Protein Structure and Function Practice Questions

Q: Most abundant amino acid found in collagen?

Glycine. ***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**.

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

Primary. ***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.

Q: Prion disease is caused by

Misfolding of proteins. ***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.

Q: Maximum buffering capacity at physiological pH is for:

Histidine. ***Correct Option: Histidine*** - Histidine possesses an **imidazole side chain** with a pKa value of approximately 6.0-6.5, which is close to the physiological pH of 7.4 - Maximum buffering capacity occurs when **pH ≈ pKa** (Henderson-Hasselbalch principle) - This proximity of its pKa to physiological pH allows histidine to effectively **accept and donate protons** as pH changes, providing the strongest buffering capacity among amino acids - Histidine residues are critical in **hemoglobin buffering** and protein buffer systems *Incorrect Option: Glycine* - Glycine's pKa values are around 2.3 for the carboxyl group and 9.6 for the amino group - Neither pKa value is close to **physiological pH 7.4**, making it a poor buffer at this pH - It lacks a side chain with a pKa near neutral pH to provide significant buffering capacity *Incorrect Option: Valine* - Valine has a **nonpolar aliphatic side chain** that does not ionize - Only the α-amino (pKa ~9.6) and α-carboxyl groups (pKa ~2.3) can buffer, both far from physiological pH - Provides minimal buffering capacity at **pH 7.4** *Incorrect Option: Cysteine* - Cysteine contains a **thiol group (-SH)** with a pKa of approximately 8.3 - While closer to physiological pH than glycine or valine, its pKa is still ~1 pH unit away from 7.4 - Its buffering capacity at **pH 7.4** is significantly less effective than histidine's imidazole group

Q: Which of the following amino acids has maximum buffering capacity at pH 7?

Histidine. ***Histidine*** - Histidine has a side chain with a **pKa** of approximately **6.0**, which is close to physiological pH (7.4). - This proximity allows histidine to effectively **donate and accept protons** at pH 7, thus providing significant buffering capacity. *Arginine* - Arginine has a side chain with a very **high pKa** of approximately 12.5, making it a strong base. - It would be ineffective as a buffer at pH 7 because it would exist almost entirely in its **protonated form**. *Lysine* - Lysine has a side chain with a **pKa** of approximately 10.5, making it a strong base. - At pH 7, it would also be almost completely **protonated** and therefore have very limited buffering capacity. *Glycine* - Glycine is the simplest amino acid with **no ionizable side chain** (only a hydrogen atom as its R group). - Its buffering capacity at pH 7 relies solely on its **amino and carboxyl groups**, which have pKa values far from 7 (around 2.3 and 9.6, respectively), making it a poor buffer in this range.

Q: Free radicals are chemical species which have

Single unpaired electron in an outer orbital. ***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.

Q: Which of the following are supportive proteins?

All of the options. ***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.

Q: GnRH is classified as a:

Peptide. ***Correct: Peptide*** - **Gonadotropin-releasing hormone (GnRH)** is a **decapeptide**, meaning it is composed of ten amino acids. - As a **peptide hormone**, it exerts its effects by binding to specific receptors on target cells, typically initiating intracellular signaling cascades. *Incorrect: Steroid* - **Steroid hormones** are lipid-soluble molecules derived from cholesterol, such as **estrogen**, **testosterone**, or **cortisol**. - They typically bind to intracellular receptors and modulate gene expression, a mechanism distinct from GnRH. *Incorrect: Amine* - **Amine hormones** are derived from amino acids but are not peptides; examples include **catecholamines** (like epinephrine) and **thyroid hormones**. - These hormones often act via cell surface receptors, but their chemical structure is simpler than complex peptides like GnRH. *Incorrect: None of the options* - This option is incorrect because **peptide** accurately describes the biochemical nature of GnRH.

Question 1

Chaperones are:

Accepted Answer

Mediators of post-translational assembly of protein complexes

Answer

Antigen presenting cells

Answer

Purine metabolism mediators

Answer

None of the above

Question 2

Most abundant amino acid found in collagen?

Accepted Answer

Glycine

Answer

Hydroxyproline

Answer

Proline

Answer

Lysine

Question 3

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

Accepted Answer

Primary

Answer

Quaternary

Answer

Tertiary

Answer

Secondary

Question 4

Prion disease is caused by

Accepted Answer

Misfolding of proteins

Answer

Denaturation of protein

Answer

Reduced formation of proteins

Answer

Excess formation of proteins

Question 5

Maximum buffering capacity at physiological pH is for:

Accepted Answer

Histidine

Answer

Glycine

Answer

Valine

Answer

Cysteine

Question 6

Which of the following amino acids has maximum buffering capacity at pH 7?

Accepted Answer

Histidine

Answer

Arginine

Answer

Lysine

Answer

Glycine

Question 7

Free radicals are chemical species which have

Accepted Answer

Single unpaired electron in an outer orbital

Answer

Double paired electron in an outer orbital

Answer

Single paired electron in an outer orbital

Answer

Double unpaired electron in an outer orbital

Question 8

Which of the following are supportive proteins?

Accepted Answer

All of the options

Answer

Titin

Answer

Dystropin

Answer

Desmin

Question 9

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?

Accepted Answer

The protein contains many amino acid residues with different pKa values.

Answer

The amino and carboxyl ends of albumin can donate and accept protons in the range of physiologic pH.

Answer

Albumin contains peptide bonds that readily hydrolyze, consuming hydrogen and hydroxyl ions.

Answer

The protein contains a large number of amino acids.

Question 10

GnRH is classified as a:

Accepted Answer

Peptide

Answer

Steroid

Answer

Amine

Answer

None of the options

Protein Structure and Function — MCQs

Protein Structure and Function — MCQs

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