Protein Structure and Function Practice Questions

Q: A person on total parenteral nutrition was given 62.5 g of protein. Calculate the amount of nitrogen present in this amount of protein.

10 g. ### Explanation **1. Why Option C (10 g) is Correct:** The calculation is based on the fundamental biochemical principle that average dietary protein contains approximately **16% nitrogen** by weight. This relationship is a cornerstone for assessing nitrogen balance in clinical nutrition. To find the nitrogen content, we use the following formula: * **Nitrogen (g) = Protein (g) ÷ 6.25** (since 100/16 = 6.25) * **Calculation:** 62.5 g (Protein) ÷ 6.25 = **10 g of Nitrogen**. Alternatively, calculating 16% of 62.5 g (0.16 × 62.5) also yields 10 g. **2. Why Other Options are Incorrect:** * **Option A (32 g):** This is an overestimation. Nitrogen only constitutes a small fraction of the amino acid structure (the amino group); the rest is the carbon skeleton. * **Option B (6.25 g):** This is the **conversion factor** itself, not the result. Students often confuse the factor with the final value. * **Option D (100 g):** This is mathematically impossible as the nitrogen content cannot exceed the total mass of the protein provided. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Nitrogen Balance:** This is calculated as *Nitrogen Intake - Nitrogen Output*. It is used to evaluate whether a patient is in an anabolic state (positive balance, e.g., growth, pregnancy) or catabolic state (negative balance, e.g., burns, trauma, starvation). * **Kjeldahl Method:** This is the standard laboratory technique used to determine the nitrogen content of biological samples. * **TPN Requirement:** In Total Parenteral Nutrition, the non-protein calorie to nitrogen ratio (NPC:N) is crucial for optimal protein synthesis, typically ranging from 100:1 to 150:1 in stressed patients.

Q: The characteristic triple helix structure is observed in which of the following molecules?

Collagen. **Explanation:** **Collagen** is the correct answer because it is characterized by a unique **triple helix** structure (tropocollagen). This structure consists of three polypeptide alpha-chains wound around each other. The stability of this helix is maintained by a repeating amino acid sequence: **Gly-X-Y**, where Glycine (the smallest amino acid) fits into the restricted central core, while X and Y are frequently **Proline** and **Hydroxyproline**. Hydroxyproline is essential for interchain hydrogen bonding, which stabilizes the triple helix. **Why other options are incorrect:** * **DNA:** Typically exists as a **double helix** (B-DNA), consisting of two antiparallel polynucleotide strands. * **Elastin:** Unlike collagen, elastin does not have a regular repeating unit or a triple helical structure. It exists as an **amorphous, random-coil** conformation that allows it to stretch and recoil. * **RNA:** Generally exists as a **single-stranded** molecule, though it can fold into complex secondary structures (like the cloverleaf in tRNA), it does not form a triple helix. **Clinical Pearls for NEET-PG:** * **Vitamin C Deficiency (Scurvy):** Vitamin C is a cofactor for prolyl and lysyl hydroxylase. Lack of hydroxyproline leads to unstable collagen triple helices, resulting in capillary fragility and poor wound healing. * **Osteogenesis Imperfecta:** Often caused by mutations substituting Glycine with bulkier amino acids, preventing proper triple helix formation ("Brittle Bone Disease"). * **Ehlers-Danlos Syndrome:** A group of disorders characterized by defects in the synthesis or processing of collagen (e.g., Type III collagen in the vascular type).

Q: Which of the following amino acids does not participate in the formation of an alpha helix?

Proline. The **Alpha-helix** is a common secondary structure of proteins characterized by a right-handed coiled conformation stabilized by hydrogen bonding between the carbonyl oxygen (C=O) and the amide hydrogen (N-H) of amino acids four residues apart [1]. **Why Proline is the correct answer:** Proline is known as a **"helix breaker"** for two primary reasons: 1. **Rigid Structure:** Its side chain is cyclized back onto the backbone nitrogen, forming a secondary amino group (imino acid) [1]. This rigid ring structure prevents the rotation necessary to fit into the tight geometry of an alpha helix. 2. **Lack of Hydrogen Bonding:** Because the nitrogen in Proline is part of a ring, it lacks the amide hydrogen required to form the stabilizing hydrogen bonds that hold the helix together. When Proline is present, it creates a "kink" or bend in the polypeptide chain. **Analysis of Incorrect Options:** * **Leucine & Methionine:** These are hydrophobic amino acids with high "helix-forming propensity." They have unbranched side chains at the beta-carbon, allowing them to pack efficiently into the helical structure. * **Lysine:** This is a charged amino acid that generally favors helix formation, provided it is not clustered with too many other similarly charged residues (which would cause electrostatic repulsion). **High-Yield Clinical Pearls for NEET-PG:** * **Glycine** is also often excluded from alpha helices, but for the opposite reason: it is too flexible (due to having only a Hydrogen atom as a side chain), making the helix entropically unstable. * **Collagen Structure:** While Proline breaks alpha helices, it is essential for the **Collagen Triple Helix**, where its rigid structure helps stabilize the unique left-handed pro-alpha chains. * **Amino Acid Mnemonic:** "MALEK" (Methionine, Alanine, Leucine, Glutamate, Lysine) are the strongest helix formers.

Q: Which of the following amino acids are not coded by a triplet codon?

Hydroxyproline and Selenocysteine. **Explanation:** The correct answer is **Hydroxyproline and Selenocysteine**. This question tests the distinction between standard amino acids, non-standard amino acids, and post-translational modifications. 1. **Why Option A is correct:** * **Hydroxyproline:** It is a post-translationally modified amino acid. It is formed by the hydroxylation of Proline (catalyzed by prolyl hydroxylase) *after* the polypeptide chain is synthesized. There is no specific triplet codon for Hydroxyproline in the genetic code. * **Selenocysteine:** Known as the "21st amino acid," it is unique. While it is incorporated during translation, it does not have its own dedicated triplet codon. Instead, it is encoded by the **UGA stop codon** through a recoding mechanism involving a specific mRNA sequence called the SECIS element. 2. **Why other options are incorrect:** * **Lysine (Option B):** A standard proteinogenic amino acid coded by AAA and AAG. * **Ornithine (Options C & D):** An intermediate in the Urea Cycle. While it is an amino acid, it is **non-proteinogenic**, meaning it is never incorporated into proteins and thus has no codon. However, the question asks for a pair; since Selenocysteine *is* incorporated into proteins (unlike Ornithine), the combination in Option A is the most accurate representation of "special" coding circumstances in biochemistry. **High-Yield Clinical Pearls for NEET-PG:** * **Hydroxyproline:** Essential for collagen stability. Its synthesis requires **Vitamin C** as a co-factor; deficiency leads to Scurvy due to defective collagen cross-linking. * **Selenocysteine:** Found in key enzymes like **Glutathione Peroxidase** (antioxidant) and **Deiodinase** (thyroid hormone metabolism). * **Pyrrolysine:** The "22nd amino acid," encoded by the UAG stop codon (primarily in archaea).

Q: Which of the following amino acids shows a tendency to form a left-handed helix?

Glycine. **Explanation:** The correct answer is **Glycine**. **1. Why Glycine is Correct:** In protein biochemistry, the alpha-helix is typically right-handed due to the L-configuration of amino acids. However, **Glycine** is unique because it is the only achiral amino acid (its side chain is a single hydrogen atom). This lack of a bulky side chain provides Glycine with exceptional conformational flexibility, allowing it to occupy regions of the **Ramachandran plot** that are sterically forbidden for other amino acids. Specifically, Glycine can adopt the dihedral angles required to form a **left-handed helix**, a structure that would cause steric hindrance (clashing) between the side chains and the backbone in any other amino acid. **2. Why the Other Options are Incorrect:** * **Cysteine, Arginine, and Histidine:** These are all **L-amino acids** with distinct side chains (R-groups). Due to steric interference between the side chain and the carbonyl oxygen of the peptide backbone, these amino acids strongly prefer the right-handed helical conformation. A left-handed helix formed by these residues is energetically unstable and rarely occurs in nature. **3. NEET-PG High-Yield Facts:** * **Glycine as a "Helix Breaker":** While it can form a left-handed helix, Glycine often acts as a "helix breaker" in standard right-handed alpha-helices because its high flexibility makes the rigid helical structure entropically unfavorable. * **Proline:** Another "helix breaker" because its rigid cyclic structure lacks the NH group necessary for hydrogen bonding. * **Collagen Triple Helix:** Glycine is essential here; it occurs at every third position (**Gly-X-Y**) because only Glycine is small enough to fit into the crowded central core of the triple helix. * **Ramachandran Plot:** Glycine shows the widest range of permissible Phi ($\phi$) and Psi ($\psi$) angles.

Q: Which of the following organelles are involved in the formation of N-glycosylated products?

Golgi apparatus and RER. **Explanation:** N-glycosylation is a critical post-translational modification where a carbohydrate chain is attached to the nitrogen atom of an **Asparagine (Asn)** residue within the consensus sequence Asn-X-Ser/Thr. This process is a coordinated effort between two specific organelles: 1. **Rough Endoplasmic Reticulum (RER):** This is where N-glycosylation **begins**. A pre-formed oligosaccharide (attached to a **Dolichol** phosphate lipid carrier) is transferred en bloc to the nascent polypeptide chain as it enters the RER lumen. 2. **Golgi Apparatus:** The glycoprotein then moves to the Golgi, where the initial carbohydrate chain undergoes extensive **processing and modification** (trimming and addition of sugar residues) to form complex or high-mannose glycans. **Analysis of Incorrect Options:** * **Nucleolus:** Primarily involved in ribosomal RNA (rRNA) synthesis and ribosome biogenesis; it plays no role in protein glycosylation. * **SER (Smooth ER):** Involved in lipid synthesis, steroidogenesis, and detoxification (Cytochrome P450 system), but lacks the ribosomes and machinery required for the initial steps of N-glycosylation. * **RER and Nucleolus:** While RER is correct, the inclusion of the nucleolus makes this option incorrect. **High-Yield Clinical Pearls for NEET-PG:** * **Dolichol Phosphate:** The essential lipid carrier inhibited by **Tunicamycin**. * **I-Cell Disease (Mucolipidosis II):** A deficiency in the enzyme *N-acetylglucosamine-1-phosphotransferase* in the Golgi. This leads to a failure to tag enzymes with **Mannose-6-Phosphate**, causing lysosomal enzymes to be secreted extracellularly instead of being routed to lysosomes. * **O-glycosylation:** Unlike N-glycosylation, O-linked glycosylation (attachment to Serine/Threonine) occurs **exclusively in the Golgi apparatus**.

Q: Generalized body edema is associated with a deficiency of which of the following substances?

Albumin. **Explanation:** The correct answer is **Albumin**. **1. Why Albumin is Correct:** Albumin is the primary protein responsible for maintaining **Plasma Colloid Osmotic Pressure (COPs)**. According to Starling’s Law, this pressure acts as a "pulling force" that keeps fluid within the intravascular compartment. When albumin levels drop (hypoalbuminemia)—due to malnutrition (Kwashiorkor), liver failure (decreased synthesis), or nephrotic syndrome (increased loss)—the oncotic pressure decreases. This allows fluid to leak from the capillaries into the interstitial space, resulting in **generalized body edema (anasarca)**. **2. Why the Other Options are Incorrect:** * **Vitamin B12:** Deficiency primarily leads to Megaloblastic Anemia and neurological symptoms (Subacute Combined Degeneration of the Spinal Cord), not edema. * **Sodium:** While sodium is the major extracellular cation, its deficiency (hyponatremia) typically causes cellular swelling (especially in the brain) rather than generalized peripheral edema. In fact, sodium *excess* (retention) is more commonly associated with edema. * **Essential Fatty Acids (EFAs):** Deficiency (e.g., Linoleic acid) leads to phrynoderma (toad skin), poor wound healing, and hair loss, but does not directly cause generalized edema. **3. NEET-PG High-Yield Pearls:** * **Kwashiorkor vs. Marasmus:** Edema is the hallmark of **Kwashiorkor** (protein deficiency with adequate calories) due to hypoalbuminemia. It is absent in **Marasmus** (total calorie deficiency). * **Normal Albumin Levels:** 3.5 to 5.0 g/dL. Edema usually manifests when levels fall below **2.5 g/dL**. * **Synthesis:** Albumin is synthesized exclusively in the liver and has a half-life of approximately 20 days.

Q: What are clusters of twisted strands of β sheet called?

β barrels. **Explanation:** The correct answer is **β barrels**. This structure is a large, closed β-sheet that twists and coils to form a closed, cylinder-like structure. In this arrangement, the first strand is hydrogen-bonded to the last strand, creating a stable, pore-like architecture. These are commonly found in transmembrane proteins (porins) where the exterior is hydrophobic (interacting with lipids) and the interior is hydrophilic (allowing water/solute passage). **Analysis of Options:** * **Amphipathic helices (A):** These are α-helices with one hydrophobic face and one hydrophilic face. While important for protein-lipid interactions, they are not composed of twisted β-sheets. * **Loops and bends (C):** These are non-repetitive secondary structures that connect α-helices and β-sheets. They allow the polypeptide chain to reverse direction but do not form large "clusters" or "barrels." * **Supersecondary structures (D):** This is a broad category (also called motifs) that includes structures like β-α-β or Greek keys. While a β-barrel *is* a type of supersecondary structure, the question specifically describes the "clusters of twisted strands" forming a specific shape, for which "β barrel" is the most precise anatomical description. **NEET-PG High-Yield Pearls:** * **Porins:** Found in the outer membranes of Gram-negative bacteria and mitochondria, porins are the classic example of β-barrels. * **Green Fluorescent Protein (GFP):** A famous example of a "β-can" or barrel structure used in molecular biology. * **Stability:** The β-barrel is exceptionally stable due to the extensive hydrogen bonding between all adjacent strands, making it resistant to denaturation. * **Beta-turns:** Often contain **Proline** (induces a kink) and **Glycine** (small and flexible), and are usually stabilized by hydrogen bonds between the 1st and 4th residues.

Q: Type IV Ehlers-Danlos syndrome is characterized by which of the following?

Decreased amount of type III collagen.. **Explanation:** Ehlers-Danlos Syndrome (EDS) is a clinically and genetically heterogeneous group of disorders resulting from defects in the synthesis or structure of fibrillar collagen [1]. **Why Option B is Correct:** Type IV EDS, also known as the **Vascular Type**, is caused by mutations in the **COL3A1 gene**. This gene encodes the pro-alpha1 chains of **Type III collagen**. The mutation leads to a significantly decreased amount of normal Type III collagen or the production of defective Type III collagen. Since Type III collagen is a major structural component of blood vessels and hollow organs (like the bowel and uterus), this subtype is characterized by life-threatening arterial rupture and visceral perforation [1]. **Why Other Options are Incorrect:** * **Option A:** Deficiencies in lysyl hydroxylase are characteristic of **Type VI EDS (Kyphoscoliotic Type)**, which presents with ocular fragility and severe hypotonia. Lysyl oxidase deficiency is associated with Menkes disease, not typically EDS. * **Option C:** Fibronectin defects are not the primary underlying pathology in the classic or vascular forms of EDS. EDS is fundamentally a collagenopathy. * **Option D:** Since Options A and C are incorrect, "All of the above" is invalid. **High-Yield NEET-PG Pearls:** * **Classic EDS (Types I/II):** Defect in **Type V collagen** (COL5A1, COL5A2); features skin hyperextensibility and "cigarette paper" scarring [1]. * **Vascular EDS (Type IV):** Defect in **Type III collagen**; most serious form due to risk of **aortic rupture** [1]. * **Kyphoscoliotic EDS (Type VI):** Defect in **Lysyl hydroxylase**; look for "retinal detachment" and "kyphoscoliosis" in the stem. * **Arthrochalasia/Dermatosparaxis EDS (Type VII):** Defect in the conversion of Type I procollagen to collagen. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Genetic Disorders, pp. 154-156.

Q: What is the most abundant protein found in the human body?

Collagen. **Explanation:** **Correct Answer: C. Collagen** Collagen is the most abundant protein in the human body, accounting for approximately **25% to 35% of the total protein mass**. It is a structural fibrous protein found primarily in the extracellular matrix of connective tissues, including skin, bones, tendons, cartilage, and blood vessels. Its high abundance is due to its critical role in providing tensile strength and structural integrity to almost every organ system. **Analysis of Incorrect Options:** * **A. Globulin:** These are a group of globular proteins (like immunoglobulins) found in the blood. While vital for immunity and transport, they do not match the sheer structural mass of collagen. * **B. Albumin:** This is the most abundant **plasma** protein (making up ~60% of serum proteins). Students often confuse "most abundant protein in the body" with "most abundant protein in the plasma." * **D. Hemoglobin:** This is the most abundant protein within **red blood cells** and is essential for oxygen transport, but its total body mass is less than that of collagen. **High-Yield Clinical Pearls for NEET-PG:** * **Structure:** Collagen is a triple helix composed of three polypeptide chains. Every third amino acid is **Glycine** (the smallest amino acid, allowing tight packing). * **Post-translational modification:** Hydroxylation of Proline and Lysine requires **Vitamin C** (Ascorbic acid). Deficiency leads to **Scurvy**. * **Types to Remember:** * Type I: Bone, Skin, Tendon (90% of body collagen). * Type II: Cartilage ("Car-two-lage"). * Type III: Reticular fibers, Blood vessels (Deficient in Vascular Ehlers-Danlos). * Type IV: Basement membrane ("Under the floor").

Question 1

A person on total parenteral nutrition was given 62.5 g of protein. Calculate the amount of nitrogen present in this amount of protein.

Accepted Answer

10 g

Answer

32 g

Answer

6.25 g

Answer

100 g

Question 2

The characteristic triple helix structure is observed in which of the following molecules?

Accepted Answer

Collagen

Answer

DNA

Answer

Elastin

Answer

RNA

Question 3

Which of the following amino acids does not participate in the formation of an alpha helix?

Accepted Answer

Proline

Answer

Leucine

Answer

Methionine

Answer

Lysine

Question 4

Which of the following amino acids are not coded by a triplet codon?

Accepted Answer

Hydroxyproline and Selenocysteine

Answer

Lysine and Hydroxyproline

Answer

Hydroxyproline and Ornithine

Answer

Selenocysteine and Ornithine

Question 5

Which of the following amino acids shows a tendency to form a left-handed helix?

Accepted Answer

Glycine

Answer

Cysteine

Answer

Arginine

Answer

Histidine

Question 6

Which of the following organelles are involved in the formation of N-glycosylated products?

Accepted Answer

Golgi apparatus and RER

Answer

Nucleolus

Answer

SER

Answer

RER and Nucleolus

Question 7

Generalized body edema is associated with a deficiency of which of the following substances?

Accepted Answer

Albumin

Answer

Vitamin B12

Answer

Sodium

Answer

Essential Fatty Acids (EFAs)

Question 8

What are clusters of twisted strands of β sheet called?

Accepted Answer

β barrels

Answer

Amphipathic helices

Answer

Loops and bends

Answer

Supersecondary structures

Question 9

Type IV Ehlers-Danlos syndrome is characterized by which of the following?

Accepted Answer

Decreased amount of type III collagen.

Answer

Deficiencies in hydroxylase and lysyl oxidase.

Answer

Nonfunctioning plasma fibronectin.

Answer

All of the above.

Question 10

What is the most abundant protein found in the human body?

Accepted Answer

Collagen

Answer

Globulin

Answer

Albumin

Answer

Hemoglobin

Protein Structure and Function — MCQs

Protein Structure and Function — MCQs

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