MCQs on Amino Acids, Proteins, and Plasma Proteins

(GPAT, NIPER, AIIMS Pharmacist, Railway Pharmacist, SSC, ESIC & State Pharmacist Exams)

Quick Revision Notes: Classification and Biological Functions of Amino Acids, Proteins, and Plasma Proteins

Before attempting the MCQs, take a few minutes to revise these important concepts.

Amino Acids

Amino acids are the building blocks of proteins. Each amino acid contains an amino group (–NH₂), a carboxyl group (–COOH), a hydrogen atom, and a variable side chain (R group) attached to the α-carbon.

Classification of Amino Acids

Amino acids can be classified in several ways:

• Based on nutritional requirement: Essential, non-essential, and conditionally essential amino acids.

• Based on side-chain polarity: Non-polar, polar uncharged, acidic (negatively charged), and basic (positively charged).

• Based on metabolic fate: Glucogenic, ketogenic, and both glucogenic and ketogenic amino acids.

• Based on chemical structure: Aliphatic, aromatic, sulphur-containing, hydroxy, acidic, basic, and imino amino acids.

Physiological Roles of Amino Acids

Amino acids are not only used for protein synthesis but also perform many important functions:

• Build and repair body tissues.

• Form enzymes, hormones, antibodies, and neurotransmitters.

• Help in growth, immunity, and wound healing.

• Act as precursors of biologically important compounds such as serotonin, dopamine, nitric oxide, histamine, and thyroid hormones.

• Participate in energy production during fasting and prolonged exercise.

Structure of Proteins

Proteins are long chains of amino acids linked by peptide bonds. Their structure is organized into four levels:

• Primary structure: Sequence of amino acids.

• Secondary structure: α-Helix and β-pleated sheet formed by hydrogen bonding.

• Tertiary structure: Three-dimensional folding of a single polypeptide chain.

• Quaternary structure: Association of two or more polypeptide chains (e.g., hemoglobin).

Functions of Proteins

Proteins perform a wide variety of functions in the body:

• Catalyze biochemical reactions (enzymes).

• Provide structural support (collagen, keratin).

• Transport molecules (hemoglobin, transferrin).

• Regulate body functions (insulin and other hormones).

• Protect against infections (immunoglobulins).

• Enable muscle contraction (actin and myosin).

• Store nutrients and maintain normal growth and repair.

Plasma Proteins

Plasma proteins are mainly synthesized in the liver, except immunoglobulins, which are produced by plasma cells. The three major plasma proteins are:

• Albumin: Maintains plasma oncotic pressure and transports drugs, fatty acids, bilirubin, calcium, and hormones.

• Globulins: Include transport proteins and antibodies (γ-globulins) that provide immunity.

• Fibrinogen: Essential for blood clotting and is converted into fibrin during coagulation.

Quick Memory Tips

• Glycine: Smallest and only achiral amino acid.

• Proline: Contains a secondary amino (imino) group.

• Leucine & Lysine: Only exclusively ketogenic amino acids.

• Albumin: Most abundant plasma protein; maintains oncotic pressure.

• Transferrin: Transports iron.

• Ceruloplasmin: Transports copper.

• Fibrinogen: Blood clotting protein; absent in serum.

• Hemoglobin: Example of a protein with quaternary structure.

• Denaturation: Alters secondary, tertiary, and quaternary structures but generally does not break peptide bonds.

Exam Tip: Competitive exams often test amino acid classification, physiological roles, essential vs. non-essential amino acids, levels of protein structure, and the functions of plasma proteins through conceptual, assertion–reason, and clinical case-based questions. Understanding these concepts will help you solve even tricky MCQs with confidence.

1.Which one of the following amino acids is both ketogenic and glucogenic?

A. Leucine
B. Lysine
C. Isoleucine
D. Alanine

Answer: C. Isoleucine

Explanation: Only leucine and lysine are exclusively ketogenic. Isoleucine is both ketogenic and glucogenic.

2. Which amino acid contains a secondary amino group?

A. Glycine
B. Alanine
C. Proline
D. Lysine

Answer: C. Proline

Explanation: Proline possesses an imino (secondary amino) group, making it structurally unique.

3. The amino acid responsible for the formation of disulfide bonds in proteins is

A. Methionine
B. Cysteine
C. Serine
D. Tyrosine

Answer: B. Cysteine

Explanation: Two cysteine residues form cystine via disulfide bonds, stabilizing protein structure.

4. Which amino acid is the major precursor for the synthesis of nitric oxide (NO)?

A. Histidine
B. Arginine
C. Lysine
D. Glycine

Answer: B. Arginine

Explanation: Nitric oxide synthase converts arginine into nitric oxide and citrulline.

5. The amino acid that lacks a chiral carbon is

A. Alanine
B. Valine
C. Glycine
D. Threonine

Answer: C. Glycine

Explanation: Glycine has two hydrogen atoms attached to the α-carbon and is therefore achiral.

6. Which amino acid is classified as an aromatic amino acid?

A. Valine
B. Methionine
C. Phenylalanine
D. Arginine

Answer: C. Phenylalanine

7. The essential amino acid among the following is

A. Tyrosine
B. Glycine
C. Lysine
D. Alanine

Answer: C. Lysine

8. Which amino acid serves as the principal precursor for serotonin?

A. Tyrosine
B. Histidine
C. Tryptophan
D. Phenylalanine

Answer: C. Tryptophan

9. Phenylalanine is converted into

A. Tyrosine
B. Histidine
C. Methionine
D. Valine

Answer: A. Tyrosine

10. Histamine is synthesized from

A. Histidine
B. Tryptophan
C. Glycine
D. Arginine

Answer: A. Histidine

11. The peptide bond possesses

A. Free rotation
B. Partial double bond character
C. Triple bond character
D. Ionic bond

Answer: B. Partial double bond character

12. The primary structure of proteins refers to

A. Folding of α-helices
B. Amino acid sequence
C. Three-dimensional shape
D. Association of subunits

Answer: B. Amino acid sequence

13. The α-helix is stabilized mainly by

A. Ionic bonds
B. Hydrogen bonds
C. Peptide bonds
D. Hydrophobic bonds

Answer: B. Hydrogen bonds

14. β-Pleated sheets are stabilized by

A. Hydrogen bonds
B. Covalent bonds
C. Ionic bonds
D. Phosphodiester bonds

Answer: A. Hydrogen bonds

15. Disulfide bonds contribute mainly to

A. Primary structure
B. Secondary structure
C. Tertiary structure
D. Genetic structure

Answer: C. Tertiary structure

16. The quaternary structure of proteins is best exemplified by

A. Myoglobin
B. Albumin
C. Hemoglobin
D. Insulin

Answer: C. Hemoglobin

17. Which level of protein structure is absent in myoglobin?

A. Primary
B. Secondary
C. Tertiary
D. Quaternary

Answer: D. Quaternary

18. Denaturation of proteins primarily affects

A. Peptide bonds
B. Primary structure
C. Secondary and tertiary structures
D. Amino acid composition

Answer: C. Secondary and tertiary structures

19. Which protein is rich in α-helical structure?

A. Silk fibroin
B. Keratin
C. Collagen
D. Elastin

Answer: B. Keratin

20.Silk fibroin mainly contains

A. α-Helix
B. β-Pleated sheet
C. Triple helix
D. Random coil

Answer: B. β-Pleated sheet

21. The major transport protein for iron in plasma is

A. Albumin
B. Ceruloplasmin
C. Transferrin
D. Ferritin

Answer: C. Transferrin

22. The oxygen storage protein in skeletal muscle is

A. Hemoglobin
B. Albumin
C. Myoglobin
D. Ferritin

Answer: C. Myoglobin

23. Insulin is classified as

A. Structural protein
B. Hormonal protein
C. Storage protein
D. Transport protein

Answer: B. Hormonal protein

24. The principal structural protein in connective tissue is

A. Elastin
B. Actin
C. Collagen
D. Keratin

Answer: C. Collagen

25. Which protein acts as the body's major intracellular contractile protein?

A. Myosin
B. Albumin
C. Collagen
D. Fibrinogen

Answer: A. Myosin

26. The most abundant plasma protein is

A. Albumin
B. Globulin
C. Fibrinogen
D. Transferrin

Answer: A. Albumin

27. The major function of plasma albumin is

A. Blood clotting
B. Immune defense
C. Maintenance of colloidal osmotic pressure
D. Oxygen transport

Answer: C. Maintenance of colloidal osmotic pressure

28. Which plasma protein is absent in serum?

A. Albumin
B. Globulin
C. Fibrinogen
D. Immunoglobulin

Answer: C. Fibrinogen

29. Fibrinogen is synthesized mainly in

A. Kidney
B. Liver
C. Bone marrow
D. Spleen

Answer: B. Liver

30. The plasma proteins primarily responsible for humoral immunity are

A. Albumin
B. α-Globulins
C. β-Globulins
D. γ-Globulins

Answer: D. γ-Globulins

31. Ceruloplasmin primarily transports

A. Iron
B. Zinc
C. Copper
D. Calcium

Answer: C. Copper

32. Haptoglobin binds

A. Bilirubin
B. Free hemoglobin
C. Iron
D. Copper

Answer: B. Free hemoglobin

33. The plasma protein responsible for transporting thyroid hormones is

A. Albumin only
B. Transferrin
C. Thyroxine-binding globulin
D. Fibrinogen

Answer: C. Thyroxine-binding globulin

34. Which plasma protein acts as an acute-phase reactant?

A. Albumin
B. C-reactive protein
C. Myoglobin
D. Actin

Answer: B. C-reactive protein

35. Edema in severe protein malnutrition primarily results from decreased

A. Hemoglobin
B. Albumin
C. Immunoglobulin
D. Fibrinogen

Answer: B. Albumin

36.

Assertion (A): Albumin contributes significantly to plasma oncotic pressure.

Reason (R): Albumin is the most abundant plasma protein.

A. Both A and R are true, and R is the correct explanation.
B. Both A and R are true, but R is not the correct explanation.
C. A is true, R is false.
D. A is false, R is true.

Answer: A

37.

Assertion (A): Glycine is optically inactive.

Reason (R): Glycine lacks an asymmetric carbon atom.

A. Both A and R are true, and R is the correct explanation.
B. Both are true, but R is not the explanation.
C. A is true, R is false.
D. Both are false.

Answer: A

38.

Assertion (A): Haemoglobin exhibits quaternary structure.

Reason (R): Haemoglobin consists of four polypeptide chains.

A. Both A and R are true, and R is the correct explanation.
B. Both are true, but R is not the explanation.
C. A is true, R is false.
D. Both are false.

Answer: A

Match the Following

39.

• List I List II

• A. Albumin 1. Copper transport

• B. Ceruloplasmin 2. Colloid osmotic pressure

• C. Transferrin 3. Iron transport

• D. Fibrinogen 4. Blood coagulation

A. A-2, B-1, C-3, D-4
B. A-1, B-2, C-3, D-4
C. A-3, B-1, C-2, D-4
D. A-2, B-3, C-1, D-4

Answer: A

Clinical Case-Based MCQ

40. A patient with severe liver cirrhosis develops generalized edema. Laboratory investigations reveal marked hypoalbuminemia, while immunoglobulin levels remain normal. The edema is mainly due to

A. Increased plasma viscosity
B. Reduced plasma oncotic pressure
C. Increased fibrinogen synthesis
D. Increased globulin concentration

Answer: B. Reduced plasma oncotic pressure

Explanation: Albumin is the major determinant of plasma colloid osmotic (oncotic) pressure. A decrease in albumin reduces plasma oncotic pressure, leading to fluid movement into the interstitial space and causing edema.

Frequently Asked in GPAT/NIPER & Pharmacist Exams)

• Exclusively ketogenic amino acids: Leucine, Lysine.

• Achiral amino acid: Glycine.

• Secondary amino acid: Proline.

• Disulphide bond formation: Cysteine.

• Precursor of nitric oxide: Arginine.

• Precursor of serotonin and melatonin: Tryptophan.

• Precursor of catecholamines: Tyrosine.

• Most abundant plasma protein: Albumin.

• Plasma protein absent in serum: Fibrinogen.

• Major plasma iron transporter: Transferrin.

• Major copper transporter: Ceruloplasmin.

• Free hemoglobin-binding protein: Haptoglobin.

• Acute-phase reactant: C-reactive protein (CRP).

• Protein with quaternary structure: Hemoglobin.

• Myoglobin lacks quaternary structure.

Denaturation disrupts secondary, tertiary, and quaternary structures but generally does not break peptide bonds (primary structure).

41. Which of the following amino acids contains a sulfur atom but does not participate in disulfide bond formation?

A. Cysteine
B. Methionine
C. Cystine
D. Homocysteine

Answer: B. Methionine

Explanation: Methionine contains sulfur in a thioether (-S-) group, which cannot form disulfide bonds. Cysteine forms disulfide bonds by oxidation to cystine.

42. Which amino acid is classified as both glucogenic and ketogenic?

A. Leucine
B. Lysine
C. Isoleucine
D. Alanine

Answer: C. Isoleucine

Explanation: Isoleucine is metabolized to acetyl-CoA (ketogenic) and succinyl-CoA (glucogenic). Leucine and lysine are exclusively ketogenic.

43. The amino acid most important for maintaining acid-base balance in the kidney through ammonia production is

A. Glycine
B. Glutamine
C. Aspartate
D. Valine

Answer: B. Glutamine

Explanation: Renal cells metabolize glutamine to produce ammonia (NH₃), which buffers urinary H⁺ ions and helps maintain acid-base balance.

44. Histamine is synthesized directly from

A. Histidine
B. Tyrosine
C. Tryptophan
D. Phenylalanine

Answer: A. Histidine

Explanation: Histidine undergoes decarboxylation by histidine decarboxylase to form histamine.

45. Which amino acid is responsible for the yellow color observed in the xanthoproteic test?

A. Glycine
B. Tyrosine
C. Alanine
D. Proline

Answer: B. Tyrosine

Explanation: Aromatic amino acids, particularly tyrosine and tryptophan, undergo nitration with concentrated nitric acid producing a yellow color.

46. Which amino acid disrupts α-helix formation due to its rigid cyclic structure?

A. Glycine
B. Valine
C. Proline
D. Serine

Answer: C. Proline

Explanation: Proline lacks the amide hydrogen required for hydrogen bonding and introduces bends in protein chains.

47. The peptide bond is best described as having

A. Complete single bond character
B. Complete double bond character
C. Partial double bond character
D. Ionic bond character

Answer: C. Partial double bond character

Explanation: Resonance gives peptide bonds partial double-bond character, restricting rotation and making proteins more stable.

48. The strongest interaction stabilizing the primary structure of proteins is

A. Hydrogen bond
B. Peptide bond
C. Hydrophobic interaction
D. Ionic interaction

Answer: B. Peptide bond

Explanation: Primary structure is maintained by covalent peptide bonds linking amino acids.

49. Which level of protein structure is directly stabilised by disulphide bonds between cysteine residues?

A. Primary only
B. Secondary only
C. Tertiary
D. Quaternary only

Answer: C. Tertiary

Explanation: Disulfide bonds stabilise tertiary structure and may also contribute to quaternary structure.

50. The oxygen-binding curve of hemoglobin is sigmoidal because of

A. High affinity for oxygen
B. Cooperative binding
C. Presence of iron
D. Presence of globin chains

Answer: B. Cooperative binding

Explanation: Binding of one oxygen molecule increases the affinity of remaining subunits for oxygen.

51. The major plasma protein responsible for maintaining colloidal osmotic pressure is

A. Fibrinogen
B. Albumin
C. Transferrin
D. Globulin

Answer: B. Albumin

Explanation: Albumin contributes about 75–80% of plasma oncotic pressure.

52. Which plasma protein is absent in serum?

A. Albumin
B. Immunoglobulin
C. Fibrinogen
D. Transferrin

Answer: C. Fibrinogen

Explanation: Serum is plasma without clotting factors, particularly fibrinogen.

53. Which plasma protein primarily transports iron?

A. Albumin
B. Ceruloplasmin
C. Transferrin
D. Ferritin

Answer: C. Transferrin

Explanation: Transferrin binds ferric iron (Fe³⁺) and transports it in the blood.

54. Wilson's disease is associated with a deficiency of

A. Albumin
B. Ceruloplasmin
C. Haptoglobin
D. Transferrin

Answer: B. Ceruloplasmin

Explanation: Ceruloplasmin transports copper. Its deficiency is characteristic of Wilson's disease.

55. The plasma protein responsible for binding free hemoglobin released during intravascular hemolysis is

A. Albumin
B. Haptoglobin
C. Fibrinogen
D. α₂-Macroglobulin

Answer: B. Haptoglobin

Explanation: Haptoglobin binds free haemoglobin and prevents renal loss of iron.

56. Which amino acid lacks an asymmetric carbon atom?

A. Alanine
B. Glycine
C. Valine
D. Serine

Answer: B. Glycine

Explanation: Glycine has two hydrogen atoms attached to its α-carbon and is therefore optically inactive.

57. Which amino acid serves as the precursor for nitric oxide?

A. Arginine
B. Lysine
C. Histidine
D. Ornithine

Answer: A. Arginine

Explanation: Nitric oxide synthase converts arginine into nitric oxide and citrulline.

58. Denaturation of proteins generally does not affect

A. Primary structure
B. Secondary structure
C. Tertiary structure
D. Quaternary structure

Answer: A. Primary structure

Explanation: Denaturation disrupts non-covalent interactions while peptide bonds remain intact.

59. Which of the following is a conjugated protein?

A. Albumin
B. Collagen
C. Hemoglobin
D. Elastin

Answer: C. Hemoglobin

Explanation: Haemoglobin contains a non-protein haem prosthetic group, making it a conjugated protein.

60. Which plasma protein is synthesized almost exclusively by plasma cells?

A. Albumin
B. Fibrinogen
C. Immunoglobulin
D. Transferrin

Answer: C. Immunoglobulin

Explanation: Immunoglobulins (antibodies) are synthesized by plasma cells, whereas most other plasma proteins are produced by the liver.

61.

Assertion (A): Albumin is the major transport protein in plasma.

Reason (R): Albumin possesses multiple binding sites for endogenous and exogenous substances.

A. Both A and R are true, and R is the correct explanation.
B. Both A and R are true, but R is not the correct explanation.
C. A is true, R is false.
D. A is false, R is true.

Answer: A

Explanation: Albumin binds bilirubin, fatty acids, calcium, hormones, and many drugs due to its multiple binding sites.

62.

Assertion (A): Collagen is a fibrous protein.

Reason (R): Collagen functions primarily as an enzyme.

A. Both A and R are true.
B. Both A and R are false.
C. A is true, R is false.
D. A is false, R is true.

Answer: C

Explanation: Collagen is a structural fibrous protein and does not function as an enzyme.

Match the Following

• List I List II

• A. Albumin 1. Iron transport

• B. Transferrin 2. Copper transport

• C. Ceruloplasmin 3. Osmotic pressure

• D. Haptoglobin 4. Hemoglobin binding

Options:

A. A-3, B-1, C-2, D-4
B. A-1, B-3, C-2, D-4
C. A-2, B-1, C-3, D-4
D. A-3, B-4, C-1, D-2

Answer: A

Explanation: Albumin maintains oncotic pressure; transferrin transports iron; ceruloplasmin transports copper; haptoglobin binds free haemoglobin.

A patient with severe liver cirrhosis develops generalised oedema despite normal renal function. The most likely biochemical explanation is

A. Increased fibrinogen synthesis
B. Reduced albumin synthesis
C. Increased immunoglobulin production
D. Reduced ceruloplasmin synthesis

Answer: B

Explanation: Liver cirrhosis decreases albumin synthesis, lowering plasma oncotic pressure and causing edema.

65. Which amino acid acts as the precursor for all of the following except one: melanin, catecholamines, and thyroid hormones?

A. Tyrosine
B. Tryptophan
C. Histidine
D. Phenylalanine

Answer: A. Tyrosine

Explanation: Tyrosine is converted into melanin, dopamine, norepinephrine, epinephrine, and thyroid hormones. Phenylalanine must first be converted to tyrosine before entering these pathways.

Dr. Alok Singh