Protein Structure and Function — MCQs

Egg proteins are considered to be the best among food proteins because : I. of their biological value II. of their digestibility III. they contain all the essential amino acids IV. their calorie content is higher than in proteins of vegetable sources Which of the statements given above is/are correct?

Q366

The biological value of a given protein depends upon

Q367

Which one of the following statements about Human Immunoglobulins is not correct?

Q368

Compared to breast milk, colostrum has

Q369

Beta hCG is structurally similar to which biochemical moiety?

Q370

Chaperones are:

Protein Structure and Function Indian Medical PG Practice Questions and MCQs

Question 361: What is the pH of the zwitterion as shown in the titration curve of glycine?

A. 6 (Correct Answer)
B. 2
C. 9
D. 4.4

Explanation: ***Correct Option: 6*** - The pH of the zwitterion corresponds to the **isoelectric point (pI)**, the pH at which the amino acid has a net charge of zero. According to the curve, this occurs at pH 5.97, which is approximately 6. - The pI is calculated as the average of the two pKa values: **pI = (pKa1 + pKa2) / 2 = (2.34 + 9.60) / 2 = 5.97**. *Incorrect Option: 4.4* - At a pH of 4.4, which is between pKa1 and pI, the glycine molecule would have a net **positive charge** as the concentration of the cationic form ([NH3+-CH2-COOH]) would still be significant. - This value does not represent any specific inflection or equivalence point on the titration curve for glycine. *Incorrect Option: 2* - A pH of approximately 2 is near the **pKa1 (2.34)** of the carboxyl group, which is the point where the concentration of the fully protonated form equals the concentration of the zwitterion. - At this pH, the molecule has a net **positive charge**, as the amino group is fully protonated and the carboxyl group is only half-deprotonated. *Incorrect Option: 9* - A pH of approximately 9 is near the **pKa2 (9.60)** of the amino group, representing the buffering region for this group. - At this pH, the molecule has a net **negative charge**, as there is a significant amount of the fully deprotonated form ([NH2-CH2-COO-]) present.

Question 362: Which protein plays a role in the process shown below? (Recent NEET Pattern 2018)

A. Clathrin (Correct Answer)
B. Ubiquitin
C. Pallidin
D. Actin

Explanation: ***Clathrin*** - **Clathrin** is the primary protein involved in forming coated pits and vesicles during receptor-mediated endocytosis, which is the process shown in the image. - It forms a characteristic **triskelion structure** (three-legged complex) that assembles into a polyhedral lattice around the budding vesicle. - This protein coat provides the mechanical force needed to bend the membrane and form vesicles for internalizing substances. - Clathrin-mediated endocytosis is the **most well-characterized pathway** for receptor-mediated uptake of molecules. *Pallidin* - Pallidin is a component of the **BLOC-1 complex** (Biogenesis of Lysosome-related Organelles Complex-1). - It is involved in the biogenesis and trafficking of lysosome-related organelles, not in the primary vesicle formation during endocytosis. - Mutations in pallidin cause Hermansky-Pudlak syndrome type 9. *Ubiquitin* - Ubiquitin is a small regulatory protein that marks other proteins for degradation via the **proteasome** or targets them for endocytosis. - While ubiquitination can signal for receptor internalization, ubiquitin itself does not form vesicle coats or directly participate in vesicle budding. *Actin* - Actin is a cytoskeletal protein that plays a supporting role in endocytosis, particularly in membrane invagination and vesicle scission. - It provides mechanical support and force for vesicle movement but is **not the primary coat protein** forming the vesicle structure during initial budding.

Question 363: All are correct about the protein shown in the figure except? (Recent NEET Pattern 2016-17)

A. Three strands of polypeptide twist to the right and wrap around each other in left handed fashion (Correct Answer)
B. 3.3 residues per turn
C. Every third amino acid is glycine and $Y$ is generally proline or hydroxyproline
D. Most abundant protein in body

Explanation: ***Three strands of polypeptide twist to the right and wrap around each other in left handed fashion*** - This statement is **INCORRECT** and describes the handedness **backwards**. This is the answer to the "except" question. - **Correct structure of collagen**: Each individual polypeptide chain forms a **left-handed helix** with approximately 3.3 residues per turn. - These three **left-handed helices** then wrap around each other to form a **right-handed triple helix (superhelix)**. - The statement incorrectly says the strands "twist to the right and wrap in left-handed fashion" - this reverses the actual handedness of the superhelix, which is **right-handed**, not left-handed. *3.3 residues per turn* - This statement is **CORRECT** about collagen structure. - The individual collagen polypeptide chains (before forming the triple helix) have approximately **3.3 amino acid residues per turn** in their left-handed helical conformation. - This differs from the **α-helix**, which has **3.6 residues per turn** and is right-handed. - The polyproline II helix structure of collagen chains is extended and has this characteristic 3.3 residues per turn. *Every third amino acid is glycine and Y is generally proline or hydroxyproline* - This statement is **CORRECT** and describes the characteristic collagen sequence pattern **(Gly-X-Y)n**. - **Glycine** must occupy every third position because its small side chain (just a hydrogen atom) is the only one small enough to fit in the crowded center of the triple helix where the three chains come together. - **Proline** and **hydroxyproline** frequently occur at the X and Y positions (especially hydroxyproline at Y). Their rigid ring structures are crucial for stabilizing the triple helix through conformational constraints and additional hydrogen bonding. *Most abundant protein in body* - This statement is **CORRECT**. Collagen is the most abundant protein in the human body, comprising approximately **25-35%** of total body protein mass. - It is the major structural protein of **connective tissues** including skin, bone, tendons, ligaments, cartilage, and blood vessels. - There are multiple types of collagen (Type I being most abundant), each serving specific structural roles in different tissues.

Question 364: All of the statements are true about the titration curve of a protein except:

A. A and B represent ionization of amino and carboxyl end of the protein
B. The protein has 3 ionizable sites
C. The protein has 1 functional ion (Correct Answer)
D. Points A and B represent first and second equivalence point

Explanation: ***The protein has 1 functional ion*** - The titration curve shows **two distinct buffering regions (plateau regions)**, labeled A and B. Each buffering region corresponds to the deprotonation of a different ionizable group. - Therefore, the protein has **at least two ionizable groups**, not just one, making this statement **clearly false**. *A and B represent ionization of amino and carboxyl end of the protein* - The first buffering region (A) typically corresponds to the ionization of the **carboxyl group (-COOH)**, which has a lower pKa. - The second buffering region (B) typically corresponds to the ionization of the **amino group (-NH3+)** or a basic side chain, which has a higher pKa. - This statement is **true** for the visible buffering regions. *The protein has 3 ionizable sites* - While the curve shows **two distinct buffering regions** within the observed pH range, many proteins have **additional ionizable groups** (such as side chains of amino acids like His, Lys, Arg, Asp, Glu, Cys, Tyr) that may ionize outside the pH range shown or overlap with other ionizations. - A simple amino acid has at minimum 3 ionizable groups: **α-carboxyl, α-amino, and at least one side chain group** (if it has an ionizable R group). - This statement is **true** - the protein likely has 3 or more ionizable sites, though only 2 are clearly visible as distinct buffering regions in this particular pH range. *Points A and B represent first and second equivalence point* - Points A and B represent the **buffering regions (plateaus)** of the titration curve, where the pH changes slowly with added base, indicating pKa values where equal amounts of protonated and deprotonated forms exist. - The **equivalence points** are located at the **steep portions** (inflection points) between these plateaus, where the amount of added base exactly neutralizes all protons from an ionizable group. - This statement is **false**, but represents a common misconception. A and B mark the pKa values (midpoints of buffering regions), not equivalence points.

Question 365: Egg proteins are considered to be the best among food proteins because : I. of their biological value II. of their digestibility III. they contain all the essential amino acids IV. their calorie content is higher than in proteins of vegetable sources Which of the statements given above is/are correct?

A. II, III and IV
B. I, II and III (Correct Answer)
C. I only
D. I, III and IV

Explanation: ***I, II and III*** - Egg proteins are considered high-quality due to their **high biological value**, meaning they are efficiently utilized by the body. This is a direct consequence of their **excellent digestibility** and complete amino acid profile. - They contain **all nine essential amino acids** in proportions that closely match human needs, making them a complete protein source. This characteristic is crucial for muscle repair, enzyme production, and overall bodily functions. *II, III and IV* - While egg proteins are highly digestible and contain all essential amino acids (II and III are correct), their **calorie content is not necessarily higher than proteins from vegetable sources (IV is incorrect)**. Calorie content depends on the specific food and its overall macronutrient composition, not just the protein source. - **Biological value** is a key reason egg proteins are considered superior, and this option omits it. *I only* - While the **high biological value (I)** of egg proteins is a fundamental reason for their quality, it is not the only reason. Their excellent digestibility and comprehensive essential amino acid profile are equally important factors. - This option is incomplete as it fails to acknowledge the other critical attributes that contribute to the superiority of egg proteins. *I, III and IV* - Egg proteins do have a **high biological value (I)** and contain **all essential amino acids (III)**. However, statement IV, which claims their calorie content is higher than vegetable proteins, is generally **incorrect**. - This option is incorrect because it includes an inaccurate statement about calorie content and overlooks the crucial aspect of **digestibility (II)**.

Question 366: The biological value of a given protein depends upon

A. Digestibility and leucine content
B. Digestibility and amino acid composition (Correct Answer)
C. Amino acid composition and specific dynamic effect
D. Amino acid composition alone

Explanation: ***Digestibility and amino acid composition*** - The **biological value (BV)** of a protein measures how efficiently the body can use the absorbed protein from a food source for growth and maintenance. - It is primarily determined by two factors: the **digestibility** of the protein (how much is absorbed) and its **amino acid composition** (the proportion and availability of essential amino acids). *Digestibility and leucine content* - While **digestibility** is an important factor for biological value, **leucine content** alone is not sufficient to determine it. - The biological value depends on the **overall balance of essential amino acids**, not just one specific amino acid. *Amino acid composition and specific dynamic effect* - **Amino acid composition** is crucial for biological value, but the **specific dynamic effect (SDE)**, also known as the thermic effect of food, refers to the energy expended during the digestion, absorption, and assimilation of food. - SDE is related to energy metabolism, not directly to the protein's utility for tissue synthesis. *Amino acid composition alone* - **Amino acid composition** is a key determinant, particularly the presence and proportion of **essential amino acids**. - However, for a protein to be utilized, it must first be **digested and absorbed**, making digestibility an equally critical factor affecting its overall biological value.

Question 367: Which one of the following statements about Human Immunoglobulins is not correct?

A. The half life of IgM is about 10 days
B. IgG comprises about 75-80% of the total serum immunoglobulins
C. IgA comprises 15% of the total serum immunoglobulins and is mainly found in the body secretions
D. IgG and IgM can both cross the placenta (Correct Answer)

Explanation: ***IgG and IgM can both cross the placenta*** - This statement is incorrect because only **IgG** can actively cross the placenta, providing **passive immunity** to the fetus. - **IgM** is too large to cross the placental barrier, so its presence in a newborn suggests **congenital infection**. *The half life of IgM is about 10 days* - The approximate **half-life of IgM** is indeed around **5-10 days**, making this statement largely correct in general contexts. - This relatively short half-life means that IgM levels can change quickly in response to acute infections. *IgG comprises about 75-80% of the total serum immunoglobulins* - **IgG** is the most abundant immunoglobulin in serum, typically accounting for **75-80%** of the total immunoglobulins. - Its high concentration and long half-life contribute to its critical role in **secondary immune responses** and **long-term immunity**. *IgA comprises 15% of the total serum immunoglobulins and is mainly found in the body secretions* - **IgA** accounts for about **10-15%** of the total serum immunoglobulins, with a significant proportion found in secretions. - It plays a crucial role in **mucosal immunity**, protecting surfaces like the gastrointestinal, respiratory, and genitourinary tracts.

Question 368: Compared to breast milk, colostrum has

A. higher protein content (Correct Answer)
B. lower water content
C. higher carbohydrate content
D. higher fat content

Explanation: ***higher protein content*** - Colostrum is rich in **immunoglobulins** (antibodies) and other **protective proteins**, which are crucial for the newborn's immune system. - These proteins, including **IgA**, **lactoferrin**, and **growth factors**, contribute to its higher protein concentration (2-3 times higher) compared to mature breast milk. *lower water content* - Colostrum does have **slightly lower water content** (~87%) compared to mature breast milk (~88-90%), making it more concentrated. - However, this difference is minimal and not the most clinically significant distinguishing feature compared to the marked protein difference. *higher carbohydrate content* - Colostrum has a **lower carbohydrate content** (primarily lactose) compared to mature breast milk. - Mature milk develops a higher lactose content to support the infant's increasing energy demands. *higher fat content* - **Mature breast milk** has a significantly **higher fat content** than colostrum. - The fat in mature milk provides the primary source of energy for the rapidly growing infant, which is less critical in early colostrum feeding.

Question 369: Beta hCG is structurally similar to which biochemical moiety?

A. LH (Correct Answer)
B. Oxytocin
C. FSH
D. ACTH

Explanation: ***LH*** - **Beta-hCG** and **Luteinizing Hormone (LH)** both belong to the family of glycoprotein hormones and share a common alpha subunit. - Their **beta subunits** are also very similar, with a significant overlap in their amino acid sequences, allowing **hCG** to bind to and activate **LH receptors**. *Oxytocin* - **Oxytocin** is a **peptide hormone** primarily involved in uterine contractions and milk ejection. - It is structurally distinct from **glycoprotein hormones** like **hCG** and lacks the alpha and beta subunit structure. *FSH* - **Follicle-Stimulating Hormone (FSH)** is a glycoprotein hormone that shares the same alpha subunit with hCG, LH, and TSH. - However, its **beta subunit** is distinct from that of **hCG**, which gives it its unique biological specificity. *ACTH* - **Adrenocorticotropic Hormone (ACTH)** is a **peptide hormone** produced by the anterior pituitary gland, regulating adrenal gland function. - It has a completely different structure and function compared to **hCG** and other glycoprotein hormones.

Question 370: Chaperones are:

A. Mediators of post-translational assembly of protein complexes (Correct Answer)
B. Antigen presenting cells
C. Purine metabolism mediators
D. None of the above

Explanation: ***Mediators of post-translational assembly of protein complexes*** - **Chaperones** are proteins that assist in the proper folding of other proteins, especially new polypeptides, and in the assembly of **protein complexes** after translation. - They prevent **misfolding** and aggregation of proteins, ensuring their correct functional conformation. *Antigen presenting cells* - **Antigen-presenting cells (APCs)** are immune cells (e.g., macrophages, dendritic cells) that present **antigens** to T cells for recognition. - Their primary function is in the **immune response**, not protein folding or assembly. *Purine metabolism mediators* - **Purine metabolism mediators** are enzymes or molecules involved in the synthesis, breakdown, and recycling of **purines (adenine and guanine)**. - This function is entirely distinct from the role of chaperones in **protein folding**. *None of the above* - This option is incorrect because the first option accurately describes the function of **chaperones**.

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

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Structure-Function Relationships

Practice Questions

Hemoglobin and Myoglobin

Practice Questions

Collagen and Elastin

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Albumin and Plasma Proteins

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Post-Translational Modifications

Practice Questions

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