Protein Structure and Function Practice Questions

Q: Which of the following hormones is an example of a peptide hormone?

Parathormone (PTH). ***Parathormone (PTH)*** - **Parathormone (PTH)** is a **peptide hormone** composed of 84 amino acids, synthesized in the parathyroid glands. - Peptide hormones are typically **water-soluble** and bind to receptors on the cell surface to exert their effects. *Dopamine* - **Dopamine** is a **catecholamine** and a neurotransmitter, derived from the amino acid **tyrosine**. - It functions as a **neurotransmitter** and hormone, regulating various bodily functions but is not a peptide. *Thyroxine* - **Thyroxine** (T4) is an **amine hormone** derived from the amino acid **tyrosine**, produced by the thyroid gland. - It is an **iodinated amino acid derivative**, not a peptide. *Cortisol* - **Cortisol** is a **steroid hormone**, part of the glucocorticoid class, derived from **cholesterol**. - It is **lipid-soluble** and functions by binding to intracellular receptors, distinctly different from peptide hormones.

Q: C-peptide occurs in

Proinsulin. ***Proinsulin*** - **C-peptide** is a byproduct released during the conversion of **proinsulin** to mature **insulin** in the pancreatic beta cells. - Measuring C-peptide levels can indirectly assess the body's own **insulin secretion**, as it has a longer half-life than insulin and is produced in equimolar amounts. *Parathormone* - **Parathormone (PTH)** is a hormone primarily involved in regulating **calcium and phosphate levels** in the body. - It is not structurally related to insulin or its precursors and does not involve C-peptide. *Thyroxine* - **Thyroxine (T4)** is a thyroid hormone that plays a crucial role in **metabolism, growth, and development**. - It is an **iodinated amino acid derivative** and is not synthesized from a prohormone involving C-peptide. *Glucagon* - **Glucagon** is a hormone produced by the pancreatic alpha cells that **raises blood glucose levels**. - While it is also a pancreatic hormone, it is synthesized from **proglucagon** through a different proteolytic cleavage pathway that does not involve C-peptide.

Q: What type of collagen is found in bone?

Type 1. ***Type 1*** - **Type 1 collagen** is the most abundant collagen in the human body, forming strong fibrils crucial for tissues like **bone**, skin, tendons, and ligaments. - Its presence provides **tensile strength** and structural integrity essential for the rigidity and function of bone. *Type 3* - **Type 3 collagen** is found in **reticular fibers** and is prominent in highly distensible tissues like the skin, blood vessels, and internal organs. - While present in some connective tissues, it does not form the primary structural scaffold of **bone**. *Type 2* - **Type 2 collagen** is the main collagen type found in **cartilage**, especially hyaline and elastic cartilage. - It provides resistance to **intermittent pressure**, making it crucial for joint function, not bone structure. *Type 4* - **Type 4 collagen** is a major component of the **basal lamina** (basement membranes), forming a mesh-like network rather than fibrils. - It provides support and filtration functions in tissues such as the kidney glomeruli and epithelia, but not in **bone**.

Q: Glutathione is:

Tripeptide. ***Tripeptide*** - Glutathione is composed of **three amino acids**: **glutamate**, **cysteine**, and **glycine**. - These three amino acids are linked by **peptide bonds**, forming a small peptide. *Polypeptide* - A polypeptide is a **long chain of amino acids** typically containing **more than 10-20 amino acid residues**. - Glutathione, with only three amino acids, does not meet the criteria for a typical polypeptide. *Oligopeptide* - An oligopeptide consists of a **few amino acids**, generally defined as between **2 and 20 amino acid residues**. - While glutathione falls within this range, "tripeptide" is a more specific and accurate classification. *Dipeptide* - A dipeptide consists of **exactly two amino acids** linked by a single peptide bond. - Glutathione contains **three amino acids**, making it distinct from a dipeptide.

Q: Replacing alanine by which amino acid, will increase UV absorbance of protein at 280 nm wavelength?

Tryptophan. ***Tryptophan*** - **Tryptophan** contains an **indole ring**, which is a **chromophore** that strongly absorbs UV light at 280 nm. - Increased tryptophan content in a protein directly correlates with a higher **UV absorbance** at this wavelength. *Glycine* - **Glycine** is the simplest amino acid, with only a **hydrogen atom** as its side chain. - It does not contain any aromatic rings or other groups that absorb UV light at 280 nm, so replacing alanine with glycine would not increase UV absorbance. *Arginine* - **Arginine** is a basic amino acid with a **guanidinium group** in its side chain. - While it has a slightly complex side chain, it does not possess any **aromatic rings** that absorb significantly at 280 nm. *Lysine* - **Lysine** is another basic amino acid with a long **aliphatic chain** and an **amino group** at the end. - Similar to arginine, lysine lacks the necessary **aromatic chromophores** to contribute to UV absorbance at 280 nm.

Q: Not a monomeric intermediate filament:

Tubulin. ***Tubulin*** - **Tubulin** is the monomeric building block of **microtubules**, which are distinct from intermediate filaments. - Microtubules are involved in cell motility, intracellular transport, and maintaining cell shape, but they do not belong to the intermediate filament family. *Desmin* - **Desmin** is a type III **intermediate filament** that is primarily expressed in muscle cells. - It plays a crucial role in organizing the **sarcomeres** and providing structural integrity to muscle fibers. *Keratin* - **Keratin** is the most diverse family of **intermediate filaments** and is primarily found in epithelial cells. - It provides **mechanical strength** to cells and tissues, forming structures like hair, nails, and the outer layer of skin. *Vimentin* - **Vimentin** is a type III **intermediate filament** that is widely expressed in cells of **mesenchymal origin**, such as fibroblasts, endothelial cells, and leukocytes. - It contributes to cell shape, motility, and the integrity of the **cytoskeleton**.

Q: Which amino acid does not include post translational modification?

Selenocysteine. ***Selenocysteine*** - **Selenocysteine** is considered the 21st proteinogenic amino acid, directly incorporated into proteins **during translation** via a specialized tRNA and UGA codon (usually a stop codon). - It uses a **co-translational mechanism** involving SECIS elements (Selenocysteine Insertion Sequence) in the mRNA. - It is **NOT** formed by post-translational modification of another amino acid. *Hydroxy-lysine* - **Hydroxy-lysine** is formed by **post-translational hydroxylation** of lysine residues in collagen. - This modification requires **lysyl hydroxylase** enzyme and **Vitamin C** as a cofactor. - Essential for collagen cross-linking and stability. *γ-Carboxyglutamate* - **γ-Carboxyglutamate** is formed by **post-translational carboxylation** of glutamate residues. - Found in clotting factors (II, VII, IX, X) and requires **Vitamin K** as a cofactor. - Critical for calcium binding and proper coagulation function. *Hydroxy-proline* - **Hydroxy-proline** is formed by **post-translational hydroxylation** of proline residues in collagen. - This modification requires **prolyl hydroxylase** enzyme and **Vitamin C** as a cofactor. - Essential for the thermal stability and structural integrity of collagen.

Q: Which component is NOT involved in collagen synthesis?

Pyridoxal phosphate. ***Pyridoxal phosphate*** - **Pyridoxal phosphate** (PLP) is the active form of **vitamin B6** and serves as a coenzyme in various metabolic reactions, particularly those involving **amino acid metabolism**, but it is **not directly involved in collagen synthesis**. - While it plays a role in general protein metabolism, it does not participate in the hydroxylation of proline and lysine or the formation of the collagen triple helix, which are the critical steps in collagen synthesis. *Ascorbic acid* - **Ascorbic acid** (vitamin C) is a crucial cofactor for **prolyl hydroxylase** and **lysyl hydroxylase**, enzymes essential for the hydroxylation of proline and lysine residues in collagen. - This hydroxylation is critical for forming stable **cross-links** and the proper folding of the collagen triple helix. - Deficiency leads to scurvy with defective collagen. *Hydroxyproline* - **Hydroxyproline** is a modified amino acid formed **during collagen synthesis** through post-translational hydroxylation of proline residues by prolyl hydroxylase. - It is an essential structural component of mature collagen, strengthening the **collagen triple helix** through additional hydrogen bonding. - Its presence is used as a marker of collagen synthesis and degradation. *Glycine* - **Glycine** is the most abundant amino acid in collagen, accounting for about one-third of its total amino acid content, occurring at every third position in the α-chain sequence (Gly-X-Y). - Its small size allows the close packing of the three α-chains, which is essential for the formation of the characteristic **collagen triple helix**.

Q: Which factor stabilizes the alpha-helical structure of proteins?

Hydrogen bonds. ***Hydrogen bonds*** - Hydrogen bonds form between the **carbonyl oxygen (C=O)** of one peptide bond and the **amide hydrogen (N-H)** of a peptide bond **four residues away** along the polypeptide backbone. - These regularly spaced **intramolecular hydrogen bonds** are the primary force maintaining the characteristic **3.6 residues per turn helical structure** and stability of the alpha-helix. - This represents the fundamental stabilizing force of **secondary protein structure**. *Disulfide bonds* - Disulfide bonds are **covalent linkages** between cysteine residues that primarily stabilize **tertiary and quaternary structures**. - They are not involved in the regular, repetitive backbone structure of an alpha-helix. *Hydrophobic forces* - Hydrophobic interactions arise from **nonpolar amino acid side chains** clustering together to avoid water. - These forces are critical for **tertiary structure** stabilization and protein core formation, not secondary structure. *Ionic interactions* - Ionic interactions (salt bridges) occur between **oppositely charged side chains** (e.g., lysine and aspartate). - They contribute to **tertiary and quaternary structure** stability but are not the primary force in alpha-helix formation.

Q: Which enzyme is involved in the conversion of fibrinogen to fibrin, a key step in the blood clotting process?

Thrombin. ***Thrombin*** - **Thrombin** is a crucial serine protease in the coagulation cascade that catalyzes the conversion of soluble **fibrinogen** into insoluble **fibrin monomers**. - These fibrin monomers then polymerize to form a stable **fibrin mesh**, which is the structural basis of a blood clot, trapping red blood cells and platelets. *Plasmin* - **Plasmin** is an enzyme involved in **fibrinolysis**, the process of breaking down blood clots, rather than forming them. - It cleaves **fibrin** into soluble degradation products, helping to dissolve the clot once its purpose is served. *Elastase* - **Elastase** is a protease that breaks down **elastin**, a protein that provides elasticity to tissues. - It is primarily involved in tissue remodeling and inflammation and does not play a direct role in the conversion of fibrinogen to fibrin. *Collagenase* - **Collagenase** refers to enzymes that break down **collagen**, a major structural protein in connective tissues. - These enzymes are involved in tissue repair and remodeling but are not directly involved in the final steps of blood coagulation.

Question 1

Which of the following hormones is an example of a peptide hormone?

Accepted Answer

Parathormone (PTH)

Answer

Dopamine

Answer

Thyroxine

Answer

Cortisol

Question 2

C-peptide occurs in

Accepted Answer

Proinsulin

Answer

Glucagon

Answer

Thyroxine

Answer

Parathormone

Question 3

What type of collagen is found in bone?

Accepted Answer

Type 1

Answer

Type 3

Answer

Type 2

Answer

Type 4

Question 4

Glutathione is:

Accepted Answer

Tripeptide

Answer

Polypeptide

Answer

Oligopeptide

Answer

Dipeptide

Question 5

Replacing alanine by which amino acid, will increase UV absorbance of protein at 280 nm wavelength?

Accepted Answer

Tryptophan

Answer

Glycine

Answer

Arginine

Answer

Lysine

Question 6

Not a monomeric intermediate filament:

Accepted Answer

Tubulin

Answer

Desmin

Answer

Keratin

Answer

Vimentin

Question 7

Which amino acid does not include post translational modification?

Accepted Answer

Selenocysteine

Answer

Hydroxy-lysine

Answer

γ-Carboxyglutamate

Answer

Hydroxy-proline

Question 8

Which component is NOT involved in collagen synthesis?

Accepted Answer

Pyridoxal phosphate

Answer

Ascorbic acid

Answer

Hydroxyproline

Answer

Glycine

Question 9

Which factor stabilizes the alpha-helical structure of proteins?

Accepted Answer

Hydrogen bonds

Answer

Disulfide bonds

Answer

Hydrophobic forces

Answer

Ionic interactions

Question 10

Which enzyme is involved in the conversion of fibrinogen to fibrin, a key step in the blood clotting process?

Accepted Answer

Thrombin

Answer

Plasmin

Answer

Elastase

Answer

Collagenase

Protein Structure and Function — MCQs

Protein Structure and Function — MCQs

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