Protein Structure and Function Practice Questions

Q: Which of the following hormones is not a protein derivative?

Thyroid hormone. ***Thyroid hormone*** - **Thyroid hormone** is a **derivative of the amino acid tyrosine** and contains iodine, classifying it as an **amine hormone**, not a protein. - Its synthesis involves the iodination of tyrosine residues on thyroglobulin, followed by coupling reactions. *Growth hormone* - **Growth hormone (GH)** is a **polypeptide hormone** synthesized and secreted by the somatotroph cells of the anterior pituitary gland. - It consists of 191 amino acid residues and is therefore a protein derivative. *Insulin* - **Insulin** is a **peptide hormone** composed of two polypeptide chains linked by disulfide bonds, making it a protein derivative. - It is synthesized in the beta cells of the pancreatic islets and plays a crucial role in glucose metabolism. *Glucagon* - **Glucagon** is a **peptide hormone** secreted by the alpha cells of the pancreatic islets, and is thus a protein derivative. - It consists of 29 amino acids and has an antagonistic effect to insulin, raising blood glucose levels.

Q: Diphtheria toxin acts by:

inhibiting protein synthesis. ***inhibiting protein synthesis*** - Diphtheria toxin is an **exotoxin** produced by *Corynebacterium diphtheriae* that enters host cells. - It catalyzes the **ADP-ribosylation of elongation factor 2 (EF-2)**, which is crucial for protein synthesis, rendering it inactive and ultimately leading to cell death. *inhibiting acetylcholine release* - This mechanism is characteristic of **botulinum toxin**, which cleaves SNARE proteins involved in acetylcholine release at the neuromuscular junction. - Diphtheria toxin does not directly interfere with neurotransmitter release. *inhibiting glucose transport* - While some bacterial toxins can affect nutrient transport, **diphtheria toxin** specifically targets protein synthesis and does not primarily inhibit glucose transport. - Inhibition of glucose transport is not its main mechanism of action. *increasing levels of cyclic AMP* - This is the mechanism of action for several other toxins, such as **cholera toxin** and **pertussis toxin**, which activate adenylate cyclase, leading to increased **cAMP levels**. - Diphtheria toxin does not function by altering cyclic AMP levels.

Q: An example of a scleroprotein is

Keratin. ***Keratin*** - **Keratin** is a fibrous, structural protein and is a prime example of a **scleroprotein** (also known as a **fibrous protein**). - It provides strength and structural integrity to tissues like hair, nails, and the outer layer of skin. *Zein* - **Zein** is a prolamine protein found in corn. - It is an **alcohol-soluble protein** and is not classified as a scleroprotein. *Glutenin* - **Glutenin** is a major storage protein found in wheat. - It is classified as a **globulin** and contributes to the elasticity of dough, but is not a scleroprotein. *Ovoglobulin* - **Ovoglobulin** refers to a group of globular proteins found in egg white. - These are typically **soluble in water** or dilute salt solutions and do not share the fibrous, structural characteristics of scleroproteins.

Q: Which of the following is not a component of collagen?

Desmosine. ***Correct: Desmosine*** - **Desmosine** is a unique amino acid derivative primarily found in **elastin**, not collagen - It forms **cross-links** crucial for elastin's elastic properties, allowing tissues to stretch and recoil - This makes it the component that is **NOT found in collagen** *Incorrect: Glycine* - **Glycine** is the most abundant amino acid in collagen, accounting for approximately **one-third** of its residues - Its small size allows for tight packing of the **collagen triple helix** (Gly-X-Y repeat pattern) *Incorrect: Lysine* - **Lysine** residues in collagen are important for forming **cross-links** between collagen fibrils, providing tensile strength - They undergo **hydroxylation** to hydroxylysine, which is essential for collagen stability and glycosylation *Incorrect: Proline* - **Proline** and its hydroxylated form, **hydroxyproline**, contribute to the stability of the collagen triple helix - The rigid ring structure of proline introduces constraints that are important for the unique structure of collagen

Q: At physiological pH, the carboxy-terminal of a peptide is

Negatively charged. ***Negatively charged*** - At **physiological pH (around 7.4)**, the **carboxyl group (-COOH)** at the carboxy-terminal of a peptide chain **ionizes** by losing a proton (H+). - This deprotonation results in a negatively charged carboxylate group (**-COO-**). - The **pKa of carboxyl groups** is typically around **2-3**, meaning at pH 7.4, the group is virtually completely deprotonated. *Infinitely charged* - This option is chemically and biologically incorrect; charges are specific and quantifiable, not infinite. - The charge on a molecule depends on its **ionization state**, which is finite and determined by pH and pKa values. *Not charged* - If the carboxyl group were not charged, it would remain in its **protonated form (-COOH)**. - This occurs at very **acidic pH** levels (pH < 3), far below physiological pH. *Positively charged* - A positive charge at the C-terminus would require protonation beyond the neutral form, which does not occur for carboxyl groups. - Students may confuse this with the **amino-terminus (N-terminus)**, which carries a **positive charge (-NH3+)** at physiological pH. - The carboxy-terminus has an **acidic group** (loses H+), while the amino-terminus has a **basic group** (gains H+).

Q: A 43-year-old woman presents with a several-year history of progressive abdominal colic and constipation. Colonic biopsy stained with Congo Red and polarizing microscopy reveals a specific appearance. This is due to which of the following protein properties?

Beta pleated sheet secondary structure. ***Beta pleated sheet tertiary structure*** - The appearance seen on **Congo Red staining** indicates the presence of **amyloid deposits**, which are rich in beta-pleated sheets [1]. - This structure is responsible for the characteristic **apple-green birefringence** under polarized light, confirming amyloidosis [2]. *Electrophoretic mobility* - While electrophoretic mobility can indicate charge differences in proteins, it does not specifically relate to the **amyloid nature** revealed by Congo Red staining. - This method is more about **separation based on charge and size**, rather than structural properties like pleated sheets. *Hydroxyproline content* - Hydroxyproline content is associated with the **collagen structure**, not with the formation of amyloid deposits, which consist mainly of proteins like **light chains or serum amyloid A**. - It does not provide information on the **beta pleated sheet** conformation found in amyloid fibrils. *Ability to bind to oxygen* - This feature is more relevant to **hemoglobin** and myoglobin and is unrelated to the formation of amyloid and its specific structures. - Amyloid does not function through **oxygen binding**, but rather through its unique **beta pleated sheet** conformation. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Diseases of the Immune System, pp. 264-266. [2] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Diseases of the Immune System, pp. 268-269.

Q: Which of the following is true about alpha-1 antitrypsin?

Inhibits elastase. ***Inhibits elastase*** - Alpha-1 antitrypsin (A1AT) primarily serves to **inhibit elastase**, a protease that can damage lung tissue, helping to protect the lungs from destruction [1]. - Deficiency of A1AT leads to **emphysema** and liver disease due to unchecked activity of elastase [1][2]. *Inhibits trypsin* - A1AT specifically does not primarily inhibit **trypsin**, which is involved in protein digestion in the intestine. - Although A1AT affects proteases, its main function is related to **elastase**, not trypsin [1]. *Inhibits trypsinogen activation in pancreas* - A1AT does not have a significant role in the **inhibition of trypsinogen activation** within the pancreas. - Instead, pancreatic enzyme regulation involves other mechanisms that do not involve A1AT's function. *Inhibits chymotrypsin* - A1AT is not known for inhibiting **chymotrypsin**, a serine protease derived from trypsinogen in the gut. - It specifically targets **elastase** and similar enzymes rather than chymotrypsin [1]. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Liver and Gallbladder, pp. 856-858. [2] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. The Lung, pp. 683-684.

Q: Which of the following is LEAST COMMONLY phosphorylated by protein kinases?

Asparagine. ***Asparagine*** - **Asparagine** lacks a hydroxyl group (-OH) in its side chain, an essential prerequisite for most protein kinases to catalyze phosphorylation. - While theoretical phosphorylation of the amide nitrogen in asparagine has been proposed, it is exceedingly rare and generally not observed in biological systems compared to the hydroxyl-containing amino acids. *Threonine* - **Threonine** contains a hydroxyl group (-OH) in its side chain, making it a common substrate for phosphorylation by **serine/threonine kinases**. - Phosphorylation of threonine plays a crucial role in regulating protein activity and signaling pathways. *Serine* - **Serine** contains a hydroxyl group (-OH) in its side chain, making it the most frequently phosphorylated amino acid by **protein kinases**, particularly **serine/threonine kinases**. - Serine phosphorylation is fundamental to almost all aspects of cell regulation and signal transduction. *Tyrosine* - **Tyrosine** contains a hydroxyl group (-OH) within its phenolic ring, making it a key target for **tyrosine kinases**. - Tyrosine phosphorylation is particularly critical in growth factor signaling, cell proliferation, and immune responses.

Q: Which of the following types of non-covalent interactions is the strongest in a non-polar environment?

Electrostatic. ***Electrostatic*** - **Electrostatic interactions** (ionic bonds/salt bridges) between oppositely charged ions are the strongest non-covalent bonds, particularly **in non-polar environments** where they can reach strengths of 12-30 kJ/mol. - In **proteins**, they contribute significantly to **tertiary and quaternary structure** stabilization, though their strength is **reduced in aqueous environments** due to the high dielectric constant of water. - These interactions are crucial at **protein active sites** and in **subunit interfaces**. *Hydrogen* - **Hydrogen bonds** form between a hydrogen atom bonded to an electronegative atom (O, N) and another electronegative atom. - Strength: 12-30 kJ/mol, making them comparable to electrostatic bonds in aqueous solution. - Critical for **DNA base pairing**, **α-helix and β-sheet structures**, and **protein folding**, where their cumulative effect provides substantial stability. *Hydrophobic* - The **hydrophobic effect** is an entropy-driven phenomenon where nonpolar residues cluster together in aqueous solution to minimize unfavorable water contacts. - Not a true attractive force but crucial for **protein core formation** and **membrane assembly**. - Contributes significantly to overall protein stability through entropic effects. *van der Waals* - **van der Waals forces** are weak, transient attractions (2-4 kJ/mol) arising from temporary dipole fluctuations. - Although individually weak, they are numerous in **protein structures** and contribute to **molecular recognition** and **binding specificity**.

Q: What is the most specific function of the Golgi apparatus?

Modification and sorting of proteins. ***Modification and sorting of proteins*** - The Golgi apparatus is the primary site for **post-translational modification**, **sorting**, and **packaging of proteins** synthesized in the endoplasmic reticulum. - Key modifications include **glycosylation** (adding carbohydrate groups), **phosphorylation**, **sulfation**, and **proteolytic cleavage**. - The Golgi sorts proteins and packages them into vesicles for delivery to their final destinations: **lysosomes**, **plasma membrane**, or **secretion** outside the cell. - This is the **most specific and comprehensive** function that distinguishes the Golgi from other organelles. *Sorting and packaging of glycoproteins* - While glycosylation is an important Golgi function, this option is **too restrictive**. - The Golgi modifies and sorts **all types of proteins**, not just glycoproteins—including non-glycosylated proteins destined for various cellular locations. - Limiting the function to only glycoproteins ignores the Golgi's broader role in protein trafficking. *Transport of lipids* - Lipid transport is primarily a function of the **smooth endoplasmic reticulum** and lipid transfer proteins. - While the Golgi participates in **lipid metabolism** and some lipid modifications occur there, this is not its **most specific** function. - The Golgi's defining role is in protein processing, not lipid transport. *None of the options* - This is incorrect because "Modification and sorting of proteins" accurately describes the Golgi's most specific and well-established function. - The Golgi apparatus has a clearly defined role in the secretory pathway.

Question 1

Which of the following hormones is not a protein derivative?

Accepted Answer

Thyroid hormone

Answer

Growth hormone

Answer

Insulin

Answer

Glucagon

Question 2

Diphtheria toxin acts by:

Accepted Answer

inhibiting protein synthesis

Answer

inhibiting acetylcholine release

Answer

inhibiting glucose transport

Answer

increasing levels of cyclic AMP

Question 3

An example of a scleroprotein is

Accepted Answer

Keratin

Answer

Zein

Answer

Glutenin

Answer

Ovoglobulin

Question 4

Which of the following is not a component of collagen?

Accepted Answer

Desmosine

Answer

Glycine

Answer

Lysine

Answer

Proline

Question 5

At physiological pH, the carboxy-terminal of a peptide is

Accepted Answer

Negatively charged

Answer

Infinitely charged

Answer

Not charged

Answer

Positively charged

Question 6

A 43-year-old woman presents with a several-year history of progressive abdominal colic and constipation. Colonic biopsy stained with Congo Red and polarizing microscopy reveals a specific appearance. This is due to which of the following protein properties?

Accepted Answer

Beta pleated sheet secondary structure

Answer

Ability to bind to oxygen

Answer

Electrophoretic mobility

Answer

Hydroxyproline content

Question 7

Which of the following is true about alpha-1 antitrypsin?

Accepted Answer

Inhibits elastase

Answer

Inhibits trypsin

Answer

Inhibits chymotrypsin

Answer

Inhibits trypsinogen activation

Question 8

Which of the following is LEAST COMMONLY phosphorylated by protein kinases?

Accepted Answer

Asparagine

Answer

Threonine

Answer

Serine

Answer

Tyrosine

Question 9

Which of the following types of non-covalent interactions is the strongest in a non-polar environment?

Accepted Answer

Electrostatic

Answer

Hydrogen

Answer

Hydrophobic

Answer

van der Waals

Question 10

What is the most specific function of the Golgi apparatus?

Accepted Answer

Modification and sorting of proteins

Answer

Transport of lipids

Answer

Sorting and packaging of glycoproteins

Answer

None of the options

Protein Structure and Function — MCQs

Protein Structure and Function — MCQs

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