Protein Structure and Function Practice Questions

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

6. ***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.

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

Clathrin. ***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.

Q: 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?

I, II and III. ***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)**.

Q: The biological value of a given protein depends upon

Digestibility and amino acid composition. ***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.

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

IgG and IgM can both cross the placenta. ***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.

Q: Compared to breast milk, colostrum has

higher protein content. ***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.

Q: Beta hCG is structurally similar to which biochemical moiety?

LH. ***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 1

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

Accepted Answer

6

Answer

2

Answer

9

Answer

4.4

Question 2

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

Accepted Answer

Clathrin

Answer

Ubiquitin

Answer

Pallidin

Answer

Actin

Question 3

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

Accepted Answer

Three strands of polypeptide twist to the right and wrap around each other in left handed fashion

Answer

3.3 residues per turn

Answer

Every third amino acid is glycine and $Y$ is generally proline or hydroxyproline

Answer

Most abundant protein in body

Question 4

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

Accepted Answer

The protein has 1 functional ion

Answer

A and B represent ionization of amino and carboxyl end of the protein

Answer

The protein has 3 ionizable sites

Answer

Points A and B represent first and second equivalence point

Question 5

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?

Accepted Answer

I, II and III

Answer

II, III and IV

Answer

I only

Answer

I, III and IV

Question 6

The biological value of a given protein depends upon

Accepted Answer

Digestibility and amino acid composition

Answer

Digestibility and leucine content

Answer

Amino acid composition and specific dynamic effect

Answer

Amino acid composition alone

Question 7

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

Accepted Answer

IgG and IgM can both cross the placenta

Answer

The half life of IgM is about 10 days

Answer

IgG comprises about 75-80% of the total serum immunoglobulins

Answer

IgA comprises 15% of the total serum immunoglobulins and is mainly found in the body secretions

Question 8

Compared to breast milk, colostrum has

Accepted Answer

higher protein content

Answer

lower water content

Answer

higher carbohydrate content

Answer

higher fat content

Question 9

Beta hCG is structurally similar to which biochemical moiety?

Accepted Answer

LH

Answer

Oxytocin

Answer

FSH

Answer

ACTH

Question 10

Chaperones are:

Accepted Answer

Mediators of post-translational assembly of protein complexes

Answer

Antigen presenting cells

Answer

Purine metabolism mediators

Answer

None of the above

Protein Structure and Function — MCQs

Protein Structure and Function — MCQs

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