Part 3 — Proteins: Classification, Properties, Tests & Functional Importance (Q51–75)

Q51.

Proteins are classified into simple, conjugated, and derived proteins. Which is a conjugated protein?
A. Hemoglobin ✅
B. Albumin
C. Keratin
D. Collagen

Explanation:

A. Hemoglobin (Correct): Has a prosthetic group (heme) + globin protein.
B. Albumin: Simple protein.
C. Keratin: Fibrous simple protein.
D. Collagen: Fibrous simple protein.

Q52.

Which is a fibrous protein?
A. Hemoglobin
B. Insulin
C. Collagen ✅
D. Amylase

Explanation:

A. Hemoglobin: Globular protein.
B. Insulin: Globular hormone.
C. Collagen (Correct): Fibrous protein providing structural support.
D. Amylase: Globular enzyme.

Q53.

Globular proteins are characterized by:
A. Insolubility in water
B. Structural roles only
C. Compact, folded, water-soluble form ✅
D. Presence only in animals

Explanation:

A. Insolubility: Fibrous proteins.
B. Structural roles only: Fibrous proteins.
**C. Correct — globular proteins (enzymes, hormones) are compact and soluble.
**D. Only in animals: False, globular proteins exist in all organisms.

Q54.

Which protein is a storage protein?
A. Hemoglobin
B. Ferritin ✅
C. Collagen
D. Actin

Explanation:

A. Hemoglobin: Transport, not storage.
B. Ferritin (Correct): Stores iron in liver and spleen.
C. Collagen: Structural.
D. Actin: Contractile.

Q55.

Which protein type functions as a biological catalyst?
A. Enzyme ✅
B. Hormone
C. Immunoglobulin
D. Transport protein

Explanation:

A. Enzyme (Correct): Catalyze biochemical reactions.
B. Hormone: Regulatory signaling proteins.
C. Immunoglobulin: Defense proteins.
**D. Transport proteins: move molecules.

Q56.

Which property is common to most proteins?
A. Amphoteric nature ✅
B. Only acidic in solution
C. Only basic in solution
D. Insoluble in water always

Explanation:

A. Amphoteric (Correct): Amino acids/proteins have both acidic (COOH) and basic (NH₂) groups → act as buffers.
**B. Acidic only: false.
**C. Basic only: false.
**D. Insoluble always: fibrous proteins are insoluble, but globular proteins are soluble.

Q57.

Which test detects peptide bonds in proteins?
A. Biuret test ✅
B. Ninhydrin test
C. Molisch test
D. Benedict’s test

Explanation:

A. Biuret (Correct): Detects peptide linkages (violet color with Cu²⁺).
**B. Ninhydrin: detects free amino acids.
**C. Molisch: general carbohydrate test.
**D. Benedict’s: reducing sugars.

Q58.

Which reagent gives a purple color with proteins in Biuret test?
A. Ninhydrin
B. Copper sulfate in alkaline medium ✅
C. Iodine solution
D. Sodium hydroxide only

Explanation:

**A. Ninhydrin: amino acids.
**B. Correct — Cu²⁺ ions in alkaline medium react with peptide bonds → purple complex.
**C. Iodine: starch test.
**D. NaOH alone: no color.

Q59.

Which test is specific for detecting the amino acid tyrosine?
A. Biuret
B. Xanthoproteic test ✅
C. Molisch
D. Benedict’s

Explanation:

**A. Biuret: peptide bonds.
B. Xanthoproteic (Correct): Tests aromatic amino acids (tyrosine, tryptophan, phenylalanine) → yellow color with HNO₃.
**C. Molisch: carbohydrates.
**D. Benedict’s: reducing sugars.

Q60.

The simplest amino acid is:
A. Alanine
B. Glycine ✅
C. Serine
D. Valine

Explanation:

A. Alanine: Has a –CH₃ side chain.
B. Glycine (Correct): R group is –H → simplest, achiral.
C. Serine: Has hydroxyl group.
D. Valine: Branched chain amino acid.

Q61.

Which amino acid can form disulfide bonds?
A. Cysteine ✅
B. Glycine
C. Serine
D. Lysine

Explanation:

A. Cysteine (Correct): –SH group can form disulfide bonds (–S–S–).
**B. Glycine: no –SH group.
**C. Serine: –OH group, not disulfide.
**D. Lysine: –NH₂ group.

Q62.

Which protein acts as a structural protein in connective tissue?
A. Hemoglobin
B. Collagen ✅
C. Insulin
D. Albumin

Explanation:

**A. Hemoglobin: oxygen transport.
B. Collagen (Correct): Major fibrous protein in connective tissue.
**C. Insulin: hormone.
**D. Albumin: plasma protein.

Q63.

Which property of proteins allows them to act as buffers?
A. Hydrophobicity
B. Amphoteric nature ✅
C. Denaturation
D. Solubility

Explanation:

**A. Hydrophobicity: related to folding.
B. Amphoteric (Correct): Contain both acidic and basic groups → resist pH change.
**C. Denaturation: loss of structure, not buffering.
**D. Solubility: unrelated to buffering.

Q64.

Which test identifies sulfur-containing amino acids?
A. Lead acetate test ✅
B. Benedict’s test
C. Molisch test
D. Iodine test

Explanation:

A. Lead acetate (Correct): Detects cysteine/cystine by black precipitate of PbS.
**B. Benedict’s: sugars.
**C. Molisch: carbohydrates.
**D. Iodine: starch.

Q65.

Which protein is abundant in hair, nails, and skin?
A. Collagen
B. Keratin ✅
C. Elastin
D. Actin

Explanation:

**A. Collagen: connective tissues.
B. Keratin (Correct): Structural protein in hair/nails.
**C. Elastin: elastic tissues.
**D. Actin: contractile protein.

Q66.

Which class of proteins includes antibodies?
A. Fibrous proteins
B. Globular proteins ✅
C. Conjugated proteins
D. Derived proteins

Explanation:

**A. Fibrous: structural (keratin, collagen).
B. Globular (Correct): Antibodies (immunoglobulins) are soluble globular proteins.
**C. Conjugated: contain prosthetic groups.
**D. Derived: from denatured proteins.

Q67.

Which amino acid is an imino acid?
A. Proline ✅
B. Glycine
C. Alanine
D. Valine

Explanation:

A. Proline (Correct): Has a secondary amine (imino) group due to cyclic structure.
B–D. All are standard amino acids but not imino acids.

Q68.

Which test confirms the presence of proteins in food samples?
A. Benedict’s
B. Iodine
C. Biuret ✅
D. Molisch

Explanation:

**A. Benedict’s: sugars.
**B. Iodine: starch.
C. Biuret (Correct): Detects peptide bonds → violet color.
**D. Molisch: carbohydrates.

Q69.

The quaternary structure of a protein refers to:
A. Amino acid sequence
B. Alpha helices and beta sheets
C. 3D folding of one polypeptide
D. Association of multiple polypeptide chains ✅

Explanation:

**A. Primary = sequence.
**B. Secondary = helices/sheets.
**C. Tertiary = folding of one chain.
**D. Correct — quaternary = arrangement of subunits (e.g., hemoglobin).

Q70.

Which amino acid is neutral and nonpolar?
A. Lysine
B. Glutamic acid
C. Valine ✅
D. Serine

Explanation:

**A. Lysine: basic.
**B. Glutamic acid: acidic.
C. Valine (Correct): Neutral, hydrophobic side chain.
**D. Serine: polar.

Q71.

What is the isoelectric point (pI) of an amino acid?
A. pH where amino acid carries no net charge ✅
B. pH where protein denatures
C. pH of maximum enzyme activity
D. pH where amino acid is insoluble

Explanation:

**A. Correct — at pI, positive and negative charges balance.
**B. Denaturation unrelated.
**C. Enzyme activity depends on optimum pH, not pI.
**D. Insolubility not necessarily tied to pI.

Q72.

Which test identifies tryptophan specifically?
A. Millon’s test
B. Hopkins-Cole test ✅
C. Benedict’s test
D. Biuret test

Explanation:

**A. Millon’s: tyrosine.
B. Hopkins-Cole (Correct): Detects tryptophan → purple color with glyoxylic acid + H₂SO₄.
**C. Benedict’s: sugars.
**D. Biuret: peptide bonds.

Q73.

Which protein acts as an oxygen reserve in muscles?
A. Myosin
B. Myoglobin ✅
C. Hemoglobin
D. Actin

Explanation:

**A. Myosin: motor protein.
B. Myoglobin (Correct): Stores oxygen in muscles.
**C. Hemoglobin: transports oxygen in blood.
**D. Actin: cytoskeletal/motor protein.

Q74.

Which protein regulates blood glucose by acting as a hormone?
A. Insulin ✅
B. Glucagon
C. Collagen
D. Albumin

Explanation:

A. Insulin (Correct): Peptide hormone lowers blood glucose.
**B. Glucagon: also regulates but raises glucose (not a protein-focused Q).
**C. Collagen: structural.
**D. Albumin: plasma transport protein.

Q75.

Which protein test uses Millon’s reagent (mercuric nitrate in HNO₃)?
A. For cysteine
B. For tryptophan
C. For tyrosine ✅
D. For glycine

Explanation:

**A. Cysteine: lead acetate test.
**B. Tryptophan: Hopkins-Cole test.
C. Tyrosine (Correct): Phenolic group reacts with Millon’s reagent → red color.
**D. Glycine: no specific test here.

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Part 3 — Proteins: Classification, Properties, Tests & Functional Importance (Q51–75)