylcholine release* - This mechanism is characteristic of **botulinum toxin**, which cleaves SNARE proteins involved in acetylcholine release at the neuromuscular junction. - Diphtheria toxin does not directly interfere with neurotransmitter release. *inhibiting glucose transport* - While some bacterial toxins can affect nutrient transport, **diphtheria toxin** specifically targets protein synthesis and does not primarily inhibit glucose transport. - Inhibition of glucose transport is not its main mechanism of action. *increasing levels of cyclic AMP* - This is the mechanism of action for several other toxins, such as **cholera toxin** and **pertussis toxin**, which activate adenylate cyclase, leading to increased **cAMP levels**. - Diphtheria toxin does not function by altering cyclic AMP levels.

Question 495: An example of a scleroprotein is

A. Zein
B. Glutenin
C. Ovoglobulin
D. Keratin (Correct Answer)

Explanation: ***Keratin*** - **Keratin** is a fibrous, structural protein and is a prime example of a **scleroprotein** (also known as a **fibrous protein**). - It provides strength and structural integrity to tissues like hair, nails, and the outer layer of skin. *Zein* - **Zein** is a prolamine protein found in corn. - It is an **alcohol-soluble protein** and is not classified as a scleroprotein. *Glutenin* - **Glutenin** is a major storage protein found in wheat. - It is classified as a **globulin** and contributes to the elasticity of dough, but is not a scleroprotein. *Ovoglobulin* - **Ovoglobulin** refers to a group of globular proteins found in egg white. - These are typically **soluble in water** or dilute salt solutions and do not share the fibrous, structural characteristics of scleroproteins.

Question 496: Which of the following is not a component of collagen?

A. Glycine
B. Lysine
C. Proline
D. Desmosine (Correct Answer)

Explanation: ***Correct: Desmosine*** - **Desmosine** is a unique amino acid derivative primarily found in **elastin**, not collagen - It forms **cross-links** crucial for elastin's elastic properties, allowing tissues to stretch and recoil - This makes it the component that is **NOT found in collagen** *Incorrect: Glycine* - **Glycine** is the most abundant amino acid in collagen, accounting for approximately **one-third** of its residues - Its small size allows for tight packing of the **collagen triple helix** (Gly-X-Y repeat pattern) *Incorrect: Lysine* - **Lysine** residues in collagen are important for forming **cross-links** between collagen fibrils, providing tensile strength - They undergo **hydroxylation** to hydroxylysine, which is essential for collagen stability and glycosylation *Incorrect: Proline* - **Proline** and its hydroxylated form, **hydroxyproline**, contribute to the stability of the collagen triple helix - The rigid ring structure of proline introduces constraints that are important for the unique structure of collagen

Question 497: At physiological pH, the carboxy-terminal of a peptide is

A. Negatively charged (Correct Answer)
B. Infinitely charged
C. Not charged
D. Positively charged

Explanation: ***Negatively charged*** - At **physiological pH (around 7.4)**, the **carboxyl group (-COOH)** at the carboxy-terminal of a peptide chain **ionizes** by losing a proton (H+). - This deprotonation results in a negatively charged carboxylate group (**-COO-**). - The **pKa of carboxyl groups** is typically around **2-3**, meaning at pH 7.4, the group is virtually completely deprotonated. *Infinitely charged* - This option is chemically and biologically incorrect; charges are specific and quantifiable, not infinite. - The charge on a molecule depends on its **ionization state**, which is finite and determined by pH and pKa values. *Not charged* - If the carboxyl group were not charged, it would remain in its **protonated form (-COOH)**. - This occurs at very **acidic pH** levels (pH < 3), far below physiological pH. *Positively charged* - A positive charge at the C-terminus would require protonation beyond the neutral form, which does not occur for carboxyl groups. - Students may confuse this with the **amino-terminus (N-terminus)**, which carries a **positive charge (-NH3+)** at physiological pH. - The carboxy-terminus has an **acidic group** (loses H+), while the amino-terminus has a **basic group** (gains H+).

Question 498: Which of the following is true about alpha-1 antitrypsin?

A. Inhibits elastase (Correct Answer)
B. Inhibits trypsin
C. Inhibits chymotrypsin
D. Inhibits trypsinogen activation

Explanation: ***Inhibits elastase*** - Alpha-1 antitrypsin (A1AT) primarily serves to **inhibit elastase**, a protease that can damage lung tissue, helping to protect the lungs from destruction [1]. - Deficiency of A1AT leads to **emphysema** and liver disease due to unchecked activity of elastase [1][2]. *Inhibits trypsin* - A1AT specifically does not primarily inhibit **trypsin**, which is involved in protein digestion in the intestine. - Although A1AT affects proteases, its main function is related to **elastase**, not trypsin [1]. *Inhibits trypsinogen activation in pancreas* - A1AT does not have a significant role in the **inhibition of trypsinogen activation** within the pancreas. - Instead, pancreatic enzyme regulation involves other mechanisms that do not involve A1AT's function. *Inhibits chymotrypsin* - A1AT is not known for inhibiting **chymotrypsin**, a serine protease derived from trypsinogen in the gut. - It specifically targets **elastase** and similar enzymes rather than chymotrypsin [1]. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Liver and Gallbladder, pp. 856-858. [2] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. The Lung, pp. 683-684.

Question 499: A 43-year-old woman presents with a several-year history of progressive abdominal colic and constipation. Colonic biopsy stained with Congo Red and polarizing microscopy reveals a specific appearance. This is due to which of the following protein properties?

A. Ability to bind to oxygen
B. Electrophoretic mobility
C. Hydroxyproline content
D. Beta pleated sheet secondary structure (Correct Answer)

Explanation: ***Beta pleated sheet tertiary structure*** - The appearance seen on **Congo Red staining** indicates the presence of **amyloid deposits**, which are rich in beta-pleated sheets [1]. - This structure is responsible for the characteristic **apple-green birefringence** under polarized light, confirming amyloidosis [2]. *Electrophoretic mobility* - While electrophoretic mobility can indicate charge differences in proteins, it does not specifically relate to the **amyloid nature** revealed by Congo Red staining. - This method is more about **separation based on charge and size**, rather than structural properties like pleated sheets. *Hydroxyproline content* - Hydroxyproline content is associated with the **collagen structure**, not with the formation of amyloid deposits, which consist mainly of proteins like **light chains or serum amyloid A**. - It does not provide information on the **beta pleated sheet** conformation found in amyloid fibrils. *Ability to bind to oxygen* - This feature is more relevant to **hemoglobin** and myoglobin and is unrelated to the formation of amyloid and its specific structures. - Amyloid does not function through **oxygen binding**, but rather through its unique **beta pleated sheet** conformation. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Diseases of the Immune System, pp. 264-266. [2] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Diseases of the Immune System, pp. 268-269.

Question 500: Which of the following is LEAST COMMONLY phosphorylated by protein kinases?

A. Asparagine (Correct Answer)
B. Threonine
C. Serine
D. Tyrosine

Explanation: ***Asparagine*** - **Asparagine** lacks a hydroxyl group (-OH) in its side chain, an essential prerequisite for most protein kinases to catalyze phosphorylation. - While theoretical phosphorylation of the amide nitrogen in asparagine has been proposed, it is exceedingly rare and generally not observed in biological systems compared to the hydroxyl-containing amino acids. *Threonine* - **Threonine** contains a hydroxyl group (-OH) in its side chain, making it a common substrate for phosphorylation by **serine/threonine kinases**. - Phosphorylation of threonine plays a crucial role in regulating protein activity and signaling pathways. *Serine* - **Serine** contains a hydroxyl group (-OH) in its side chain, making it the most frequently phosphorylated amino acid by **protein kinases**, particularly **serine/threonine kinases**. - Serine phosphorylation is fundamental to almost all aspects of cell regulation and signal transduction. *Tyrosine* - **Tyrosine** contains a hydroxyl group (-OH) within its phenolic ring, making it a key target for **tyrosine kinases**. - Tyrosine phosphorylation is particularly critical in growth factor signaling, cell proliferation, and immune responses.

Practice by Chapter

Amino Acids: Structure and Properties

Practice Questions

Peptide Bond Formation

Practice Questions

Primary Structure of Proteins

Practice Questions

Secondary Structure of Proteins

Practice Questions

Tertiary and Quaternary Structures

Practice Questions

Protein Folding and Chaperones

Practice Questions

Protein Domains and Motifs

Practice Questions

Structure-Function Relationships

Practice Questions

Hemoglobin and Myoglobin

Practice Questions

Collagen and Elastin

Practice Questions

Albumin and Plasma Proteins

Practice Questions

Post-Translational Modifications

Practice Questions

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