Protein Structure and Function — MCQs

Protein Structure and Function Indian Medical PG Practice Questions and MCQs

Question 381: Which of the following is not a peptide/protein -

A. Growth hormone
B. Glucocorticoids (Correct Answer)
C. PTH
D. Insulin

Explanation: ***Glucocorticoids*** - **Glucocorticoids** are **steroid hormones** derived from cholesterol, making them **lipids**, not peptides or proteins. - They exert their effects by binding to **intracellular receptors** to regulate gene expression. *Growth hormone* - **Growth hormone (GH)** is a **peptide hormone** produced by the anterior pituitary gland. - It plays a crucial role in **growth**, cell reproduction, and metabolism. *PTH* - **Parathyroid hormone (PTH)** is a **peptide hormone** secreted by the parathyroid glands. - It primarily regulates **calcium** and phosphate levels in the blood. *Insulin* - **Insulin** is a **peptide hormone** produced by the beta cells of the pancreatic islets. - Its main function is to regulate **glucose metabolism** by facilitating glucose uptake into cells.

Question 382: Which of the following hormones is an example of a peptide hormone?

A. Adrenaline
B. Parathormone (Correct Answer)
C. Thyroxine
D. Cortisol

Explanation: ***Parathormone*** - **Parathormone**, also known as parathyroid hormone (PTH), is a **peptide hormone** secreted by the parathyroid glands. - It plays a crucial role in regulating **calcium and phosphate levels** in the blood. *Adrenaline* - **Adrenaline** (epinephrine) is a **catecholamine**, which is a type of amine hormone derived from the amino acid tyrosine. - It is involved in the "fight or flight" response, affecting the cardiovascular system and metabolism. *Thyroxine* - **Thyroxine** (T4) is a **thyroid hormone**, which is an amine hormone derived from tyrosine and iodine. - It is critical for regulating metabolism, growth, and development. *Cortisol* - **Cortisol** is a **steroid hormone**, synthesized from cholesterol in the adrenal cortex. - It is a glucocorticoid involved in stress response, metabolism, and immune function.

Question 383: A glycoprotein that promotes new attachment and increased cell proliferation is:

A. Glycogen
B. Fibronectin (Correct Answer)
C. Fibrin
D. Glycosaminoglycan

Explanation: ***Fibronectin*** - **Fibronectin** is a **glycoprotein** that plays a crucial role in **cell adhesion**, growth, migration, and differentiation by binding to integrins on the cell surface. - It promotes **new attachment** of cells to the extracellular matrix and can enhance **cell proliferation**. *Glycogen* - **Glycogen** is a branched polysaccharide of glucose that serves as the primary **storage form of glucose** in animals and fungi. - Its main function is energy storage, not cell attachment or proliferation. *Fibrin* - **Fibrin** is an insoluble protein formed from **fibrinogen** during blood coagulation, leading to the formation of a blood clot. - While it's involved in wound healing and provides a scaffold, its primary role is not to promote new cell attachment or proliferation in the general sense like fibronectin. *Glycosaminoglycan* - **Glycosaminoglycans (GAGs)** are unbranched polysaccharides that are often found covalently linked to proteins to form **proteoglycans**. - They contribute to the structural integrity of tissues, hydration, and can interact with growth factors, but their direct role in promoting new cell attachment and increased cell proliferation is less pronounced than that of fibronectin.

Question 384: In which of the following condition is SHBG synthesis decreased?

A. Increased androgen (Correct Answer)
B. Increased estrogen
C. Pregnancy
D. Hyperthyroidism

Explanation: ***Increased androgen*** - **Androgens** (e.g., testosterone, DHT) directly suppress the hepatic synthesis of **Sex Hormone-Binding Globulin (SHBG)**. - This leads to a higher proportion of **free, biologically active sex hormones** in circulation. - Conditions with excess androgens (e.g., PCOS, androgen therapy) show decreased SHBG levels. *Increased estrogen* - **Estrogens** have the opposite effect - they *increase* hepatic synthesis of **SHBG**. - This results in *lower* levels of free sex hormones in circulation. *Pregnancy* - During **pregnancy**, there is a significant *increase* in **estrogen** levels. - This surge in estrogen leads to a substantial *increase* in **SHBG** production (can increase 2-3 fold). *Hyperthyroidism* - **Hyperthyroidism** (excess thyroid hormone) *increases* hepatic protein synthesis including **SHBG**. - Conversely, hypothyroidism would decrease SHBG levels.

Question 385: The shortest hormone is:

A. Insulin
B. Glucagon
C. HCG
D. TRH (Correct Answer)

Explanation: ***TRH*** - **TRH (Thyrotropin-Releasing Hormone)** is a **tripeptide**, meaning it is composed of only three amino acids (pyroGlu-His-Pro-NH2), making it the **shortest known hormone**. - Its primary function is to stimulate the release of **TSH (Thyroid-Stimulating Hormone)** and prolactin from the anterior pituitary. *Insulin* - **Insulin** is a **peptide hormone** composed of 51 amino acids (two chains linked by disulfide bonds). - It plays a crucial role in regulating **glucose metabolism** and is significantly larger than TRH. *Glucagon* - **Glucagon** is a **polypeptide hormone** consisting of 29 amino acids. - Its main function is to **raise blood glucose levels**, and while smaller than insulin, it is still much larger than TRH. *HCG* - **HCG (Human Chorionic Gonadotropin)** is a **glycoprotein hormone** composed of two subunits (alpha and beta) with a total of 237 amino acids. - It is involved in maintaining **pregnancy** and is a very large hormone compared to TRH.

Question 386: Which of the following hormones is an example of a peptide hormone?

A. Dopamine
B. Thyroxine
C. Parathormone (PTH) (Correct Answer)
D. Cortisol

Explanation: ***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.

Question 387: C-peptide occurs in

A. Proinsulin (Correct Answer)
B. Glucagon
C. Thyroxine
D. Parathormone

Explanation: ***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.

Question 388: What type of collagen is found in bone?

A. Type 3
B. Type 2
C. Type 4
D. Type 1 (Correct Answer)

Explanation: ***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**.

Question 389: Glutathione is:

A. Polypeptide
B. Oligopeptide
C. Dipeptide
D. Tripeptide (Correct Answer)

Explanation: ***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.

Question 390: Replacing alanine by which amino acid, will increase UV absorbance of protein at 280 nm wavelength?

A. Glycine
B. Tryptophan (Correct Answer)
C. Arginine
D. Lysine

Explanation: ***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.

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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