Protein Structure and Function — MCQs

Protein Structure and Function Indian Medical PG Practice Questions and MCQs

Question 371: Most abundant amino acid found in collagen?

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

Explanation: ***Glycine*** - **Glycine** is the most abundant amino acid in collagen, making up approximately one-third of its total amino acid content. - Its small size (due to a single hydrogen atom as its side chain) is crucial for the formation of the **triple helix structure** of collagen, allowing the tight packing of the three alpha chains. *Hydroxyproline* - **Hydroxyproline** is a modified amino acid derived from proline, and while abundant in collagen, it accounts for about 13% of the total amino acids, less than glycine. - It plays a vital role in stabilizing the collagen triple helix through **hydrogen bonding**. *Proline* - **Proline** is a structurally important amino acid in collagen, contributing to the kinks and turns necessary for the formation of the triple helix. - However, its abundance is less than that of glycine, making up around 10-17% of collagen's amino acids. *Lysine* - **Lysine** is a less abundant but essential amino acid in collagen, making up about 2-5% of its composition. - It is critical for **cross-linking** collagen fibers, which provides tensile strength, and can be hydroxylated to form **hydroxylysine**.

Question 372: Which structure of protein is not denatured after heating up to 100 degrees Celsius?

A. Primary (Correct Answer)
B. Quaternary
C. Tertiary
D. Secondary

Explanation: ***Primary*** - The **primary structure** refers to the specific linear sequence of **amino acids** forming the polypeptide chain, linked by **covalent peptide bonds**. - These strong **peptide bonds** are generally resistant to heat denaturation at 100°C, meaning the amino acid sequence remains intact. *Quaternary* - The **quaternary structure** involves the arrangement of multiple polypeptide subunits and is maintained by weaker interactions like **hydrophobic interactions**, hydrogen bonds, and salt bridges. - These interactions are highly susceptible to disruption by heat, causing the subunits to dissociate and the quaternary structure to be lost. *Tertiary* - The **tertiary structure** describes the three-dimensional folding of a single polypeptide chain, stabilized by various non-covalent interactions (e.g., hydrogen bonds, ionic bonds, hydrophobic interactions) and **disulfide bonds**. - Heat disrupts these weaker non-covalent interactions and can even break disulfide bonds, leading to the unfolding and loss of the specific 3D shape. *Secondary* - The **secondary structure** (e.g., **alpha-helices** and **beta-pleated sheets**) arises from hydrogen bonds between the backbone atoms of the polypeptide chain. - While peptide bonds remain intact, these vital **hydrogen bonds** are easily broken by heat, causing the unraveling of helices and sheets.

Question 373: Prion disease is caused by

A. Denaturation of protein
B. Reduced formation of proteins
C. Misfolding of proteins (Correct Answer)
D. Excess formation of proteins

Explanation: ***Misfolding of proteins*** - Prion diseases are caused by the accumulation of abnormally **misfolded prion proteins (PrPSc)**, which convert normal cellular prion proteins (PrPC) into the harmful form. - This **conformational change** leads to the aggregation of these misfolded proteins, forming amyloid plaques that result in neurodegeneration. *Denaturation of protein* - **Denaturation** refers to the loss of a protein's tertiary and secondary structure, often caused by external stressors like heat or pH changes, but it doesn't describe the specific, self-propagating misfolding seen in prion diseases. - While misfolding involves a change in protein structure, denaturation itself is a broader term and doesn't fully capture the infectious nature of prions. *Reduced formation of proteins* - Prion diseases are generally not characterized by a **reduction in protein synthesis**; instead, they involve an alteration in the existing protein structure and its subsequent accumulation. - A decrease in protein formation would lead to different pathological outcomes, such as deficiencies or loss of function, rather than neurodegenerative plaques. *Excess formation of proteins* - Although there is an accumulation of misfolded prion proteins, the disease is not primarily caused by the **overproduction of normal prion protein (PrPC)**. - The critical event is the **conversion** of correctly folded PrPC into the misfolded PrPSc, not merely an increased quantity of the native form.

Question 374: Which of the following is an amine hormone?

A. FSH
B. TSH
C. Insulin
D. T4 (Correct Answer)

Explanation: ***T4*** - **Thyroxine (T4)** is a **thyroid hormone** derived from the amino acid **tyrosine**. - It is classified as an **amine hormone** (also called **amino acid-derived hormone**) because it is synthesized from a single amino acid and contains an **amine group**. - Other amine hormones include **T3, epinephrine, norepinephrine, and dopamine**. *FSH* - **Follicle-stimulating hormone (FSH)** is a **glycoprotein hormone**, not an amine hormone. - It is composed of **alpha and beta subunits** and plays a role in **gonad development** and **reproductive function**. *TSH* - **Thyroid-stimulating hormone (TSH)** is also a **glycoprotein hormone**. - It stimulates the **thyroid gland** to produce thyroid hormones (T3 and T4). *Insulin* - **Insulin** is a **peptide hormone**, consisting of **amino acid chains** (51 amino acids in two chains) linked by **disulfide bonds**. - It is critical for **glucose regulation** and is significantly larger and more complex than amine hormones.

Question 375: Free radicals are chemical species which have

A. Double paired electron in an outer orbital
B. Single paired electron in an outer orbital
C. Single unpaired electron in an outer orbital (Correct Answer)
D. Double unpaired electron in an outer orbital

Explanation: ***Single unpaired electron in an outer orbit*** - Free radicals are defined by having at least one **unpaired electron** in their outermost orbital. - This unpaired electron makes them highly **reactive** and unstable, seeking to pair with an electron from another molecule. *Double paired electron in an outer orbit* - This describes a **stable molecule** where all electrons in the outer shell are paired, forming covalent bonds. - Such molecules are not free radicals and do not exhibit the same high reactivity. *Single paired electron in an outer orbit* - An electron is generally either paired (meaning two electrons occupying one orbital) or unpaired (meaning one electron occupying an orbital). A "single paired electron" is a conceptual contradiction. - If an orbital has a single electron, it is by definition unpaired; if it's paired, there are two electrons. *Double unpaired electron in an outer orbit* - While it's possible for some species to have two unpaired electrons (e.g., diradicals), the defining characteristic of *any* free radical is the presence of *at least one* **single unpaired electron**. - This option incorrectly suggests that two unpaired electrons are necessary or universally defining, rather than just one.

Question 376: Which of the following are supportive proteins?

A. Titin
B. Dystropin
C. All of the options (Correct Answer)
D. Desmin

Explanation: ***Correct: All of the options*** - **Titin**, **Dystrophin**, and **Desmin** are all well-known supportive proteins within muscle tissue, playing crucial roles in maintaining structural integrity and function. - These proteins contribute to the elasticity, stability, and transmission of force within muscle fibers. **Titin** - A giant sarcomeric protein responsible for muscle elasticity and maintaining the structural integrity of myofibrils. - Acts as a molecular spring, anchoring myosin thick filaments to the Z-disk and contributing to passive muscle force. **Dystrophin** - Provides a critical link between the muscle cytoskeleton and the extracellular matrix. - Its absence or malfunction leads to muscle degeneration, as seen in **Duchenne muscular dystrophy**. **Desmin** - An intermediate filament protein that forms a scaffold around sarcomeres. - Connects sarcomeres to each other, to the sarcolemma, and to organelles like mitochondria. - Essential for maintaining alignment and mechanical integration of myofibrils within muscle cells.

Question 377: Human serum albumin, the most abundant blood protein, has multiple roles, including acting as a buffer to help maintain blood pH. Albumin can act as a buffer because of which one of the following?

A. The amino and carboxyl ends of albumin can donate and accept protons in the range of physiologic pH.
B. Albumin contains peptide bonds that readily hydrolyze, consuming hydrogen and hydroxyl ions.
C. The protein contains many amino acid residues with different pKa values. (Correct Answer)
D. The protein contains a large number of amino acids.

Explanation: ***The protein contains many amino acid residues with different pKa values.*** - **Albumin's buffering capacity** primarily arises from the diverse **pKa values** of its numerous ionizable side chains. These groups can **donate or accept protons** over a range of physiological pH. - This allows albumin to effectively resist changes in blood pH by buffering both **acids and bases** that enter the bloodstream. *The amino and carboxyl ends of albumin can donate and accept protons in the range of physiologic pH.* - While the **N-terminal amino group** and **C-terminal carboxyl group** of any protein can ionize, their contribution to the overall buffering capacity of a large protein like albumin is minimal compared to the numerous **ionizable side chains**. - These two groups only provide a **single buffering site** each, which is insufficient for the broad buffering capability observed with albumin. *Albumin contains peptide bonds that readily hydrolyze, consuming hydrogen and hydroxyl ions.* - **Peptide bonds** are generally very stable and do not readily hydrolyze under physiological conditions. - If they did hydrolyze frequently, it would lead to protein degradation, not buffering, and would consume water rather than directly buffering H+ or OH- ions. *The protein contains a large number of amino acids.* - While albumin is a large protein, containing many amino acids, its **size alone** does not explain its buffering capacity. - The key factor is the **chemical nature** of these amino acids, specifically the presence of **ionizable side chains** with varied pKa values, not merely the quantity of amino acids.

Question 378: Who was awarded the Nobel Prize for determining the amino acid sequence of insulin?

A. Banting & Macleod
B. Paul Berg
C. Charles Best
D. Sanger (Correct Answer)

Explanation: ***Sanger*** - **Frederick Sanger** was awarded the Nobel Prize in Chemistry in 1958 for his work on the **structure of proteins**, specifically for determining the **amino acid sequence of insulin**. - His method involved breaking down the protein into smaller fragments and then sequencing these fragments to reconstruct the entire protein structure. *Banting & Macleod* - **Frederick Banting** and **John Macleod** received the Nobel Prize in Physiology or Medicine in 1923 for the **discovery of insulin** itself. - Their work focused on isolating and demonstrating the therapeutic effects of insulin in treating diabetes. *Paul Berg* - **Paul Berg** was awarded the Nobel Prize in Chemistry in 1980 for his fundamental studies of the **biochemistry of nucleic acids**, particularly for his work on **recombinant DNA technology**. - His contributions were pivotal in the development of genetic engineering. *Charles Best* - **Charles Best** was a medical scientist who assisted Frederick Banting in the **discovery of insulin**. - While central to the discovery, he was not included in the Nobel Prize awarded to Banting and Macleod, though Banting shared his prize money with Best.

Question 379: A 60 year old lady is concerned about the wrinkles around her eyes. This is primarily due to alterations in

A. Fibrillin
B. Collagen cross linking (Correct Answer)
C. Collagenase
D. Desmosine

Explanation: ***Collagen cross linking*** - With aging, the **collagen fibers** in the skin undergo structural changes, including a decrease in efficient cross-linking. - This reduction in **collagen cross-linking** leads to a loss of skin elasticity and tensile strength, contributing to the formation of wrinkles. *Fibrillin* - **Fibrillin** is a glycoprotein that is essential for the formation of elastic fibers, not primarily collagen. - Defects in fibrillin are associated with conditions like **Marfan syndrome**, which affects connective tissue integrity but is not the primary cause of age-related wrinkles. *Collagenase* - **Collagenase** is an enzyme that breaks down collagen; an increase in its activity can contribute to collagen degradation. - While collagenase activity plays a role in skin aging, the *deficiency* or *defect* is not the primary cause of wrinkles, rather it is the overall degradation and altered structure of collagen. *Desmosine* - **Desmosine** is a unique amino acid that is part of **elastin fibers**, not collagen. - It is crucial for the elasticity of tissues but a defect in desmosine itself is not the direct cause of age-related wrinkles, which are more directly related to collagen structure.

Question 380: During eukaryotic protein synthesis, phosphorylation of which of the following is enhanced by insulin?

A. eIF2
B. eIF4A
C. eIF4G
D. eIF4E (Correct Answer)

Explanation: ***eIF4E*** - Insulin activates the **mTOR pathway**, which leads to activation of **Mnk1/2 kinases** that phosphorylate eIF4E at **Ser209**. - This phosphorylation enhances eIF4E's **affinity for the 5' cap structure** and increases **cap-dependent translation initiation** efficiency. *eIF4G* - While eIF4G is essential for **eIF4F complex formation**, its phosphorylation is not the primary target enhanced by insulin signaling. - Insulin's effect on eIF4G is mainly **indirect through 4E-BP1 phosphorylation**, which releases eIF4E to bind eIF4G. *eIF2* - **eIF2 phosphorylation** by kinases like **PERK, PKR, and GCN2** inhibits translation initiation during stress conditions. - This is **opposite to insulin's anabolic effects**, as insulin signaling typically promotes conditions that reduce eIF2 phosphorylation. *eIF4A* - eIF4A functions as an **RNA helicase** in the eIF4F complex, unwinding mRNA secondary structures. - While important for translation, **direct phosphorylation enhancement by insulin** is not a primary mechanism for eIF4A regulation.

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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