Protein found in a biological system with a unique three dimensional structure and biological activity is call

NCERT Exemplar · Q39

Q. Protein found in a biological system with a unique three dimensional structure and biological activity is called a native protein. When a protein in its native form, is subjected to a physical change like change in temperature or a chemical change like change in pH, denaturation of protein takes place. Explain the cause.

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Concept understanding — Protein Structure Levels

Protein Structure Levels: From a String to a Working Machine

Imagine you have a long string of beads. Each bead is a different colour, and the order of colours is fixed. If you just lay that string on a table, it's a floppy, useless line. But if you could somehow make that string fold itself into a tiny, precise 3D shape — say, a key that fits a specific lock — you'd have something that actually does a job. That's exactly what a protein is.

A protein starts as a long chain of smaller units called amino acids. There are 20 different kinds, each with a unique side chain (the "colour" of the bead). The exact sequence of these amino acids is determined by your DNA. But a protein isn't just a chain — it's a chain that folds into a specific shape, and that shape determines what the protein does. If the shape is wrong, the protein can't work.

The folding happens in stages, and we call these stages the four levels of protein structure.

Level 1: Primary Structure — The Sequence

This is the simplest level: just the linear order of amino acids in the chain, linked by peptide bonds. Think of it as the sentence written in the language of proteins.

Primary structure = the sequence of amino acids from the N-terminus (start) to the C-terminus (end).

Why does this matter? Because the sequence determines everything else. Change one amino acid in a critical spot, and the entire protein can misfold. Example: sickle cell anaemia is caused by a single amino acid swap in haemoglobin — valine replaces glutamic acid at position 6. One bead out of hundreds changes colour, and the whole protein folds wrong.

Level 2: Secondary Structure — Local Folding Patterns

The chain doesn't stay straight. Hydrogen bonds form between the backbone atoms (not the side chains) of nearby amino acids. These bonds cause the chain to twist or fold into regular, repeating patterns.

Two common patterns:

Alpha helix ( $\alpha$ -helix): The chain coils like a spring or a spiral staircase. Hydrogen bonds form between every 4th amino acid, holding the coil tight.
Beta sheet ( $\beta$ -sheet): The chain folds back and forth like a pleated fan. Hydrogen bonds form between adjacent segments, creating a flat, sheet-like structure.

Note

Secondary structure is stabilised entirely by hydrogen bonds between the carbonyl oxygen of one amino acid and the amide hydrogen of another — both part of the peptide backbone. Side chains stick out and don't participate.

These patterns are local — they happen in short stretches of the chain. A single protein can have multiple $\alpha$ -helices and $\beta$ -sheets separated by loops.

Level 3: Tertiary Structure — The Global 3D Shape

Now the whole chain folds into its final, compact, three-dimensional shape. This is where the protein becomes functional. The tertiary structure is stabilised by interactions between the side chains of amino acids that may be far apart in the sequence but come close in space.

What holds it together?

Hydrophobic interactions: Nonpolar side chains cluster together in the protein's interior, away from water.
Hydrogen bonds: Between polar side chains.
Ionic bonds: Between positively and negatively charged side chains.
Disulfide bridges: Covalent bonds between the sulfur atoms of two cysteine amino acids — these are strong and lock parts of the chain together.
Van der Waals forces: Weak attractions between closely packed atoms.

Watch out

A common mistake: thinking tertiary structure is just "more secondary structure." It's not. Secondary structure is local folding; tertiary structure is the global arrangement of the entire chain, including how helices and sheets pack together.

Level 4: Quaternary Structure — Multiple Chains Working Together

Some proteins are made of more than one polypeptide chain. Each chain is a separate subunit, and the quaternary structure describes how these subunits assemble into a functional complex. …

Why this formula?

Protein Structure Levels: Understanding the "Why" Behind the Hierarchy

Protein structure is not defined by a single formula, but by a logical hierarchy of organization. Each level builds on the previous one, and the "formulas" here are really principles of molecular interaction that explain why proteins fold the way they do.

Let's break down each level and the reasoning behind its key features.

1. Primary Structure: The Sequence "Formula"

What it is: The linear sequence of amino acids linked by peptide bonds.

Key "formula":

\text{Protein} = \text{NH}_2\text{-[Amino Acid]}_1\text{-[AA]}_2\text{-...-[AA]}_n\text{-COOH}

Why this holds:

Peptide bond formation is a condensation reaction:

\text{-COOH} + \text{NH}_2\text{-} \rightarrow \text{-CO-NH-} + \text{H}_2\text{O}

This bond is rigid and planar due to resonance (partial double-bond character). This restricts rotation, which directly influences higher-order folding.
The sequence is determined by DNA (genetic code). Every change in sequence can alter the entire structure — this is why a single mutation (e.g., sickle cell anemia: Glu → Val at position 6) can cause disease.

Exam insight: The primary structure is the only level that is covalently determined. All higher levels are non-covalent interactions.

2. Secondary Structure: Local Folding Patterns

Key patterns: $\alpha$ -helix and $\beta$ -pleated sheet.

Why these form — the hydrogen bond "formula":

The $\alpha$ -helix

Hydrogen bonds form between the carbonyl oxygen ( $\text{C=O}$ ) of residue $n$ and the amide hydrogen ( $\text{N-H}$ ) of residue $n+4$ .
Why $n+4$ ? This spacing allows the backbone to coil into a right-handed helix with exactly 3.6 amino acids per turn.
Reasoning: The peptide bond's planar nature forces the backbone into a specific geometry. The $n+4$ pattern maximizes H-bonding while minimizing steric clashes.

The $\beta$ -sheet

Hydrogen bonds form between adjacent strands (either parallel or antiparallel).
Why not $n+4$ ? The backbone is extended (pleated), so H-bonds occur between different segments, not within the same chain.

Key formula (Ramachandran plot):

Only certain backbone dihedral angles ( $\phi, \psi$ ) are allowed:

$\alpha$ -helix: $\phi \approx -57^\circ$ , $\psi \approx -47^\circ$
$\beta$ -sheet: $\phi \approx -130^\circ$ , $\psi \approx +130^\circ$

Why these angles? Steric hindrance — atoms cannot overlap. The Ramachandran plot shows the only regions where no two atoms clash.

3. Tertiary Structure: The 3D Fold

Key "formula": The hydrophobic effect drives folding.

Why this holds:

Water molecules form a cage-like structure around nonpolar (hydrophobic) side chains. This is entropically unfavorable (water loses freedom).
To minimize this, hydrophobic side chains cluster together in the protein's core, away from water.
Result: The protein collapses into a compact globule, with polar/charged residues on the surface.

Supporting interactions (the "glue"):

Interaction	Why it matters
Hydrogen bonds	Between side chains (e.g., Ser–Glu)
Ionic bonds	Between charged groups (e.g., Lys–Asp)
Van der Waals forces	Close packing of atoms
Disulfide bridges	Covalent S–S bonds (only in oxidizing environments)

Why not just one formula? Tertiary structure is unique to each protein — it's the sum of all these interactions, not a single equation.

4. Quaternary Structure: Multiple Subunits

Key "formula":

\text{Functional protein} = \sum_{i=1}^{n} \text{Subunit}_i

Why this holds:

Some proteins need multiple polypeptide chains to function (e.g., hemoglobin: $\alpha_2\beta_2$ ). …

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Denaturation disrupts the non-covalent interactions (hydrogen bonds, ionic bonds, hydrophobic interactions) and disulfide bridges that stabilise a protein’s unique 3D structure, causing loss of biological activity while the primary structure remains intact.

Why a protein has a “native” shape — and why it matters

A protein’s biological function depends entirely on its precise three-dimensional shape. This shape is not random; it is the native conformation — the most stable, lowest-energy arrangement the polypeptide chain can adopt under normal physiological conditions. Think of it like a key: only when it is cut exactly right does it fit the lock. The “cutting” is done by a hierarchy of structural levels:

Primary structure — the linear sequence of amino acids.
Secondary structure — local folding into $\alpha$ -helices and $\beta$ -pleated sheets, stabilised by hydrogen bonds between backbone amide and carbonyl groups.
Tertiary structure — the overall 3D folding of a single polypeptide, held together by side-chain interactions: hydrogen bonds, ionic bonds, hydrophobic packing, van der Waals forces, and sometimes covalent disulfide ( $\ce{-S-S-}$ ) bridges.
Quaternary structure — assembly of multiple polypeptide subunits (not always present).

The native state is a delicate balance. All those non-covalent interactions are individually weak, but collectively they lock the protein into a specific, functional shape. Change the environment — temperature or pH — and you tip that balance.

What denaturation actually does

Denaturation is the unfolding of the protein from its native conformation into a disordered, often random-coil state. The key point: the primary structure (covalent peptide bonds) remains unchanged. Only the higher-order structure is lost.

Temperature increase — Heat adds kinetic energy. Molecules vibrate faster. At some point, the thermal motion overcomes the hydrogen bonds and hydrophobic interactions that hold the secondary and tertiary structure together. The protein “melts” into a tangled, non-functional chain. For most proteins, this happens sharply at a characteristic temperature (like egg white turning opaque when boiled — the albumin denatures).
pH change — Every amino acid side chain has an optimal pH range where it carries the right charge for ionic bonds and hydrogen bonds to form. Move the pH away from the protein’s isoelectric point, and:
- Carboxyl groups ( $\ce{-COOH}$ ) lose a proton to become $\ce{-COO-}$ (or gain one if pH is very low).
- Amino groups ( $\ce{-NH2}$ ) gain a proton to become $\ce{-NH3+}$ (or lose one if pH is very high). …

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🧠 The Core Concept

A native protein has a unique three-dimensional structure (secondary, tertiary, or quaternary) that is essential for its biological activity.

Denaturation = loss of this native structure → loss of biological activity.

✗ Common Mistake #1: Confusing cause with effect

What students write:

"Denaturation happens because the protein loses its biological activity."

Why it's wrong:

This reverses cause and effect. Loss of activity is a result of denaturation, not the cause.

✓ How to avoid:

Always remember the sequence:

Physical/chemical change → disruption of bonds → loss of 3D structure → loss of biological activity

The cause is the disruption of hydrogen bonds, disulfide bridges, ionic bonds, and hydrophobic interactions that maintain the native conformation.

✗ Common Mistake #2: Saying "all bonds break" during denaturation

What students write:

"All bonds in the protein break, including peptide bonds."

Why it's wrong:

Denaturation does not break peptide bonds — those are covalent bonds that hold the primary structure together. Only secondary, tertiary, and quaternary structures are disrupted.

✓ How to avoid:

Make a clear distinction:

Structure level	Bonds affected in denaturation?
Primary (peptide bonds)	✗ No
Secondary (H-bonds)	✓ Yes
Tertiary (H-bonds, ionic, disulfide, hydrophobic)	✓ Yes
Quaternary (same as tertiary, between subunits)	✓ Yes

Key line to remember:

"Denaturation disrupts non-covalent interactions and sometimes disulfide bridges, but never peptide bonds."

✗ Common Mistake #3: Thinking denaturation is always irreversible

What students write:

"Denaturation is always permanent."

Why it's wrong:

Some proteins can renature (regain native structure) if the denaturing agent is removed gently — e.g., ribonuclease (Anfinsen's experiment).

✓ How to avoid:

Use the correct terminology:

Reversible denaturation → possible if only weak bonds are disrupted (e.g., mild heat, pH change)
Irreversible denaturation → when disulfide bonds break or aggregation occurs (e.g., boiling an egg)

Exam tip: If the question asks "Explain the cause," focus on bond disruption, not reversibility — but don't claim it's always irreversible.

✗ Common Mistake #4: Vague or incomplete explanation of "cause"

What students write:

"Change in temperature or pH causes denaturation."

Why it's wrong:

This is just restating the question. You must explain how these changes cause denaturation.

✓ How to avoid:

Give a mechanism:

Temperature increase → provides kinetic energy → breaks hydrogen bonds and hydrophobic interactions → unfolds the protein
pH change → alters ionization of amino acid side chains → disrupts ionic bonds and hydrogen bonds → loss of 3D structure

Example answer structure: …

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Same / Similar Concept — real previous-year questions on the same or a closely similar concept, not this exact question.

Showing the 12 most recent of 17 on this concept.

AP EAPCET 2026Set ap-2026-05-19-FN1 markMCQ
$Q. Choose the correct statements A) The first amino acid in a protein chain is in the left end and termed as C-terminal amino acids. B) Long protein chain folded upon itself like a hollow woolen ball is called tertiary structure. C) In general, only right handed helices are observed in protein. D) In general, polysaccharides will have glycosidic bond formed by dehydration. (A) A, B and C (B) B, C and D (C) A, C and D (D) A, B and D$
›Reveal solutionSolution
Of four claims about protein/polysaccharide structure, only the "first amino acid = C-terminal" claim is wrong (it should be N-terminal); tertiary structure, right-handed helices, and dehydration-formed glycosidic bonds are all correct.

Concept and Intuition

A polypeptide has directionality: one end carries a free –NH₂ group (the N-terminus, conventionally drawn/numbered first, "left" end) and the other a free –COOH group (the C-terminus, "right" end). Beyond primary structure (sequence) and secondary structure (regular local folding like the α-helix, predominantly right-handed in natural proteins), further compact 3-D folding into a globular shape is tertiary structure. Separately, polysaccharides are built by joining monosaccharide units via glycosidic bonds, formed through dehydration (loss of a water molecule per bond), just like peptide bonds in proteins.

Step-by-Step Solution

A: the residue with the free amino group is by convention the FIRST/N-terminal amino acid (drawn on the left) — calling it "C-terminal" is a direct mix-up of the two termini. FALSE.

B: further folding of an already-folded (secondary structure) polypeptide chain into a compact, roughly spherical, hollow-ball-like shape is precisely the description used for tertiary structure. TRUE. …
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AP EAPCET 2025Set eng-2025-05-23-FN1 markMCQ
$\alpha$
›Reveal solutionSolution
This tests the protein structure hierarchy: primary = sequence (constitution), secondary = local folding patterns like $\alpha$ -helix/ $\beta$ -sheet, tertiary = overall 3-D folding. Statement-I is right; Statement-II wrongly calls a secondary-structure feature 'tertiary'.

Concept and Intuition

Proteins are described at four structural levels: primary (linear amino-acid sequence — the actual covalent 'constitution' of the molecule), secondary (regular, repeating local folding patterns held by hydrogen bonds along the peptide backbone — the $\alpha$ -helix and $\beta$ -pleated sheet are the two classic examples), tertiary (the overall three-dimensional folding of the whole chain, stabilised by disulfide bonds, hydrogen bonds, and hydrophobic interactions among side chains), and quaternary (arrangement of multiple polypeptide subunits). Recognising which level a named feature belongs to is the entire test here.

Step-by-Step Solution

Statement-I: the primary structure is defined as the sequence in which amino acids are linked via peptide bonds — this is indeed the molecule's basic constitution (which amino acid, in which order). True. …
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AP EAPCET 2025Set ap-2025-05-19-AN1 markMCQ
$2^\circ$
›Reveal solutionSolution
Denaturation breaks the weak interactions maintaining secondary/tertiary folding, but the covalent peptide-bond backbone (primary structure) survives untouched.

Concept and Intuition

Protein structure is organised in levels: primary (amino-acid sequence, held by strong covalent peptide bonds), secondary (α-helices/β-sheets, held by hydrogen bonds), and tertiary (overall 3-D fold, held by a mix of hydrogen bonds, hydrophobic effects, ionic interactions, and disulfide bridges). Denaturing agents (heat, extreme pH, chaotropic agents) disrupt these weak non-covalent interactions but do not have enough energy to break the strong covalent peptide bonds of the backbone.

Step-by-Step Solution

Denaturation unfolds the protein by breaking the hydrogen bonds/hydrophobic interactions responsible for secondary structure.

It also destroys tertiary structure (loss of the specific 3-D globular shape), since tertiary structure depends on similarly weak interactions (plus occasionally disulfide bonds, which some strong denaturants can also break). …
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AP EAPCET 2025Set ap-2025-05-19-FN1 markMCQ
$Q. Match the following List-I A) Glycosidic bond B) Hollow woollen ball folding C) Enzyme D) Esterbond List-II I) Trypsin II) Bond between phosphate and 5th carbon of sugar III) tertiary structure of protein IV) formed by dehydration (A) A-II, B-I, C-IV, D-III (B) A-IV, B-III, C-I, D-II (C) A-III, B-II, C-I, D-IV (D) A-IV, B-I, C-II, D-III$
›Reveal solutionSolution
Matching each biomolecule term to its correct description gives A-IV, B-III, C-I, D-II — answer (B).

Concept and Intuition

Glycosidic bond: forms between two monosaccharides via a dehydration (condensation) reaction, releasing a water molecule.

The image of a protein folding "like a hollow woollen ball" is a common descriptive analogy for a protein's tertiary structure — the overall 3D folding of a single polypeptide chain.

Trypsin is a well-known digestive protease and is the standard textbook example cited for "enzyme."

The ester bond joining a phosphate group to the 5′-carbon hydroxyl of a sugar (as in nucleotides) is a phosphoester linkage.

Step-by-Step Solution

A) Glycosidic bond → IV (formed by dehydration).

B) "Hollow woollen ball" folding → III (tertiary structure of protein).

C) Enzyme → I (Trypsin, a textbook enzyme example). …
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AP EAPCET 2025Set ap-2025-05-19-FN1 markMCQ
$Q. Choose the correct statements among the following I. In exothermic reactions 'P' is at a lower level than 'S'. II. Enzyme activity can be affected by temperature etc. with alteration of tertiary structure of protein. III. Low temperature denature the protein. IV. Ligases can remove the groups from substrate (A) I, II (B) III, IV (C) II, IV (D) I, III$
›Reveal solutionSolution
Exothermic reactions do place products at lower energy than substrates, and temperature does affect activity via tertiary-structure changes — both true. Low temperature inactivates (not denatures) enzymes, and ligases join (not remove) — both false. Correct set: I, II — option (A).

Concept and Intuition

Enzyme-catalysed reaction energetics: in an exothermic reaction, the products (P) are at a lower free-energy level than the substrates (S), with the difference released as heat — this is basic thermodynamics of catalysed reactions and is independent of the enzyme itself (the enzyme only lowers the activation energy, not the net energy change). Enzyme activity is highly temperature-sensitive because temperature affects the enzyme's three-dimensional (tertiary) structure, which determines the shape of its active site; this is why activity rises with temperature up to an optimum and then falls off as heat begins to disrupt this structure (denaturation) at high temperatures. Crucially, LOW temperature does not denature an enzyme — it merely reduces the kinetic energy of molecules, slowing the reaction reversibly (activity resumes when temperature rises again); denaturation (irreversible loss of structure) is a HIGH-temperature (or extreme pH) phenomenon. Ligases are enzymes that join two molecules together (often using ATP, e.g., DNA ligase sealing nicks) — they do not remove groups from a substrate; that role belongs to hydrolases/lyases.

Step-by-Step Solution

Statement I: In exothermic reactions, product (P) is at a lower energy level than substrate (S) — TRUE, by definition of an exothermic process (energy released, product lower in energy). …
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AP EAPCET 2024Set eng-2024-05-21-FN1 markMCQ
$Q. Which of the following structure of proteins represents its constitution? (A) Secondary structure (B) Quaternary structure (C) Primary structure (D) Tertiary structure$
›Reveal solutionSolution
A protein's constitution — the linear sequence of amino acid residues — is defined as its primary structure.

Concept and Intuition

Protein structure is described at four levels: primary (amino-acid sequence, held by peptide bonds), secondary (local folding like α-helix/β-sheet, via H-bonds), tertiary (overall 3-D fold, via various side-chain interactions), and quaternary (arrangement of multiple polypeptide subunits). "Constitution" refers to which atoms/residues are bonded to which — i.e. connectivity — which is captured entirely by the primary structure; the higher-order structures describe spatial conformation, not constitution.

Step-by-Step Solution

Recall the four levels of protein structure and what each describes.

"Constitution" = the sequence/connectivity of amino acid units — matches the definition of primary structure exactly. …
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AP EAPCET 2023Set ap-2023-05-22-AN1 markMCQ
$Q. Enzymes are denaturated by (A) Low pH (B) High substrate concentration (C) Low temperature (D) heat$
›Reveal solutionSolution
Enzymes lose their catalytic 3-D shape and activity when heated — heat is the classic, universally correct denaturing agent among the given options.

Concept and Intuition

An enzyme's activity depends entirely on its precise tertiary (3-D) structure, especially the shape of its active site. This structure is held together by relatively weak, non-covalent interactions (hydrogen bonds, ionic bonds, hydrophobic interactions) that are easily disrupted by excess thermal energy, causing the protein to unfold and lose its functional shape — this is denaturation.

Step-by-Step Solution

(A) Low pH can denature enzymes only in extreme cases; mild pH shifts merely reduce activity reversibly rather than denaturing outright, so it's not the cleanest universal answer.

(B) High substrate concentration does not denature an enzyme; at most it can cause substrate inhibition, a different, reversible phenomenon.

(C) Low temperature slows enzyme activity (reduces molecular motion/collision frequency) but does not denature the enzyme — activity resumes on warming. …
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AP EAPCET 2023Set eng-2023-05-15-FN1 markMCQ
$\mathrm{H_2N-CH(CH_3)-CO-}$
›Reveal solutionSolution
The structure of Gly-Ala is a dipeptide where glycine (no side chain) is the N‑terminal residue and alanine (methyl side chain) is the C‑terminal residue. The correct option is (B).

Concept and Intuition

Proteins and peptides are built from amino acids linked by peptide (amide) bonds. The name of a dipeptide is written from the N‑terminus (free amino group) to the C‑terminus (free carboxyl group). So “Gly-Ala” means glycine is at the left (N‑terminal) end, and alanine is at the right (C‑terminal) end.

Glycine (Gly) has a hydrogen atom as its side chain — so its α‑carbon has no branch (just –H).

Alanine (Ala) has a methyl group (–CH₃) as its side chain — so its α‑carbon carries a small branch.

Thus, in the correct structure:

The leftmost part (H₂N–) is attached to a carbon with no side chain (glycine).

That carbon is linked via an amide bond (–CO–NH–) to the α‑carbon of alanine, which does have a methyl branch.

The rightmost part is the free carboxyl group (–CO₂H) of alanine.

Step‑by‑Step Reasoning

Identify the residues from the name

“Gly-Ala” tells us the sequence: Glycine first, then Alanine.

Glycine: H₂N–CH₂–COOH (side chain = H)

Alanine: H₂N–CH(CH₃)–COOH (side chain = CH₃)

Form the peptide bond

The carboxyl group of glycine reacts with the amino group of alanine, releasing water. The resulting dipeptide is:

$\mathrm{H_2N-CH_2-C(=O)-NH-CH(CH_3)-CO_2H}$
Notice:

Left side (N‑terminus): H₂N–CH₂– (glycine, no branch)

Middle: amide bond (–CO–NH–)

Right side (C‑terminus): –CH(CH₃)–CO₂H (alanine, methyl branch)

Match with the given options

Option (A): Left carbon has a methyl branch (alanine), right carbon has no branch (glycine) → Ala-Gly, not Gly-Ala.

Option (B): Left carbon has no branch (glycine), right carbon has a methyl branch (alanine) → Gly-Ala. …
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AP EAPCET 2023Set eng-2023-05-17-AN1 markMCQ
$Q. Which of the following are fibrous proteins? Keratin (A) Insulin (B) Myosin (C) Albumin (D) (A) A, B (B) A, C (C) B, D (D) C, D$
›Reveal solutionSolution
Fibrous proteins are structural, elongated molecules; keratin and myosin are fibrous, while insulin and albumin are globular.

Concept and Intuition

Proteins are broadly classed by shape/function: fibrous proteins are long thread-like molecules (often helices/sheets) giving mechanical strength; globular proteins fold compactly and perform dynamic biochemical roles.

Step-by-Step Solution

Keratin (A): structural protein in hair/nails/wool - fibrous.

Insulin (B): small globular hormone regulating blood glucose - globular.

Myosin (C): motor protein of muscle, forms long filaments - fibrous. …
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AP EAPCET 2022Set ap-2022-07-11-FN1 markMCQ
$Q. Hemoglobin in man is made up of (A) 4 Polypeptide chains and 4 haeme molecules (B) 2 Polypeptide chains and 2 haeme molecules (C) 4 Polypeptide chains and one haeme molecules (D) 2 Polypeptide chains and 1 haeme molecule$
›Reveal solutionSolution
This tests the quaternary structure of haemoglobin; the answer is 4 polypeptide chains and 4 haeme molecules.

Concept and Intuition

Haemoglobin is a globular, tetrameric protein. Each of its 4 globin (polypeptide) subunits folds around one haem prosthetic group, whose central Fe2+ ion reversibly binds one O2 molecule. Because oxygen-binding happens per haem group and there are 4 haem groups (one per chain), a single haemoglobin molecule can carry up to 4 O2 molecules — this is also the structural basis of cooperative binding (the sigmoidal O2-dissociation curve).

Step-by-Step Solution

Adult haemoglobin (HbA) = α2β2, i.e., 2 alpha-globin chains + 2 beta-globin chains = 4 polypeptide chains total.

Each chain has exactly one haem group embedded in it — so 4 chains → 4 haem groups.

This directly matches option (A): 4 polypeptide chains and 4 haeme molecules. …
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AP EAPCET 2022Set ap-2022-07-12-AN1 markMCQ
$Q. Which among the following are peptide based hormones?$
(i) Estrogen
(ii) Insulin
(iii) Endorphin
(iv) Androgen (A)
(i) and
(iv) (B)
(ii) and
(iii) (C)
(i) and
(iii) (D)
(ii) and (iv)
›Reveal solutionSolution
Among the four, only insulin and endorphin are peptide hormones; estrogen and androgen are steroids.

Concept and Intuition

Hormones are chemically classified as steroids (derived from cholesterol, e.g., sex hormones), peptides/proteins (chains of amino acids, e.g., insulin, glucagon), or amino-acid derivatives (e.g., thyroxine, adrenaline). Knowing this classification directly answers such questions.

Step-by-Step Solution

Estrogen (i) — a steroid hormone (female sex hormone), not peptide-based.

Insulin (ii) — a well-known peptide hormone made of two polypeptide chains (A and B chains).

Endorphin (iii) — an endogenous opioid peptide, hence peptide-based. …
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AP EAPCET 2022Set eng-2022-07-06-AN1 markMCQ
$2^\circ$
›Reveal solutionSolution
Denaturation unfolds a protein by breaking the weak interactions maintaining its 2°/3° shape and destroys its biological activity, but it leaves the covalent peptide-bond backbone (the primary structure) intact.

Concept and Intuition

A protein's structure is organised in levels: primary (the amino-acid sequence, held together by strong covalent peptide bonds), secondary (α-helices/β-sheets, held by H-bonds along the backbone), and tertiary (the overall 3-D fold, held by H-bonds, hydrophobic interactions, ionic bonds, and disulfide bridges between side chains). Denaturation — by heat, extreme pH, heavy metals, or other agents — supplies enough energy/disruption to break the weak interactions holding 2°/3° structure, so the protein unfolds into a random coil and loses its specific shape. Since biological activity (e.g. enzyme catalysis) depends entirely on that specific 3-D shape, activity is lost. But the sequence of amino acids joined by covalent peptide bonds is not affected by these mild conditions — that would require actually breaking covalent bonds (hydrolysis), a chemically much more demanding process.

Step-by-Step Solution

Recall what denaturation physically does: it disrupts H-bonds, hydrophobic interactions, and other weak forces stabilising the folded state. …
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Why a protein has a “native” shape — and why it matters

What denaturation actually does

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