Protein Structure and Function Practice Questions

Q: What is the primary protein that binds to thyroxine?

Thyroxine-binding globulin. ***Thyroxine-binding globulin*** - **TBG** is a single chain glycoprotein that is the primary transporter, binding approximately 70-80% of circulating **T4** (thyroxine) and a smaller proportion of T3. - It serves as a high-affinity reservoir, maintaining hormonal homeostasis and contributing to the long **half-life** of T4. *Follistatin* - **Follistatin** binds and inhibits the activity of **activin** and is primarily involved in regulating the release of **FSH** (follicle-stimulating hormone) from the pituitary. - It has no functional role in the systemic transport or binding of thyroid hormones. *Transthyretin* - **Transthyretin** (or prealbumin) is the second most abundant carrier, binding about 10-15% of circulating T4, and is the principal carrier of **retinol** (Vitamin A). - Although it transports T4, its binding affinity and capacity are significantly lower than those of **TBG**, thus it is not the primary binder. *Transferrin* - **Transferrin** is the main plasma protein responsible for transporting **ferric iron (Fe3+)** throughout the body. - Its binding specificity is strictly for iron and does not involve the transport of **thyroxine**.

Q: A patient has a long arm span, hypermobile joints and ectopia lentis. What is the defective protein?

Fibrillin-1. ***Fibrillin-1***- **Marfan syndrome**, characterized by a **long arm span (arachnodactyly)**, **joint hypermobility**, and superiorly displaced **ectopia lentis (hypermobile lens)**, results from mutations in the *FBN1* gene, which codes for **Fibrillin-1**.- **Fibrillin-1** is a critical component of connective tissue **microfibrils** found in the suspensory ligaments of the eye, periosteum, and aortic media, explaining the systemic clinical manifestations.*Collagen*- Defects in **Collagen** (e.g., Type I, III, or V) are associated with **Osteogenesis Imperfecta** (brittle bones) or **Ehlers-Danlos syndrome** (severe skin and joint hyperlaxity) [1].- While Ehlers-Danlos syndrome can cause joint hypermobility, the classic combination with **ectopia lentis** strongly differentiates Marfan syndrome from primary collagenopathies [1].*Elastin*- Mutations in the **Elastin** gene (ELN) are primarily linked to conditions like **Williams syndrome**, which typically presents with **supravalvular aortic stenosis** and characteristic facial features [2].- **Elastin** provides rubber-like properties to tissues, but its primary defect does not explain the specific loss of structural integrity leading to superiorly displaced **ectopia lentis** seen in Marfan syndrome [2].*Keratin*- **Keratin** proteins are intermediate filaments crucial for mechanical stability in **epithelial cells** (skin, hair, nails).- Defects in keratin primarily cause **epidermolysis bullosa** or various forms of **ichthyosis** (skin blistering/scaling), and do not lead to the systemic skeletal or ocular connective tissue anomalies observed here.

Q: Which of the following amino acids primarily acts as a buffer in blood due to its ability to accept and donate protons at physiological pH?

Histidine. ***Histidine***- The side chain of **histidine**, the **imidazole group**, has a pKa of approximately 6.0, which is close to the physiological pH of **7.4**, making it an effective buffer.- This property is especially vital for the **buffering capacity of hemoglobin** in red blood cells, contributing significantly to pH homeostasis (Bohr effect).*Arginine*- Arginine possesses a **guanidinium group** in its side chain with a very high pKa (~12.5).- This high pKa means its side chain is almost always positively charged and protonated at physiological pH, rendering it ineffective as a physiological **acid-base buffer**.*Tryptophan*- Tryptophan has a large, non-polar **indole ring** side chain, which is chemically inert and lacks an ionizable group within the physiological pH range.- Since it cannot accept or donate protons near pH 7.4, it does not contribute to the **buffering system** of the blood.*Tyrosine*- Tyrosine contains a **phenolic hydroxyl group** with a pKa of approximately 10.5.- Because its pKa is significantly higher than physiological pH, it is largely neutral and incapable of mediating proton exchange effectively in the **blood plasma**.

Q: Which among the following helps in the maturation of collagen?

Ascorbic acid. ***Ascorbic acid***- Ascorbic acid (Vitamin C) is an essential cofactor for **prolyl hydroxylase** and **lysyl hydroxylase** enzymes, which hydroxylate proline and lysine residues in procollagen.- This **hydroxylation** is critical for stabilizing the collagen triple helix structure, a necessary step for proper collagen maturation and secretion.*Proline*- **Proline** (along with glycine and lysine) is a major amino acid component of the **collagen triple helix structure**.- While essential for collagen synthesis, proline itself is a substrate, not the **cofactor** required for the crucial hydroxylation steps necessary for maturation stabilization.*Copper and zinc*- **Copper** is a required cofactor for **lysyl oxidase**, the enzyme responsible for creating *covalent cross-links* between collagen molecules, which is a late-stage event for tensile strength.- Although cross-linking is part of the overall maturation process (extracellular), **ascorbic acid's** role in the intracellular hydroxylation of procollagen is a more fundamental step in maturation.*Phenylalanine*- **Phenylalanine** is an essential aromatic amino acid primarily used in protein synthesis and as a precursor for **tyrosine**.- It plays no direct role as a **cofactor** or required structural component in the specialized post-translational modification and subsequent maturation of collagen.

Q: IGF-1 and IGF-2 are structurally most similar to which of the following molecules?

Insulin. ***Insulin*** - **Insulin-like growth factors (IGFs)**, including IGF-1 and IGF-2, belong to the same peptide growth factor superfamily as insulin, sharing notable **sequence homology** (~50% amino acid similarity) and a similar three-dimensional structure. - Both IGFs and insulin are small peptides stabilized by **disulfide bonds** forming A and B domains, and act via similar receptor tyrosine kinases (the insulin receptor and IGF-1 receptor can cross-react). - Mature insulin represents the **structurally closest molecule** to the secreted, functional IGFs. *Preproinsulin* - **Preproinsulin** is the earliest precursor and includes an N-terminal **signal peptide** (the 'pre' sequence) which is cleaved off upon entry into the endoplasmic reticulum. - Since IGFs mature into secreted proteins without this transient signal sequence in their final structure, preproinsulin is structurally less similar than mature insulin. *Proinsulin* - **Proinsulin** consists of the A chain and B chain linked by the connecting **C-peptide** domain. - Although IGFs are derived from a single precursor chain like proinsulin, **mature insulin** (two chains, A and B) is structurally closer to the secreted IGFs than proinsulin (which has three domains: A, B, and C). *C-peptide* - The **C-peptide** is the connecting segment linking the A and B chains of proinsulin, which is cleaved and removed before insulin becomes mature. - It is a short, linear peptide with little structural resemblance to the complex, disulfide-bonded domains of functional IGFs or insulin.

Q: What is the pH of the zwitterion as shown in the titration curve of glycine?

6. ***Correct Option: 6*** - The pH of the zwitterion corresponds to the **isoelectric point (pI)**, the pH at which the amino acid has a net charge of zero. According to the curve, this occurs at pH 5.97, which is approximately 6. - The pI is calculated as the average of the two pKa values: **pI = (pKa1 + pKa2) / 2 = (2.34 + 9.60) / 2 = 5.97**. *Incorrect Option: 4.4* - At a pH of 4.4, which is between pKa1 and pI, the glycine molecule would have a net **positive charge** as the concentration of the cationic form ([NH3+-CH2-COOH]) would still be significant. - This value does not represent any specific inflection or equivalence point on the titration curve for glycine. *Incorrect Option: 2* - A pH of approximately 2 is near the **pKa1 (2.34)** of the carboxyl group, which is the point where the concentration of the fully protonated form equals the concentration of the zwitterion. - At this pH, the molecule has a net **positive charge**, as the amino group is fully protonated and the carboxyl group is only half-deprotonated. *Incorrect Option: 9* - A pH of approximately 9 is near the **pKa2 (9.60)** of the amino group, representing the buffering region for this group. - At this pH, the molecule has a net **negative charge**, as there is a significant amount of the fully deprotonated form ([NH2-CH2-COO-]) present.

Q: Match the following molecules with their carriers in plasma: MOLECULES: 1. Thyroxine 2. Fatty acid 3. Hemoglobin 4. Heme CARRIERS: A. Hemopexin B. Prealbumin C. Albumin D. Haptoglobin

1-B, 2-C, 3-D, 4-A. ***1-B, 2-C, 3-D, 4-A*** - **Thyroxine** is primarily transported by **prealbumin (transthyretin)**, which has high affinity for thyroid hormones and prevents their rapid clearance. - **Fatty acids** bind to **albumin** at multiple binding sites, allowing efficient transport of these hydrophobic molecules in plasma. - **Hemoglobin** released from lysed red blood cells is bound by **haptoglobin** to prevent renal loss and iron wastage. - **Heme** groups are specifically transported by **hemopexin**, which has the highest binding affinity for heme among plasma proteins. *1-B, 2-C, 3-A, 4-D* - Incorrectly pairs **hemoglobin** with hemopexin; hemoglobin binds to **haptoglobin** to form stable complexes. - Incorrectly pairs **heme** with haptoglobin; free heme has highest affinity for **hemopexin**, not haptoglobin. *1-A, 2-B, 3-C, 4-D* - Incorrectly pairs **thyroxine** with hemopexin; thyroxine is primarily carried by **prealbumin** and thyroxine-binding globulin. - Incorrectly pairs **fatty acids** with prealbumin; fatty acids bind predominantly to **albumin** due to its multiple hydrophobic binding sites. *1-A, 2-D, 3-B, 4-C* - Incorrectly pairs **heme** with albumin; while albumin can bind heme weakly, **hemopexin** is the specific high-affinity carrier. - Incorrectly pairs **hemoglobin** with prealbumin; free hemoglobin forms specific complexes with **haptoglobin** for recycling.

Q: Beta hCG is structurally similar to which biochemical moiety?

LH. ***LH*** - **Beta-hCG** and **Luteinizing Hormone (LH)** both belong to the family of glycoprotein hormones and share a common alpha subunit. - Their **beta subunits** are also very similar, with a significant overlap in their amino acid sequences, allowing **hCG** to bind to and activate **LH receptors**. *Oxytocin* - **Oxytocin** is a **peptide hormone** primarily involved in uterine contractions and milk ejection. - It is structurally distinct from **glycoprotein hormones** like **hCG** and lacks the alpha and beta subunit structure. *FSH* - **Follicle-Stimulating Hormone (FSH)** is a glycoprotein hormone that shares the same alpha subunit with hCG, LH, and TSH. - However, its **beta subunit** is distinct from that of **hCG**, which gives it its unique biological specificity. *ACTH* - **Adrenocorticotropic Hormone (ACTH)** is a **peptide hormone** produced by the anterior pituitary gland, regulating adrenal gland function. - It has a completely different structure and function compared to **hCG** and other glycoprotein hormones.

Question 1

What is the primary protein that binds to thyroxine?

Accepted Answer

Thyroxine-binding globulin

Answer

Follistatin

Answer

Transthyretin

Answer

Transferrin

Question 2

A patient has a long arm span, hypermobile joints and ectopia lentis. What is the defective protein?

Accepted Answer

Fibrillin-1

Answer

Keratin

Answer

Elastin

Answer

Collagen

Question 3

Which of the following amino acids primarily acts as a buffer in blood due to its ability to accept and donate protons at physiological pH?

Accepted Answer

Histidine

Answer

Arginine

Answer

Tryptophan

Answer

Tyrosine

Question 4

Which among the following helps in the maturation of collagen?

Accepted Answer

Ascorbic acid

Answer

Proline

Answer

Copper and zinc

Answer

Phenylalanine

Question 5

IGF-1 and IGF-2 are structurally most similar to which of the following molecules?

Accepted Answer

Insulin

Answer

Preproinsulin

Answer

Proinsulin

Answer

C-peptide

Question 6

What is the pH of the zwitterion as shown in the titration curve of glycine?

Accepted Answer

6

Answer

2

Answer

9

Answer

4.4

Question 7

All are correct about the protein shown in the figure except? (Recent NEET Pattern 2016-17)

Accepted Answer

Three strands of polypeptide twist to the right and wrap around each other in left handed fashion

Answer

3.3 residues per turn

Answer

Every third amino acid is glycine and $Y$ is generally proline or hydroxyproline

Answer

Most abundant protein in body

Question 8

All of the statements are true about the titration curve of a protein except:

Accepted Answer

The protein has 1 functional ion

Answer

A and B represent ionization of amino and carboxyl end of the protein

Answer

The protein has 3 ionizable sites

Answer

Points A and B represent first and second equivalence point

Question 9

Match the following molecules with their carriers in plasma:

MOLECULES:
1. Thyroxine
2. Fatty acid
3. Hemoglobin
4. Heme

CARRIERS:
A. Hemopexin
B. Prealbumin
C. Albumin
D. Haptoglobin

Accepted Answer

1-B, 2-C, 3-D, 4-A

Answer

1-B, 2-C, 3-A, 4-D

Answer

1-A, 2-B, 3-C, 4-D

Answer

1-A, 2-D, 3-B, 4-C

Question 10

Beta hCG is structurally similar to which biochemical moiety?

Accepted Answer

LH

Answer

Oxytocin

Answer

FSH

Answer

ACTH

Protein Structure and Function — MCQs

Protein Structure and Function — MCQs

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