Protein Structure and Function Practice Questions

Q: What type of protein is keratin classified as?

Fibrous protein. ***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.

Q: Amino acid with aliphatic side chain is?

Leucine. ***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.

Q: What is the major site of protein glycosylation?

ER and Golgi body. ***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.

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

Hydroxylation of proline occurs in Golgi apparatus. ***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.

Q: Which of the following is a neutral amino acid?

Glycine. ***Glycine*** - **Glycine** has a hydrogen atom as its side chain, making it the **simplest amino acid** and electrically neutral at physiological pH. - Its **nonpolar side chain** contributes to its neutral charge and allows it to fit into various protein structures. *Aspartate* - **Aspartate** is an **acidic amino acid** with a carboxyl group in its side chain. - This **carboxyl group** can lose a proton, giving aspartate a net negative charge at physiological pH. *Arginine* - **Arginine** is a **basic amino acid** characterized by a guanidinium group in its side chain. - The **guanidinium group** contains multiple nitrogen atoms that can accept protons, making arginine positively charged at physiological pH. *Histidine* - **Histidine** is classified as a **basic amino acid** due to the imidazole ring in its side chain. - The **imidazole ring** has a pKa close to physiological pH, allowing it to be protonated and positively charged, but it is not neutral.

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

20 days. ***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.

Q: Which of the following statements about chaperones is false?

Are lipid in nature. ***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.

Q: What is the half-life of Prealbumin?

2 days. ***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.

Q: Which of the following is a ribozyme?

Peptidyl transferase. ***Peptidyl transferase*** - This enzyme is an integral part of the **large ribosomal subunit** and is responsible for catalyzing the formation of peptide bonds during protein synthesis. - While historically thought to be purely proteinaceous, it is now known that the **catalytic activity** of peptidyl transferase comes from its **rRNA component**, specifically the 23S rRNA in prokaryotes and 28S rRNA in eukaryotes, making it a ribozyme. *Elongation factor 2* - **Elongation Factor 2 (EF2)** is a **GTPase** that facilitates the translocation of the ribosome along the mRNA during protein synthesis. - It is a **protein**, not an RNA molecule, and thus does not possess catalytic activity as a ribozyme. *Primase* - **Primase** is an **RNA polymerase** that synthesizes short RNA primers required for the initiation of DNA replication. - It is a **protein enzyme** and not an RNA molecule with catalytic activity. *RNA polymerase* - **RNA polymerase** is a **protein enzyme** responsible for synthesizing RNA from a DNA template during transcription. - It uses a DNA template to produce an RNA strand, but its own catalytic activity is derived from its **protein structure**, not from an RNA component.

Q: Protein refolding is carried out by?

Chaperone. ***Chaperone*** - **Chaperone proteins** assist in the proper folding of other proteins, particularly during stress conditions like heat shock, by preventing **aggregation** and promoting correct conformation. - They do not become part of the final functional protein but transiently bind during the folding process, thus facilitating **protein refolding** and assembly. *Valine* - **Valine** is an **essential amino acid** and a building block for proteins, but it does not play a direct role in protein refolding. - It contributes to the **hydrophobic core** of proteins due to its non-polar side chain, influencing protein structure but not managing the folding process. *Threonine* - **Threonine** is an **essential amino acid** with a polar side chain, often involved in **glycosylation** and phosphorylation, but not in the complex process of protein refolding. - Its hydroxyl group can participate in **hydrogen bonding**, influencing protein stability and interactions, but not acting as a folding catalyst. *Aspartate* - **Aspartate** is a **non-essential acidic amino acid** that can be involved in various metabolic pathways and is a component of proteins. - Its acidic side chain can form **salt bridges** and hydrogen bonds, contributing to the protein's overall charge and structure, but it does not actively oversee protein refolding.

Question 1

What type of protein is keratin classified as?

Accepted Answer

Fibrous protein

Answer

Conjugated protein

Answer

Globular protein

Answer

Cylindrical protein

Question 2

Amino acid with aliphatic side chain is?

Accepted Answer

Leucine

Answer

Serine

Answer

Threonine

Answer

Aspartate

Question 3

What is the major site of protein glycosylation?

Accepted Answer

ER and Golgi body

Answer

Ribosome and Golgi body

Answer

ER and Ribosome

Answer

Ribosome and Cytoplasm

Question 4

Which of the following statements regarding collagen synthesis is incorrect?

Accepted Answer

Hydroxylation of proline occurs in Golgi apparatus

Answer

Hydroxylation of lysine occurs in ER

Answer

Synthesized in ribosomes as preprocollagen

Answer

Triple helix assembly occurs in ER

Question 5

Which of the following is a neutral amino acid?

Accepted Answer

Glycine

Answer

Aspartate

Answer

Arginine

Answer

Histidine

Question 6

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

Accepted Answer

20 days

Answer

30 days

Answer

3 days

Answer

7 days

Question 7

Which of the following statements about chaperones is false?

Accepted Answer

Are lipid in nature

Answer

Cause folding of proteins

Answer

Include heat shock proteins

Answer

May have ATPase activity

Question 8

What is the half-life of Prealbumin?

Accepted Answer

2 days

Answer

10 days

Answer

20 days

Answer

40 days

Question 9

Which of the following is a ribozyme?

Accepted Answer

Peptidyl transferase

Answer

Elongation factor 2

Answer

Primase

Answer

RNA polymerase

Question 10

Protein refolding is carried out by?

Accepted Answer

Chaperone

Answer

Valine

Answer

Threonine

Answer

Aspartate

Protein Structure and Function — MCQs

Protein Structure and Function — MCQs

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