Amino Acids: Structure and Properties — MCQs
Which amino acid among the following has significant UV absorption at 280 nm used in protein quantification?
The pH of body fluids is stabilized by buffer systems. Which of the following compounds is the most effective buffer at physiologic pH?
Which of the following amino acids has maximum buffering capacity at pH 7?
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?
Amino acid with its pKa value within physiological range is:
Which one of the following antibiotics inhibits transpeptidation in bacterial cell wall synthesis?
Which of the following amino acids is suitably accommodated within the first turn of an alpha helix?
If a sequence of 4 nucleotides codes for 1 amino acid, how many amino acids can be theoretically formed?
Which of the following is a non-essential amino acid?
Which type of bond is primarily responsible for the primary structure of a protein?
Amino Acids: Structure and Properties Indian Medical PG Practice Questions and MCQs
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
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
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
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.
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
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.
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)
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.
Question 5: Amino acid with its pKa value within physiological range is:
- A. Histidine (Correct Answer)
- B. Arginine
- C. Glycine
- D. Lysine
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.
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
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.
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
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**.
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)
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.
Question 9: Which of the following is a non-essential amino acid?
- A. Tyrosine (Correct Answer)
- B. Phenylalanine
- C. Lysine
- D. Threonine
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.
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
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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