Protein Structure and Function — MCQs

10 questions

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

Which factor stabilizes the alpha-helical structure of proteins?

Which of the following changes can be made in insulin structure so that there is least change in the function of insulin:

What is the primary physiological effect of increased 2,3-DPG on hemoglobin?

Which of the following statements about chaperones is false?

What sequence on the template strand of DNA corresponds to the first amino acid inserted into a protein?

Which type of bond is primarily responsible for the primary structure of a protein?

Maldigestion of protein and fat is manifested in chronic pancreatitis only if the damage to pancreatic tissue exceeds?

Western blot is used for:

Q10

Which of the following can be absorbed without being broken down, especially in infants?

Protein Structure and Function Indian Medical PG Practice Questions and MCQs

Question 1: 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.

Question 2: Which factor stabilizes the alpha-helical structure of proteins?

A. Disulfide bonds
B. Hydrophobic forces
C. Ionic interactions
D. Hydrogen bonds (Correct Answer)

Explanation: ***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.

Question 3: Which of the following changes can be made in insulin structure so that there is least change in the function of insulin:

A. Breaking disulphide linkages
B. Interchange of B29 and B30 (Correct Answer)
C. Interchange of A5 & A6
D. Interchange of A1 & A4

Explanation: ***Interchange of B29 and B30*** - Interchanging amino acids at positions B29 and B30 in the **B-chain of insulin** typically causes the **least disruption** to its overall three-dimensional structure and biological activity. - These positions are often found at the **C-terminus of the B chain** and are less critical for receptor binding and activity compared to other regions. *Breaking disulphide linkages* - **Disulphide linkages** are crucial for maintaining the **tertiary structure** of insulin, connecting its A and B chains and stabilizing its folded state. - Breaking these bonds would lead to **denaturation** and a significant loss of function, as the molecule would unfold and be unable to bind to its receptor effectively. *Interchange of A5 & A6* - Amino acids at positions **A5 and A6** are located in a region of the **A-chain** that is important for the structural integrity and receptor binding of insulin. - Interchanging these amino acids would likely cause a **significant change in the precise folding** of the insulin molecule, potentially impairing its ability to interact with the insulin receptor. *Interchange of A1 & A4* - Positions **A1 and A4** are located at the N-terminus of the **A-chain**, a region known for its critical role in **receptor recognition and binding**. - Swapping these amino acids would introduce **substantial structural changes** in this vital domain, leading to a major reduction or complete loss of insulin's biological activity.

Question 4: What is the primary physiological effect of increased 2,3-DPG on hemoglobin?

A. Increased affinity of hemoglobin to oxygen
B. Decreased affinity of hemoglobin to oxygen (Correct Answer)
C. Left shift of oxygen-hemoglobin dissociation curve
D. Right shift of oxygen-hemoglobin dissociation curve

Explanation: ***Decreased affinity of hemoglobin to oxygen*** - **2,3-Diphosphoglycerate (2,3-DPG)** binds to the beta subunits of deoxyhemoglobin, stabilizing the **deoxygenated state** and thus **reducing hemoglobin's affinity for oxygen**. - This is the **primary molecular mechanism** by which 2,3-DPG exerts its effect, facilitating **oxygen unloading** in peripheral tissues. - This decreased affinity manifests graphically as a **right shift** in the oxygen-hemoglobin dissociation curve. *Increased affinity of hemoglobin to oxygen* - This is incorrect because 2,3-DPG specifically works to **decrease hemoglobin's affinity** for oxygen, promoting oxygen release. - Increased affinity would mean oxygen is held more tightly, which is counterproductive for **oxygen delivery** to tissues. *Left shift of oxygen-hemoglobin dissociation curve* - A **left shift** indicates **increased affinity** of hemoglobin for oxygen, meaning oxygen is held more tightly. - Since 2,3-DPG decreases affinity, it causes a **right shift**, not a left shift. *Right shift of oxygen-hemoglobin dissociation curve* - While this is the **graphical representation** of 2,3-DPG's effect, it is a **consequence** of the primary molecular mechanism (decreased affinity). - A right shift signifies that for any given partial pressure of oxygen, hemoglobin is **less saturated** with oxygen, reflecting the decreased affinity caused by 2,3-DPG binding.

Question 5: Which of the following statements about chaperones is false?

A. Are lipid in nature (Correct Answer)
B. Cause folding of proteins
C. Include heat shock proteins
D. May have ATPase activity

Explanation: ***Are lipid in nature*** - Chaperones are **proteins** (typically **heat shock proteins** or **chaperonins**), not lipids. - Their function involves assisting in the proper **folding and assembly of other proteins**, and they are composed of amino acids. *Cause folding of proteins* - Chaperones **do not cause** proteins to fold; rather, they **assist in proper folding** and refolding by preventing aggregation or misfolding. - They bind to nascent or partially unfolded proteins to guide them towards their correct three-dimensional structure. *May have ATPase activity* - Many chaperones, especially **Hsp70** and **chaperonins** like GroEL/GroES, utilize **ATP hydrolysis** for their function. - This **ATPase activity** drives conformational changes essential for binding, release, and refolding of their client proteins. *Include heat shock proteins* - The **heat shock protein (Hsp)** families (e.g., Hsp70, Hsp90, Hsp60) are a major class of chaperones. - Hsps are upregulated in response to stress (like heat) to help refold damaged proteins and prevent aggregation.

Question 6: What sequence on the template strand of DNA corresponds to the first amino acid inserted into a protein?

A. 3' TAC 5' (Correct Answer)
B. 3' TAG 5'
C. 3' TAA 5'
D. 3' ATG 5'

Explanation: ***3' TAC 5'*** - The **start codon** for protein synthesis on **mRNA** is **5'-AUG-3'**, which codes for **methionine** (or N-formylmethionine in prokaryotes) and signals the initiation of translation. - To produce an mRNA codon of **5'-AUG-3'**, the complementary sequence on the **template DNA strand** must be **3'-TAC-5'** (adenine pairs with uracil/thymine, guanine pairs with cytosine, and the strands are antiparallel). - During transcription, RNA polymerase reads the template strand in the 3' to 5' direction and synthesizes mRNA in the 5' to 3' direction. *3' TAG 5'* - This template DNA sequence would be transcribed to produce the mRNA codon **5'-AUC-3'**, which codes for **isoleucine**, not methionine. - Therefore, this sequence does not correspond to the first amino acid inserted into a protein. *3' TAA 5'* - This template DNA sequence would be transcribed to produce the mRNA codon **5'-AUU-3'**, which also codes for **isoleucine**, not methionine. - This is not the initiation codon sequence. *3' ATG 5'* - While **ATG** appears in this sequence, when presented as the **template strand** in the 3' to 5' orientation, it would be transcribed to produce mRNA **5'-UAC-3'**, which codes for **tyrosine**, not methionine. - The sequence **ATG** on the **coding strand** (non-template strand) corresponds to the start codon, but this option incorrectly presents it as the template strand sequence.

Question 7: Which type of bond is primarily responsible for the primary structure of a protein?

A. Hydrogen bond
B. Disulfide bond
C. Peptide bond (Correct Answer)
D. Electrostatic bond

Explanation: ***Peptide bond*** - The **primary structure** of a protein is defined by the unique linear sequence of **amino acids** linked together by **peptide bonds**. - These are **amide bonds** formed between the carboxyl group of one amino acid and the amino group of another, with the elimination of water. *Hydrogen bond* - **Hydrogen bonds** are crucial for the **secondary structure** (e.g., alpha-helices and beta-sheets) and **tertiary/quaternary structures** of proteins, stabilizing their 3D folds. - They involve interactions between polar atoms, not the direct linkage of amino acids in the primary sequence. *Disulfide bond* - **Disulfide bonds** are **covalent bonds** formed between the sulfur atoms of two **cysteine residues**, contributing to the **tertiary** and sometimes **quaternary structure** stability. - They are not involved in forming the linear sequence of amino acids, which is the primary structure. *Electrostatic bond* - **Electrostatic bonds**, or **ionic bonds**, occur between oppositely charged amino acid side chains and are important for **tertiary** and **quaternary structure** stability. - They do not form the backbone of the protein's primary sequence.

Question 8: Maldigestion of protein and fat is manifested in chronic pancreatitis only if the damage to pancreatic tissue exceeds?

A. 30%
B. 50%
C. 90% (Correct Answer)
D. 75%

Explanation: ***90%*** - **Maldigestion** of protein and fat in chronic pancreatitis typically occurs when there is extensive damage to the pancreatic tissue, specifically affecting more than **90%** of its functional capacity. - This threshold is critical because the pancreas has a significant reserve capacity for enzyme production, meaning a large portion must be damaged before **exocrine insufficiency** becomes clinically apparent. *30%* - Damage to only **30%** of pancreatic tissue is generally below the threshold for significant clinical manifestations of maldigestion. - The remaining **70%** of functional tissue can still adequately produce digestive enzymes to prevent widespread nutrient malabsorption. *50%* - While **50%** damage is substantial, it usually does not lead to overt clinical symptoms of maldigestion, particularly fat malabsorption (**steatorrhea**). - The body's compensatory mechanisms and the remaining functional pancreatic mass can still maintain relatively normal digestion at this stage. *75%* - Although **75%** damage represents significant pancreatic loss, it often does not fully manifest as severe maldigestion of protein and fat. - Significant **steatorrhea** and **protein malabsorption** typically require an even greater reduction in exocrine function.

Question 9: Western blot is used for:

A. RNA
B. Maternal DNA
C. DNA
D. Proteins (Correct Answer)

Explanation: ***Proteins*** - **Western blot** (also known as protein immunoblot) is a widely used analytical technique in molecular biology and immunogenetics to **detect specific proteins** in a given sample. - It involves separating proteins by size using gel electrophoresis, transferring them to a membrane, and then detecting the protein of interest using specific **antibodies**. *RNA* - The technique used to detect RNA is called **Northern blot**, which involves separating RNA fragments by size and then detecting specific RNA sequences using nucleic acid probes. - While both Northern and Western blots involve electrophoresis and transfer to a membrane, the target molecule is different. *Maternal DNA* - Detection of specific DNA sequences, including maternal DNA or fetal DNA, is typically performed using techniques like **Southern blot** or, more commonly now, **PCR-based methods** and **next-generation sequencing**. - Maternal DNA itself is not the specific target of a Western blot; proteins derived from any source of DNA, maternal or otherwise, would be the target. *DNA* - The technique primarily used for the detection of specific DNA sequences is **Southern blot**, which involves separating DNA fragments by size and using labeled probes to identify target sequences. - Western blot is fundamentally designed for protein analysis, relying on antibody-antigen recognition rather than DNA hybridization.

Question 10: Which of the following can be absorbed without being broken down, especially in infants?

A. a-Dextrins
B. Protein (Correct Answer)
C. Sucrose
D. Triglycerides

Explanation: ***Protein*** - In infants, particularly during the neonatal period, the intestinal epithelium exhibits increased **permeability** allowing for the absorption of intact proteins. - This phenomenon is crucial for the passive transfer of **maternal antibodies** (immunoglobulins) present in breast milk, providing temporary immunity to the infant. - This mechanism of intact protein absorption is known as **pinocytosis** and is especially prominent in the first few days of life. *a-Dextrins* - These are oligosaccharides derived from starch and require further enzymatic breakdown by **maltase-glucoamylase** before they can be absorbed as monosaccharides. - They cannot be absorbed intact, as their molecular size is too large to pass through the intestinal epithelial cells directly. *Sucrose* - Sucrose is a disaccharide that must be hydrolyzed into its constituent monosaccharides, **glucose and fructose**, by the enzyme **sucrase** in the brush border before absorption. - Intact sucrose molecules are too large to be absorbed across the intestinal wall. *Triglycerides* - Triglycerides are complex lipids that are first emulsified by bile salts and then hydrolyzed into **monoglycerides and free fatty acids** by pancreatic lipase. - These smaller components are then absorbed and re-esterified within the intestinal cells, rather than being absorbed as intact triglycerides.

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