Protein Structure and Function — MCQs

Protein Structure and Function Indian Medical PG Practice Questions and MCQs

Question 441: What is the approximate half-life of albumin in the human body?

A. 30 days
B. 20 days (Correct Answer)
C. 3 days
D. 7 days

Explanation: ***20 days*** - The **half-life of albumin** in the human body is approximately **20 days**, reflecting the time it takes for half of the circulating albumin to be catabolized or excreted. - This relatively long half-life means that changes in albumin levels, such as those due to malnutrition or liver disease, may take several weeks to become evident. *3 days* - A half-life of 3 days is too short for albumin, which is a major, long-lasting plasma protein. - Proteins with such a short half-life typically include more rapidly turnover proteins or small peptides. *7 days* - A half-life of 7 days is also too short for albumin, which plays a critical role in maintaining plasma oncotic pressure and transporting various substances. - While some proteins have a 7-day half-life, albumin's is considerably longer. *30 days* - A half-life of 30 days is longer than the typical half-life of albumin. - While some proteins may have half-lives in this range, 20 days is the more commonly accepted value for albumin.

Question 442: Which of the following statements regarding collagen synthesis is incorrect?

A. Hydroxylation of lysine occurs in ER
B. Synthesized in ribosomes as preprocollagen
C. Triple helix assembly occurs in ER
D. Hydroxylation of proline occurs in Golgi apparatus (Correct Answer)

Explanation: ***Hydroxylation of proline occurs in Golgi apparatus*** - This statement is incorrect because the **hydroxylation of proline** residues occurs in the **endoplasmic reticulum** (ER), not the Golgi apparatus. - This step is critical for forming stable **triple helix** structures of collagen and requires **vitamin C**. *Synthesized in ribosomes as preprocollagen* - This statement is correct. Collagen synthesis begins in the cytoplasm, where mRNA is translated by **ribosomes** into **preprocollagen**, which contains a signal peptide. - The signal peptide directs the nascent polypeptide chain into the lumen of the **endoplasmic reticulum**. *Hydroxylation of lysine occurs in ER* - This statement is correct. Following entry into the ER, specific **lysine** residues are hydroxylated by **lysyl hydroxylase** to form hydroxylysine. - This hydroxylation, along with that of proline, is crucial for **cross-linking** and stability of the collagen molecule. *Triple helix assembly occurs in ER* - This statement is correct. After hydroxylation and glycosylation of some residues, three procollagen alpha chains self-assemble to form a **triple helix** within the **endoplasmic reticulum**. - This assembly is stabilized by **disulfide bonds** at the C-terminal ends and molecular chaperones.

Question 443: What type of protein is keratin classified as?

A. Conjugated protein
B. Globular protein
C. Cylindrical protein
D. Fibrous protein (Correct Answer)

Explanation: ***Fibrous protein*** - **Keratin** is a structural protein characterized by its **elongated, filament-like structure**, which is typical of fibrous proteins. - Fibrous proteins like keratin provide **mechanical strength** and play a significant role in the structure of tissues such as skin, hair, and nails. - Other examples of fibrous proteins include collagen, elastin, and myosin. *Globular protein* - **Globular proteins** have a **compact, spherical shape** and are often water-soluble, serving functions like enzymes, transporters, or receptors (e.g., hemoglobin or albumin). - Keratin's primary role is structural, not catalytic or transport, and its shape is not compact or spherical. *Cylindrical protein* - While some proteins might have a somewhat elongated or tube-like structure, **"cylindrical protein" is not a standard biochemical classification** of protein type. - This term does not accurately describe the characteristic fibrous nature and function of keratin. *Conjugated protein* - **Conjugated proteins** contain a non-protein component (prosthetic group) such as a carbohydrate, lipid, or metal ion attached to the protein (e.g., glycoproteins, lipoproteins, hemoglobin). - Keratin is a **simple fibrous protein** composed only of amino acids without prosthetic groups, so it is not classified as a conjugated protein.

Question 444: What is the half-life of Prealbumin?

A. 2 days (Correct Answer)
B. 10 days
C. 20 days
D. 40 days

Explanation: ***2 days*** - Prealbumin, also known as transthyretin, has a **short half-life** of approximately 2-3 days, making it a sensitive indicator of recent changes in **nutritional status**. - Its rapid turnover allows for prompt reflection of improvement or deterioration in protein synthesis. *10 days* - A half-life of 10 days would make prealbumin less responsive to acute changes in nutrition compared to its actual turnover rate. - This duration is longer than the typical half-life of proteins used to monitor **short-term nutritional status**. *20 days* - A 20-day half-life would indicate a protein with a much slower turnover, unsuitable for monitoring **acute nutritional interventions**. - Proteins with such long half-lives, like **albumin**, reflect more chronic states rather than rapid changes. *40 days* - A half-life of 40 days is characteristic of proteins like **albumin**, which are influenced by longer-term nutritional and inflammatory processes. - Such a long half-life would not be useful for assessing immediate responses to **nutritional support** or acute disease states.

Question 445: Which of the following statements about chaperones is false?

A. Are lipid in nature (Correct Answer)
B. Cause folding of proteins
C. Include heat shock proteins
D. May have ATPase activity

Explanation: ***Are lipid in nature*** - Chaperones are **proteins** (typically **heat shock proteins** or **chaperonins**), not lipids. - Their function involves assisting in the proper **folding and assembly of other proteins**, and they are composed of amino acids. *Cause folding of proteins* - Chaperones **do not cause** proteins to fold; rather, they **assist in proper folding** and refolding by preventing aggregation or misfolding. - They bind to nascent or partially unfolded proteins to guide them towards their correct three-dimensional structure. *May have ATPase activity* - Many chaperones, especially **Hsp70** and **chaperonins** like GroEL/GroES, utilize **ATP hydrolysis** for their function. - This **ATPase activity** drives conformational changes essential for binding, release, and refolding of their client proteins. *Include heat shock proteins* - The **heat shock protein (Hsp)** families (e.g., Hsp70, Hsp90, Hsp60) are a major class of chaperones. - Hsps are upregulated in response to stress (like heat) to help refold damaged proteins and prevent aggregation.

Question 446: Which of the following is activated by calmodulin?

A. Muscle phosphorylase
B. Calcium/calmodulin-dependent protein kinase (Correct Answer)
C. Phospholipase C
D. Adenylyl cyclase

Explanation: ***Calcium/calmodulin-dependent protein kinase*** - **Calmodulin** is a **calcium-binding messenger protein** that, when bound to calcium, undergoes a conformational change allowing it to activate various enzymes, including **calcium/calmodulin-dependent protein kinases** (CaMKs). - CaMKs play crucial roles in many cellular processes, including **metabolism**, **gene expression**, and **neurotransmission**, by phosphorylating target proteins. *Muscle phosphorylase* - **Muscle phosphorylase** (glycogen phosphorylase) is primarily activated by **epinephrine**, **AMP**, and **nerve stimulation** (via calcium), but not directly by calmodulin. - Its activation leads to the breakdown of **glycogen** into glucose-1-phosphate. *Phospholipase C* - **Phospholipase C (PLC)** is typically activated by **G protein-coupled receptors** and **tyrosine kinase receptors**, leading to the production of **inositol trisphosphate (IP3)** and **diacylglycerol (DAG)**. - While it plays a role in calcium signaling upstream (releasing calcium from stores), it is not directly activated by calmodulin. *Adenylyl cyclase* - **Adenylyl cyclase (AC)** is a key enzyme in generating **cyclic AMP (cAMP)**, and is commonly regulated by **G proteins** (specifically Gs and Gi subunits). - While certain isoforms (AC1, AC3, AC8) can be directly activated by calcium/calmodulin, **CaMK** remains the most classical and direct example of calmodulin activation.

Question 447: Which of the following statements about Glutathione is false?

A. Tripeptide
B. Act as antioxidant in reduced state
C. Formed from glutamic acid, glycine, cysteine
D. All of the above are true statements (Correct Answer)

Explanation: ***All of the above are true statements*** - Since the question asks which statement is **FALSE**, and all the listed properties of glutathione are **TRUE**, the correct answer indicates that none of the statements are false. - All three statements accurately describe glutathione's structure and function. *Tripeptide* - Glutathione is indeed a **tripeptide** composed of three amino acids: **γ-glutamyl-cysteinyl-glycine**. - The unique γ-peptide bond (between glutamate's γ-carboxyl and cysteine's amino group) makes it resistant to peptidases. *Act as antioxidant in reduced state* - Glutathione functions as an **antioxidant** in its **reduced form (GSH)**, donating electrons to neutralize reactive oxygen species (ROS). - The **thiol group (-SH) of cysteine** is the active site for antioxidant activity. - Enzyme **glutathione reductase** maintains GSH levels by reducing oxidized glutathione (GSSG). *Formed from glutamic acid, glycine, cysteine* - Glutathione is synthesized from **glutamate, cysteine, and glycine** in two ATP-dependent steps. - First, **γ-glutamylcysteine synthetase** links glutamate and cysteine. - Then, **glutathione synthetase** adds glycine to form the complete tripeptide.

Question 448: Which amino acid has two chiral centers?

A. Threonine (Correct Answer)
B. Tyrosine
C. Tryptophan
D. Phenylalanine

Explanation: ***Threonine*** - Threonine is unique among the standard 20 amino acids because it possesses **two chiral centers**: one at the **alpha-carbon** and another at the **beta-carbon**. - The presence of two chiral centers means that threonine can exist as **four stereoisomers** (2^n, where n is the number of chiral centers). *Tryptophan* - Tryptophan has only **one chiral center**, which is the **alpha-carbon** bonded to the amino group, carboxyl group, hydrogen atom, and the side chain. - Its side chain, an **indole ring**, does not contain an additional chiral center. *Tyrosine* - Tyrosine, like most amino acids, possesses only **one chiral center** at its **alpha-carbon**. - The aromatic ring system (phenol group) in its side chain does not introduce another chiral center. *Phenylalanine* - Phenylalanine also has only **one chiral center** located at its **alpha-carbon**. - Its benzyl side chain, consisting of a methylene group and a benzene ring, is not chiral.

Question 449: What is the primary action of metalloproteinases in the extracellular matrix?

A. Modification of collagen structure
B. Degradation of extracellular matrix components, including collagen (Correct Answer)
C. Formation of collagen
D. Activation of collagen synthesis

Explanation: ***Degradation of extracellular matrix components, including collagen*** - **Metalloproteinases (MMPs)** are a family of zinc-dependent endopeptidases that are crucial for breaking down various components of the **extracellular matrix (ECM)**. - This degradation is essential for processes like **tissue remodeling**, development, wound healing, and also plays a role in disease pathogenesis such as metastasis and inflammation. *Formation of collagen* - The formation of collagen is primarily mediated by **fibroblasts** and involves a complex process of synthesis, hydroxylation, glycosylation, and assembly of procollagen molecules, not MMPs. - MMPs act to break down existing collagen, not to create new collagen fibers. *Modification of collagen structure* - While collagen undergoes post-translational modifications (e.g., hydroxylation, glycosylation) within cells, MMPs are involved in cleaving the peptide bonds, leading to **degradation**, rather than structural modification of intact collagen. - Enzymes like **lysyl hydroxylase** and **prolyl hydroxylase** are responsible for modifying collagen structure. *Activation of collagen synthesis* - Collagen synthesis is primarily regulated by various **growth factors (e.g., TGF-β)** and hormones that stimulate fibroblasts to produce collagen. - MMPs are involved in the breakdown of collagen, which is the opposite of activating its synthesis.

Question 450: Protein segregation occurs in which organelle?

A. Peroxisomes
B. ER
C. Mitochondria
D. Golgi apparatus (Correct Answer)

Explanation: ***Golgi apparatus*** - The **Golgi apparatus** is a central organelle for **protein modification, sorting, and packaging** into vesicles for delivery to various cellular destinations. - It acts as a "post office" of the cell, directing proteins to their correct locations through **segregation** into specific secretory or transport pathways. *Peroxisomes* - **Peroxisomes** are involved in **metabolic processes** such as fatty acid oxidation and detoxification. - While they import some proteins, their primary role is not in the overall **segregation** and trafficking of proteins for diverse cellular destinations. *ER* - The **endoplasmic reticulum (ER)** is where proteins are synthesized (rough ER) and undergo initial folding and modification, including glycosylation. - However, the ER's main function is protein synthesis and early modification, not the final **segregation** and sorting for transport to different cellular locations. *Mitochondria* - **Mitochondria** are primarily responsible for **ATP production** through cellular respiration and houses its own genome. - While mitochondria import specific proteins necessary for their function, they are not involved in the general **segregation** of proteins destined for other organelles or secretion.

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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