Protein Structure and Function — MCQs

Protein Structure and Function Indian Medical PG Practice Questions and MCQs

Question 441: What is the major site of protein glycosylation?

A. Ribosome and Golgi body
B. ER and Ribosome
C. Ribosome and Cytoplasm
D. ER and Golgi body (Correct Answer)

Explanation: ***ER and Golgi body*** - The **endoplasmic reticulum (ER)** is the primary site for **N-linked glycosylation**, where carbohydrates are added to the asparagine residues of nascent proteins. - The **Golgi apparatus** is crucial for further modification and processing of these N-linked glycans, as well as the site for **O-linked glycosylation**, where sugars are added to serine or threonine residues. *Ribosome and Golgi body* - **Ribosomes** are responsible for **protein synthesis (translation)** but do not directly perform glycosylation, which is a post-translational modification. - While the **Golgi body** is a site of glycosylation, the ribosome's inclusion makes this option incorrect as the ribosome's role precedes glycosylation. *ER and Ribosome* - The **ER** is a major site of protein glycosylation, especially N-linked glycosylation. - However, **ribosomes** are involved in protein synthesis and lack the enzymatic machinery for adding sugar moieties to proteins. *Ribosome and Cytoplasm* - **Ribosomes** synthesize proteins, but glycosylation does not occur there. - The **cytoplasm** is the site for many metabolic pathways, but major protein glycosylation events mostly occur within the ER and Golgi.

Question 442: Amino acid with aliphatic side chain is?

A. Serine
B. Leucine (Correct Answer)
C. Threonine
D. Aspartate

Explanation: ***Leucine*** - Leucine has an **isobutyl group** (-CH2CH(CH3)2) as its side chain, making it a **nonpolar aliphatic amino acid**. - **Aliphatic amino acids** (glycine, alanine, valine, leucine, isoleucine, proline) have side chains consisting of only carbon and hydrogen atoms in straight or branched chains, with **no polar functional groups**. - These amino acids are **hydrophobic** and typically found in the interior of proteins. *Serine* - Serine has a **hydroxyl group** (-OH) in its side chain (-CH2OH), classifying it as a **polar uncharged amino acid**, not an aliphatic amino acid. - The hydroxyl group makes the side chain **hydrophilic** and capable of hydrogen bonding. - The presence of the polar functional group distinguishes it from aliphatic amino acids. *Threonine* - Threonine also contains a **hydroxyl group** (-OH) in its side chain (-CH(OH)CH3), making it a **polar uncharged amino acid**, not an aliphatic amino acid. - Like serine, the hydroxyl group provides **polarity and hydrogen bonding capacity**. - This functional group places it in a different classification from aliphatic amino acids. *Aspartate* - Aspartate has a **carboxyl group** (-COOH) in its side chain (-CH2COOH), making it an **acidic (negatively charged) amino acid**. - At physiological pH, this group is deprotonated (COO⁻), making aspartate **negatively charged**. - This clearly distinguishes it from nonpolar aliphatic amino acids.

Question 443: 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 444: 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 445: 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 446: 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 447: Which type of bond is primarily responsible for the primary structure of a protein?

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

Explanation: ***Peptide bond*** - The **primary structure** of a protein is defined by the unique linear sequence of **amino acids** linked together by **peptide bonds**. - These are **amide bonds** formed between the carboxyl group of one amino acid and the amino group of another, with the elimination of water. *Hydrogen bond* - **Hydrogen bonds** are crucial for the **secondary structure** (e.g., alpha-helices and beta-sheets) and **tertiary/quaternary structures** of proteins, stabilizing their 3D folds. - They involve interactions between polar atoms, not the direct linkage of amino acids in the primary sequence. *Disulfide bond* - **Disulfide bonds** are **covalent bonds** formed between the sulfur atoms of two **cysteine residues**, contributing to the **tertiary** and sometimes **quaternary structure** stability. - They are not involved in forming the linear sequence of amino acids, which is the primary structure. *Electrostatic bond* - **Electrostatic bonds**, or **ionic bonds**, occur between oppositely charged amino acid side chains and are important for **tertiary** and **quaternary structure** stability. - They do not form the backbone of the protein's primary sequence.

Question 448: 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 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 glycosylation occurs in:

A. Peroxisomes
B. Endoplasmic reticulum (ER) (Correct Answer)
C. Mitochondria
D. Golgi bodies

Explanation: ***ER*** - **N-linked glycosylation**, the most common type of protein glycosylation, initiates in the **endoplasmic reticulum (ER)**, where an oligosaccharide precursor is transferred to asparagine residues of newly synthesized proteins. - The ER environment facilitates protein folding and quality control, ensuring correctly folded glycoproteins are transported further. *Golgi bodies* - While **further modification and processing** of glycosylated proteins occur in the Golgi apparatus, the initial step of N-linked glycosylation begins in the ER. - The Golgi is responsible for trimming and adding different sugar residues to complete the **glycan chains** and for sorting the glycoproteins to their final destinations. *Mitochondria* - Mitochondria are primarily involved in **cellular respiration** and **ATP production**. - They do not play a significant role in protein glycosylation; most mitochondrial proteins are imported from the cytoplasm in an unglycosylated state. *Peroxisomes* - Peroxisomes are involved in various **metabolic processes**, including fatty acid oxidation and detoxification. - They are not known to be sites of protein glycosylation.

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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