Protein Structure and Function — MCQs

Protein Structure and Function Indian Medical PG Practice Questions and MCQs

Question 481: Which of the following can be a homologous substitution for valine in hemoglobin?

A. Phenylalanine
B. Lysine
C. Glutamic acid
D. Isoleucine (Correct Answer)

Explanation: ***Isoleucine*** - **Isoleucine** is an **aliphatic amino acid** with a branched hydrocarbon side chain, making it structurally similar to valine. - This structural similarity allows it to fit into the same hydrophobic pocket in hemoglobin without significantly altering its function. *Glutamic acid* - **Glutamic acid** is an **acidic amino acid** with a carboxyl group in its side chain, making it polar and negatively charged. - Its substitution would introduce a significant charge difference and alter the local environment, potentially affecting hemoglobin's structure and function. *Phenylalanine* - **Phenylalanine** is an **aromatic amino acid** with a large, bulky benzene ring in its side chain. - This large side chain would likely cause **steric hindrance** if it were to replace valine, disrupting the normal conformation of hemoglobin. *Lysine* - **Lysine** is a **basic amino acid** with a long side chain containing an amine group, making it polar and positively charged. - Replacing valine with lysine would introduce a positive charge and a different side chain geometry, which could significantly impact hemoglobin's stability and function.

Question 482: The protein content in 100 g of cow's milk is

A. 2.2 g
B. 3.2 g (Correct Answer)
C. 4.2 g
D. 1.2 g

Explanation: ***3.2 g*** - **Cow's milk** typically contains approximately **3.2 to 3.5 grams** of protein per 100 grams, making this the correct option. - The primary proteins in cow's milk are **casein** and **whey proteins**, both offering essential amino acids. *2.2 g* - This value is **too low** for the typical protein content found in 100 grams of cow's milk. - Such a low protein content would generally mean the milk has been diluted or processed significantly. *4.2 g* - This value is **higher than the average** protein content for 100 grams of regular cow's milk. - While some specialized or concentrated milk products might have higher protein, it's not typical for standard cow's milk. *1.2 g* - This value is **significantly lower** than the usual protein content of cow's milk. - This would indicate a highly diluted product or a different food item altogether.

Question 483: Which of the following is not a hemoprotein?

A. Myoglobin
B. Elastin (Correct Answer)
C. Cytochrome P450
D. Catalase

Explanation: ***Correct: Elastin*** - **Elastin** is a structural protein primarily found in **connective tissues** that provides elasticity to organs and tissues (skin, blood vessels, lungs) - It does NOT contain a **heme group** and is therefore not classified as a hemoprotein - Functions purely as a structural component without any prosthetic groups *Incorrect: Myoglobin* - **Myoglobin** is an iron- and oxygen-binding protein found in muscle tissue - Contains a single **heme group** with an iron atom, making it a quintessential hemoprotein - Functions in oxygen storage and delivery in muscle cells *Incorrect: Cytochrome P450* - **Cytochrome P450** enzymes are a superfamily of hemoproteins containing a **heme prosthetic group** - The heme iron is crucial for their role in **drug metabolism** and detoxification - Involved in metabolism of endogenous and exogenous compounds in the liver *Incorrect: Catalase* - **Catalase** is an enzyme that catalyzes decomposition of hydrogen peroxide (H₂O₂) into water and oxygen - Contains a **heme prosthetic group** with an **iron atom** essential for its enzymatic activity - One of the most efficient enzymes, protecting cells from oxidative damage

Question 484: The comparison of the amino acid sequence of cytochrome C across different species shows many variations. Where are these variations predominantly found?

A. Only in strand regions.
B. Mainly in loop regions. (Correct Answer)
C. Only in helical regions.
D. Randomly distributed throughout the sequence.

Explanation: ***Mainly in loop regions.*** - **Loop regions** in proteins are generally more flexible and less constrained than helical or strand regions. - This higher flexibility allows for greater variation in amino acid sequence without significantly altering the protein's overall structure or function, as they often don't participate directly in the active site or structural core. *Only in helical regions.* - **Helical regions** (alpha-helices) are fundamental to the protein's stable tertiary structure, and amino acid substitutions here are more likely to disrupt secondary and tertiary structure. - The packing of amino acids within helices is precise, and changes can destabilize the protein. *Only in strand regions.* - **Strand regions** (beta-sheets) are also crucial for maintaining the protein's stable structural core. - Alterations in these regions can significantly impact inter-strand hydrogen bonding and overall protein folding, leading to functional impairment. *Randomly distributed throughout the sequence.* - Variation is not entirely random; evolution selects against changes in functionally and structurally critical regions. - While some random mutations occur, those that persist and are observed across species tend to be in less critical areas, such as **loop regions**, to preserve function.

Question 485: Which of the following is structurally similar to growth hormone?

A. Insulin
B. Human Placental lactogen (Correct Answer)
C. Somatostatin
D. Human chorionic gonadotropin

Explanation: ***Human Placental lactogen*** - **Human placental lactogen (hPL)**, also known as **chorionic somatomammotropin**, is highly similar in structure to **growth hormone** due to a close evolutionary relationship. - Both hormones are single-chain polypeptides with significant sequence homology and share some biological functions, including **metabolic effects** and promotion of **growth**. *Insulin* - **Insulin** is a peptide hormone consisting of two polypeptide chains linked by disulfide bonds, which is structurally distinct from the single-chain **growth hormone**. - While both regulate metabolism, their primary functions and receptor binding mechanisms are different, reflecting their distinct structures. *Somatostatin* - **Somatostatin** is a small peptide hormone that functions primarily as an inhibitory neurohormone and gastrointestinal hormone, acting to suppress the secretion of many other hormones, including **growth hormone**. - Its molecular structure is significantly smaller and distinct from the larger, single-chain structure of **growth hormone**. *Human chorionic gonadotropin* - **Human chorionic gonadotropin (hCG)** is a glycoprotein hormone composed of two distinct subunits (alpha and beta), which is structurally different from the single-chain polypeptide structure of **growth hormone**. - **hCG** primarily functions in maintaining pregnancy by stimulating the corpus luteum, a function unrelated to the primary actions of **growth hormone**.

Question 486: Following are polar amino acids, except:

A. Arginine
B. Histidine
C. Serine
D. Isoleucine (Correct Answer)

Explanation: ***Isoleucine*** - Isoleucine is classified as a **non-polar amino acid** due to its **hydrocarbon side chain**. - Its side chain (–CH(CH₃)CH₂CH₃) consists entirely of carbon and hydrogen atoms, making it **hydrophobic** and unable to form hydrogen bonds. *Serine* - Serine is a **polar, uncharged amino acid** because of its **hydroxyl (-OH) group** in the side chain. - The hydroxyl group allows serine to participate in **hydrogen bonding**, making it hydrophilic. *Arginine* - Arginine is a **polar, basic (positively charged) amino acid** due to its **guanidinium group** in the side chain. - The guanidinium group is highly basic and **positively charged at physiological pH**, making it very hydrophilic. *Histidine* - Histidine is a **polar amino acid** with an **imidazole ring** in its side chain. - The imidazole ring can be uncharged or positively charged depending on the pH, making it capable of forming **hydrogen bonds** and acting as a proton donor or acceptor.

Question 487: Which amino acid among the following has the maximum tendency to bind phosphate?

A. Serine (Correct Answer)
B. Alanine
C. Phenylalanine
D. Tryptophan

Explanation: ***Serine*** - **Serine** contains a **hydroxyl group (-OH)** in its side chain, making it a primary target for **phosphorylation** by kinases. - The hydroxyl group forms a **phosphate ester bond** with phosphate, playing a crucial role in **cell signaling and enzyme regulation**. - Along with **threonine and tyrosine**, serine is one of the **three amino acids** commonly phosphorylated in eukaryotic proteins. - **Most common** phosphorylation site among the three due to abundance and accessibility. *Alanine* - **Alanine** has a simple **methyl group (-CH₃)** as its side chain, which is chemically inert. - Lacks a **reactive functional group** (hydroxyl, sulfhydryl, or phenolic) capable of forming covalent bonds with phosphate. - Cannot undergo phosphorylation. *Phenylalanine* - **Phenylalanine** contains a **benzyl side chain** (phenyl ring attached to CH₂), which is non-polar and aromatic. - Does not possess a **hydroxyl or other reactive group** needed for phosphate ester bond formation. - Not a substrate for kinases. *Tryptophan* - **Tryptophan** has a large **indole ring system** in its side chain, making it bulky, aromatic, and hydrophobic. - Although the indole contains nitrogen, the side chain does **not undergo direct phosphorylation**. - Not recognized as a substrate by protein kinases.

Question 488: Which of the following is a polar amino acid?

A. Tryptophan
B. Serine (Correct Answer)
C. Valine
D. Tyrosine

Explanation: ***Serine*** - Serine is a **polar, uncharged amino acid** due to the presence of a **hydroxyl group (-OH)** in its side chain. - The hydroxyl group allows serine to participate in hydrogen bonding, making it hydrophilic and thus polar. - Among the polar amino acids, serine is **unambiguously classified** as polar with no aromatic characteristics. *Tryptophan* - Tryptophan is an **aromatic amino acid** with a large, nonpolar indole ring in its side chain. - While it contains a nitrogen atom, the overall bulk and structure of its side chain render it largely **nonpolar** or slightly amphipathic. *Valine* - Valine is a **branched-chain amino acid** with an isopropyl group in its side chain. - This side chain consists entirely of **hydrocarbon atoms**, making it distinctly **nonpolar** and hydrophobic. *Tyrosine* - Tyrosine is technically a **polar amino acid** due to its phenolic hydroxyl group that can participate in hydrogen bonding. - However, it is primarily classified as an **aromatic amino acid** due to its benzene ring structure. - In many classification schemes, tyrosine's **dual nature** (aromatic + polar) makes it less straightforward than serine, which is unambiguously polar. - In this context, **serine** is the clearest example of a polar amino acid without aromatic characteristics.

Question 489: Ubiquitin is involved in what process?

A. Protein folding
B. Protein degradation (Correct Answer)
C. Synthesis of nucleic acid
D. Glycosylation of proteins

Explanation: ***Protein degradation*** - **Ubiquitin** is a small regulatory protein that attaches to other proteins as a signal, primarily for their **degradation** by the **proteasome**. - This process, known as **ubiquitination**, marks misfolded, damaged, or no longer needed proteins for targeted destruction. *Protein folding* - This process is primarily mediated by **chaperone proteins**, which assist in the correct three-dimensional structuring of polypeptides. - While ubiquitin can sometimes influence protein folding indirectly by marking misfolded proteins for degradation, its direct role is not in the folding itself. *Synthesis of nucleic acid* - The synthesis of **nucleic acids** (DNA and RNA) is carried out by **DNA polymerases** and **RNA polymerases**, respectively. - Ubiquitin is not involved in the enzymatic processes of replication or transcription. *Glycosylation of proteins* - **Glycosylation** is the enzymatic addition of carbohydrate moieties to proteins, typically occurring in the **endoplasmic reticulum** and **Golgi apparatus**. - This process is crucial for protein function, trafficking, and cell-cell recognition, but ubiquitin has no direct role in it.

Question 490: Which of the following is a type of covalent bond?

A. Hydrogen bond
B. Disulfide bond (Correct Answer)
C. Ionic bond
D. Electrostatic bond

Explanation: ***Correct: Disulfide bond*** - A **disulfide bond** is formed by the oxidation of two **thiol** (sulfhydryl) groups, creating a strong **covalent bond** between two sulfur atoms. - These bonds are crucial for stabilizing the **tertiary and quaternary structures of proteins**, contributing significantly to their overall shape and function. *Incorrect: Hydrogen bond* - A **hydrogen bond** is a **weak electrostatic attraction** between a hydrogen atom covalently bonded to a highly electronegative atom (like oxygen or nitrogen) and another electronegative atom. - It is an **intermolecular force** or a weak intramolecular force, not a covalent bond that involves the sharing of electrons. *Incorrect: Ionic bond* - An **ionic bond** is formed by the **complete transfer of electrons** from one atom to another, resulting in the formation of oppositely charged ions that attract each other. - This bond involves **electrostatic attraction** between ions, rather than the sharing of electrons characteristic of covalent bonds. *Incorrect: Electrostatic bond* - An **electrostatic bond** is a general term for the attractive force between oppositely charged particles, encompassing **ionic bonds** and other weaker interactions. - This term describes the **nature of the attraction** rather than the specific type of chemical bond (like covalent, which involves electron sharing).

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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