Proteins and Amino Acids: Analysis, digestion, physico-chemical properties MCQs With Answer

Introduction

This collection of MCQs on Proteins and Amino Acids: Analysis, Digestion, Physico‑chemical Properties is tailored for M.Pharm students preparing for MPA 104T Food Analysis. The questions focus on core analytical techniques (Kjeldahl, Biuret, Lowry, Bradford, HPLC, electrophoresis, mass spectrometry), enzymatic digestion pathways in the gastrointestinal tract, amino acid chemistry (pKa, isoelectric point, side‑chain reactivity), and physical behavior of proteins (folding, denaturation, solubility, buffering). Each item tests conceptual understanding and applied knowledge relevant to laboratory practice, method selection, interferences, and interpretation of results. Use these MCQs for revision, classroom quizzes, and exam preparation to strengthen both theory and practical problem solving.

Q1. Which method provides a direct quantitative measure of total nitrogen content in a food protein sample and is commonly used to estimate crude protein?

• Kjeldahl method
• Bradford assay
• Biuret test
• SDS-PAGE

Correct Answer: Kjeldahl method

Q2. Which reagent is the basis of the Bradford protein assay that binds primarily to basic and aromatic amino acid residues, producing a blue shift?

• Coomassie Brilliant Blue G-250
• Folin–Ciocalteu reagent
• Copper sulfate (Biuret reagent)
• Ninhydrin

Correct Answer: Coomassie Brilliant Blue G-250

Q3. In SDS-PAGE, which property of proteins is primarily equalized by SDS so that separation is based on molecular weight?

• Charge-to-mass ratio
• Isoelectric point
• Hydrophobic surface area
• Native conformation

Correct Answer: Charge-to-mass ratio

Q4. Which technique is most appropriate for determining the sequence of N‑terminal amino acids in a small purified peptide?

• Edman degradation
• Bradford assay
• MALDI-TOF mass spectrometry without fragmentation
• Kjeldahl digestion

Correct Answer: Edman degradation

Q5. Which amino acid side chain has an ionizable pKa near physiological pH and contributes significantly to protein buffering around pH 6–8?

• Histidine imidazole
• Alanine methyl
• Phenylalanine aromatic ring
• Valine isopropyl

Correct Answer: Histidine imidazole

Q6. Which assay relies on reduction of the Folin–Ciocalteu reagent by tyrosine and tryptophan residues and is more sensitive than the Biuret method?

• Lowry assay
• Kjeldahl method
• Bradford assay
• Ninhydrin assay

Correct Answer: Lowry assay

Q7. Which enzyme initiates protein digestion in the stomach by cleaving peptide bonds next to aromatic and acidic residues at low pH?

• Pepsin
• Trypsin
• Chymotrypsin
• Carboxypeptidase A

Correct Answer: Pepsin

Q8. During amino acid analysis by HPLC, which pre‑column derivatization reagent is commonly used to allow UV or fluorescence detection?

• OPA (o-phthalaldehyde) combined with a thiol
• SDS
• Folin–Ciocalteu reagent
• Kjeldahl catalyst

Correct Answer: OPA (o-phthalaldehyde) combined with a thiol

Q9. The isoelectric point (pI) of a protein is defined as the pH at which the protein has:

• Net zero electrical charge
• Maximum solubility
• Maximum net positive charge
• Maximum net negative charge

Correct Answer: Net zero electrical charge

Q10. Which peptide bond cleavage specificity is characteristic of trypsin?

• Cleaves at the carboxyl side of lysine and arginine residues
• Cleaves at the carboxyl side of aromatic residues like phenylalanine
• Cleaves only at proline residues
• Catalyzes only N-terminal amino acid removal

Correct Answer: Cleaves at the carboxyl side of lysine and arginine residues

Q11. Which phenomenon describes irreversible loss of native structure and biological activity of a protein upon heating or chemical treatment?

• Denaturation
• Renaturation
• Fractionation
• Glycation

Correct Answer: Denaturation

Q12. Which measurement approach distinguishes proteins based on their mass-to-charge ratios and is useful for intact mass determination and peptide identification?

• Mass spectrometry (e.g., MALDI-TOF, ESI-MS)
• Biuret test
• Isoelectric focusing by 2D-PAGE only
• Bradford colorimetric assay

Correct Answer: Mass spectrometry (e.g., MALDI-TOF, ESI-MS)

Q13. Which reagent reacts with free amino groups to produce a colored product and is commonly used in thin-layer detection of amino acids?

• Ninhydrin
• Biuret reagent
• Bromophenol blue
• Silver nitrate

Correct Answer: Ninhydrin

Q14. What is a key limitation of the Kjeldahl method when estimating protein in food samples?

• It measures total nitrogen, including non‑protein nitrogen, leading to overestimation
• It cannot detect sulfur-containing amino acids
• It requires fluorescent derivatization
• It directly sequences peptides

Correct Answer: It measures total nitrogen, including non‑protein nitrogen, leading to overestimation

Q15. Which structural level of protein organization is stabilized mainly by hydrogen bonds between backbone amide and carbonyl groups?

• Secondary structure (alpha helices and beta sheets)
• Primary structure
• Quaternary structure
• Post-translational modifications

Correct Answer: Secondary structure (alpha helices and beta sheets)

Q16. Which buffer property of proteins allows them to resist pH changes near physiological pH?

• Presence of ionizable side chains with pKa values near the pH of interest
• High molecular weight
• Hydrophobic core packing
• Disulfide bond formation

Correct Answer: Presence of ionizable side chains with pKa values near the pH of interest

Q17. In reverse-phase HPLC used for peptide separation, what is the predominant basis for retention time differences?

• Hydrophobicity of peptides
• Ionic charge only
• Molecular weight exclusively
• Optical rotation

Correct Answer: Hydrophobicity of peptides

Q18. Which analytical method separates proteins based on their isoelectric points using a pH gradient?

• Isoelectric focusing (IEF)
• Size-exclusion chromatography
• SDS-PAGE
• Thin-layer chromatography

Correct Answer: Isoelectric focusing (IEF)

Q19. Which amino acid undergoes oxidative deamination in metabolic pathways and plays a central role in nitrogen transfer through transamination reactions?

• Glutamate
• Glycine
• Proline
• Cysteine

Correct Answer: Glutamate

Q20. When assessing protein solubility as a function of pH for formulation studies, solubility typically reaches a minimum at which condition?

• At the protein’s isoelectric point
• At extremely high pH regardless of pI
• At very low ionic strength only
• When protein is fully denatured by heat

Correct Answer: At the protein’s isoelectric point

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