Amino Acids: Structure and Properties Indian Medical PG Practice Questions and MCQs
Practice Indian Medical PG questions for Amino Acids: Structure and Properties. These multiple choice questions (MCQs) cover important concepts and help you prepare for your exams.
Amino Acids: Structure and Properties Indian Medical PG Question 1: Which amino acid among the following has significant UV absorption at 280 nm used in protein quantification?
- A. Tyrosine (Correct Answer)
- B. Alanine
- C. Histidine
- D. Arginine
Amino Acids: Structure and Properties Explanation: ***Correct Option: Tyrosine***
- Tyrosine contains a **phenol functional group** (aromatic ring with hydroxyl group), giving it **significant UV absorption at 280 nm** (specifically ~274 nm)
- Along with **tryptophan** and **phenylalanine**, tyrosine is one of the three aromatic amino acids used for **protein quantification via UV spectroscopy**
- The aromatic side chain with conjugated double bonds enables strong UV light absorption
*Incorrect Option: Alanine*
- Alanine has a **methyl group** as its side chain (non-polar, aliphatic)
- **Lacks aromatic rings** or conjugated systems
- Does **not absorb UV light** at 280 nm
*Incorrect Option: Histidine*
- Histidine has an **imidazole ring** (heterocyclic aromatic) in its side chain
- While technically aromatic, it has **minimal UV absorption at 280 nm** (weak absorption around 210-230 nm)
- **Not used for protein quantification** at 280 nm due to insignificant absorption at this wavelength
*Incorrect Option: Arginine*
- Arginine contains a **guanidinium group** (highly basic, polar)
- **Non-aromatic structure** without conjugated double bonds
- Does **not exhibit UV absorption** at wavelengths used for protein analysis
Amino Acids: Structure and Properties Indian Medical PG Question 2: The pH of body fluids is stabilized by buffer systems. Which of the following compounds is the most effective buffer at physiologic pH?
- A. NH4OH, pKa = 9.24
- B. Na2HPO4, pKa = 12.32
- C. NaH2PO4, pKa = 7.21 (Correct Answer)
- D. CH3CO2H, pKa = 4.75
Amino Acids: Structure and Properties Explanation: ***NaH2PO4, pKa = 7.21***
- A buffer's maximum effectiveness is typically within 1 pH unit of its **pKa value**.
- With a **pKa of 7.21**, the H2PO4⁻/HPO4²⁻ buffer system (phosphate buffer) is optimally positioned to buffer fluctuations around the physiologic pH of **7.35-7.45**.
- This makes the phosphate buffer system highly effective in intracellular and urinary pH regulation.
*NH4OH, pKa = 9.24*
- This compound is a **weak base** with a pKa of 9.24, meaning it would be effective at a pH much higher than the physiologic range.
- Its buffering capacity would be minimal at **pH 7.4**, as the system would be predominantly in one form, reducing its ability to resist pH changes.
*Na2HPO4, pKa = 12.32*
- This represents the **second dissociation** of phosphoric acid (HPO4²⁻ ⇌ PO4³⁻ + H⁺) with a very high **pKa of 12.32**.
- This dissociation occurs at extremely alkaline pH levels, far above the physiological range.
- At physiologic pH, this equilibrium would be almost entirely shifted to HPO4²⁻, providing no buffering capacity.
*CH3CO2H, pKa = 4.75*
- **Acetic acid** has a pKa of 4.75, making it an effective buffer in the acidic range (around pH 3.75-5.75).
- It would be almost entirely dissociated at **physiologic pH**, offering very little buffering capacity against pH changes in body fluids.
Amino Acids: Structure and Properties Indian Medical PG Question 3: Which of the following amino acids has maximum buffering capacity at pH 7?
- A. Arginine
- B. Lysine
- C. Histidine (Correct Answer)
- D. Glycine
Amino Acids: Structure and Properties Explanation: ***Histidine***
- Histidine has a side chain with a **pKa** of approximately **6.0**, which is close to physiological pH (7.4).
- This proximity allows histidine to effectively **donate and accept protons** at pH 7, thus providing significant buffering capacity.
*Arginine*
- Arginine has a side chain with a very **high pKa** of approximately 12.5, making it a strong base.
- It would be ineffective as a buffer at pH 7 because it would exist almost entirely in its **protonated form**.
*Lysine*
- Lysine has a side chain with a **pKa** of approximately 10.5, making it a strong base.
- At pH 7, it would also be almost completely **protonated** and therefore have very limited buffering capacity.
*Glycine*
- Glycine is the simplest amino acid with **no ionizable side chain** (only a hydrogen atom as its R group).
- Its buffering capacity at pH 7 relies solely on its **amino and carboxyl groups**, which have pKa values far from 7 (around 2.3 and 9.6, respectively), making it a poor buffer in this range.
Amino Acids: Structure and Properties Indian Medical PG Question 4: A 32-year-old male is on a weight-maintenance diet, so he does not want to lose or gain any weight. Which amino acid must be present in the diet to prevent the patient from going into a negative nitrogen balance?
- A. Alanine
- B. Arginine
- C. Glycine
- D. Threonine (Correct Answer)
Amino Acids: Structure and Properties Explanation: ***Threonine***
- **Threonine** is an **essential amino acid**, meaning the body cannot synthesize it and it must be obtained from the diet.
- To maintain a **neutral nitrogen balance** and prevent a **negative nitrogen balance** (loss of body protein), all essential amino acids, including threonine, must be supplied in adequate amounts.
*Alanine*
- **Alanine** is a **non-essential amino acid**, which means the body can synthesize it from other compounds; therefore, its absence from the diet would not directly cause a negative nitrogen balance.
- It plays a significant role in **gluconeogenesis** and the **glucose-alanine cycle**.
*Arginine*
- **Arginine** is considered a **conditionally essential amino acid**, meaning it can be synthesized by the body, but sometimes not in sufficient amounts to meet needs (e.g., during rapid growth, illness, or trauma).
- Under normal weight-maintenance conditions, the body can typically synthesize enough arginine.
*Glycine*
- **Glycine** is a **non-essential amino acid** and is the smallest amino acid, often easily synthesized by the body.
- Its presence in the diet, while important, is not critical for preventing negative nitrogen balance because the body can produce it.
Amino Acids: Structure and Properties Indian Medical PG Question 5: Amino acid with its pKa value within physiological range is:
- A. Histidine (Correct Answer)
- B. Arginine
- C. Glycine
- D. Lysine
Amino Acids: Structure and Properties Explanation: ***Histidine***
- Histidine's side chain has a **pKa of 6.0**, which is very close to physiological pH (7.4), allowing it to act as both a **proton donor and acceptor** at physiological conditions.
- This property makes histidine crucial for **enzyme catalysis** and maintaining **pH buffering** in various biological systems.
*Arginine*
- Arginine has a **pKa of 12.5**, which is significantly higher than physiological pH, meaning its side chain is almost always **protonated and positively charged** at physiological conditions.
- Its high pKa makes it a strong base and less likely to buffer effectively within the physiological range.
*Glycine*
- Glycine is a **non-polar amino acid** with no ionizable side chain; its pKa values are for the **alpha-carboxyl** (around 2.34) and **alpha-amino** (around 9.60) groups.
- While these groups are ionized at physiological pH, they are part of the peptide backbone in proteins and do not provide buffering capacity within the physiological range.
*Lysine*
- Lysine has a **pKa of 10.5**, which is also significantly higher than physiological pH, meaning its side chain is predominantly **protonated and positively charged** at this pH.
- Like arginine, its high pKa makes it a strong base that does not readily buffer within the physiological pH range.
Amino Acids: Structure and Properties Indian Medical PG Question 6: Which one of the following antibiotics inhibits transpeptidation in bacterial cell wall synthesis?
- A. Penicillin (Correct Answer)
- B. Chloramphenicol
- C. Amphotericin
- D. Vancomycin
Amino Acids: Structure and Properties Explanation: ***Penicillin***
- Penicillin is a **beta-lactam antibiotic** that targets bacterial cell wall synthesis by inhibiting the enzyme **transpeptidase** (penicillin-binding proteins).
- This inhibition prevents the cross-linking of **peptidoglycan strands**, leading to a weakened cell wall and subsequent bacterial lysis.
*Chloramphenicol*
- **Chloramphenicol** inhibits bacterial protein synthesis by binding to the **50S ribosomal subunit**, thereby preventing peptide bond formation (peptidyl transferase activity).
- It does not act on the bacterial cell wall.
*Vancomycin*
- **Vancomycin** is a **glycopeptide antibiotic** that inhibits bacterial cell wall synthesis by binding to the **D-Ala-D-Ala terminus** of peptidoglycan precursors.
- It prevents the **transglycosylation and transpeptidation steps** by blocking substrate access, but it does not directly inhibit the transpeptidase enzyme itself like beta-lactams do.
- Its mechanism is distinct from penicillin's direct enzyme inhibition.
*Amphotericin*
- **Amphotericin B** is an **antifungal agent** that targets the fungal cell membrane by binding to **ergosterol**, forming pores that disrupt membrane integrity.
- It has no activity against bacterial cell wall synthesis.
Amino Acids: Structure and Properties Indian Medical PG Question 7: Which of the following amino acids is suitably accommodated within the first turn of an alpha helix?
- A. Tyrosine
- B. Glycine
- C. Alanine (Correct Answer)
- D. Aspartic acid
Amino Acids: Structure and Properties Explanation: ***Alanine***
- Alanine's **small, nonpolar side chain** (-CH3) makes it ideal for alpha-helix formation as it minimizes steric hindrance and fits well within the helix's compact structure.
- Its intrinsic helical propensity is among the highest, promoting the formation of **hydrogen bonds** that stabilize the alpha helix.
*Aspartic acid*
- Aspartic acid has a **negatively charged side chain** (-CH2COO⁻) that can cause electrostatic repulsion within the helix, making it less favorable, especially at physiological pH.
- The charge can also interfere with the formation of the crucial **hydrogen bonds** in the backbone, destabilizing the helix.
*Tyrosine*
- Tyrosine possesses a **large, bulky aromatic side chain** that can create significant steric hindrance within the tightly packed structure of an alpha helix.
- The **hydroxyl group** on its side chain can potentially form hydrogen bonds, but its overall size and rigidity disfavor its inclusion in the initial turns of a helix.
*Glycine*
- Glycine has the **smallest side chain (a hydrogen atom)**, which gives it too much conformational flexibility, making it a **helix breaker**.
- Its high flexibility allows for many conformations, making it difficult to maintain the rigid helical structure and form stable **hydrogen bonds**.
Amino Acids: Structure and Properties Indian Medical PG Question 8: If a sequence of 4 nucleotides codes for 1 amino acid, how many amino acids can be theoretically formed?
- A. 4
- B. 64
- C. 16
- D. 256 (Correct Answer)
Amino Acids: Structure and Properties Explanation: ***256***
- With **4 distinct nucleotides** and a code sequence of **4 nucleotides** per amino acid, the number of possible unique combinations is calculated as 4^4.
- This results in 4 × 4 × 4 × 4 = **256 theoretically possible amino acids**.
- This is a mathematical combinatorics calculation: with 4 choices at each of 4 positions, total combinations = 4^4 = 256.
*64*
- This number represents the combinations if **3 nucleotides** coded for one amino acid (4^3 = 64), which is the actual case in the **standard genetic code** (triplet codons).
- However, the question specifies a hypothetical sequence of **4 nucleotides** per amino acid, making this option incorrect.
*16*
- This number would be correct if **2 nucleotides** coded for one amino acid (4^2 = 16).
- The problem explicitly states that **4 nucleotides** code for each amino acid in this theoretical scenario.
*4*
- This would only be the case if each **single nucleotide** coded for one amino acid (4^1 = 4).
- Given **4 distinct nucleotides** and a sequence length of 4, the potential for combinations is much higher.
Amino Acids: Structure and Properties Indian Medical PG Question 9: Which of the following is a non-essential amino acid?
- A. Tyrosine (Correct Answer)
- B. Phenylalanine
- C. Lysine
- D. Threonine
Amino Acids: Structure and Properties Explanation: ***Tyrosine***
- **Tyrosine** is considered a **non-essential amino acid** because the human body can synthesize it from the essential amino acid **phenylalanine**.
- This synthesis occurs via the enzyme **phenylalanine hydroxylase**, making its dietary intake not strictly necessary if phenylalanine is available.
*Phenylalanine*
- **Phenylalanine** is an **essential amino acid**, meaning the human body **cannot synthesize it** and it must be obtained through the diet.
- It serves as a precursor for various important molecules, including tyrosine, contributing to neurotransmitter synthesis.
*Lysine*
- **Lysine** is an **essential amino acid** that the human body **cannot synthesize** and must be acquired from dietary sources.
- It plays a crucial role in **protein synthesis**, calcium absorption, and the production of hormones and enzymes.
*Threonine*
- **Threonine** is another example of an **essential amino acid** that the human body is **unable to produce** on its own.
- It is important for the formation of **collagen** and elastin, and contributes to immune function.
Amino Acids: Structure and Properties Indian Medical PG Question 10: 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
Amino Acids: Structure and Properties 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.
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