Amino Acids and Peptide Bonds — AQA A-level Biology

SPEC 3.1.4.1 - PROTEINS

Proteins

Amino acids, peptide bonds, and the four levels of protein structure — from primary sequence to quaternary organisation

Amino Acid Structure

AQA SPEC (3.1.4.1)

Amino acids are the monomers from which proteins are made. The general structure of an amino acid has an amine group (–NH₂), a carboxyl group (–COOH), a hydrogen atom, and an R-group (side chain) — all attached to a central carbon atom. The twenty amino acids that are common in all organisms differ only in their R-group.

Examiner Alert

All 20 amino acids have the same central structure. The only difference between them is the R-group (side chain). The R-group determines the amino acid's chemical properties (charged, polar, non-polar, acidic, basic) and therefore determines how the protein folds. You must be able to draw or interpret this general structure.

Peptide Bond Formation

AQA SPEC (3.1.4.1)

A condensation reaction between two amino acids forms a peptide bond. Dipeptides are formed by the condensation of two amino acids. Polypeptides are formed by the condensation of many amino acids.

The peptide bond forms between the –COOH group of one amino acid and the –NH₂ group of the next:

Key Facts

Peptide bond = –CO–NH– (also written –C(=O)–NH–)

Formed by condensation: one H₂O eliminated per bond

A polypeptide of n amino acids has (n−1) peptide bonds

Broken by hydrolysis (e.g. by proteases during digestion)

From Polypeptide to Protein

AQA SPEC (3.1.4.1)

A functional protein may contain one or more polypeptides.

1

Amino acids (monomers) — joined by condensation reactions

2

Dipeptide — two amino acids joined by one peptide bond

3

Polypeptide — many amino acids joined by peptide bonds; the sequence of amino acids constitutes the primary structure

4

Protein — one or more polypeptides folded into a specific 3D shape; a functional protein may also contain non-protein (prosthetic) groups

Globular Proteins

Roughly spherical; soluble in water (hydrophilic R-groups on surface); metabolically active — enzymes, antibodies, hormones, transport proteins (haemoglobin)

Fibrous Proteins

Long, parallel chains forming fibres/sheets; insoluble; structural roles — collagen (connective tissue), keratin (hair, nails), elastin

Primary Structure

DEFINITION

Primary structure is the specific sequence of amino acids in a polypeptide chain.

The primary structure is:

Determined by the base sequence of DNA (via transcription and translation)

Held together by covalent peptide bonds — the strongest bonds in protein structure

Unique to each protein — even one amino acid change can alter function

Examiner Alert — Sickle Cell Anaemia

AQA frequently uses sickle cell anaemia as an example. A single amino acid substitution in the β-globin chain of haemoglobin (glutamic acid → valine at position 6) alters the tertiary structure of the β-globin subunit and consequently the quaternary structure of haemoglobin, causing molecules to aggregate and red blood cells to sickle. This demonstrates how a change in primary structure → changed tertiary and quaternary structure → loss of function.

Secondary Structure

DEFINITION

Secondary structure is the regular folding of a polypeptide chain, stabilised by hydrogen bonds between the –C=O and –N–H groups of different peptide bonds along the chain backbone.

α-Helix

Polypeptide chain coils into a right-handed helix

Held by hydrogen bonds between C=O of one peptide bond and N–H of another, approximately 4 residues further along

Found in: keratin (hair), haemoglobin subunits, many enzyme active site regions

β-Pleated Sheet

Polypeptide chain folds back and forth in a sheet-like arrangement

Held by hydrogen bonds between C=O and N–H of adjacent strands

Found in: silk fibroin, immunoglobulins (antibodies), β-keratin in feathers and scales

Critical Distinction

Secondary structure H-bonds form between groups on the peptide bond backbone (–C=O and –N–H), NOT between R-groups. R-group interactions determine tertiary structure. This distinction is tested directly in AQA mark schemes — confusing the two levels loses marks.

Tertiary Structure

DEFINITION

Tertiary structure is the precise three-dimensional folding of a single polypeptide chain, determined by interactions between R-groups.

The AQA spec (3.1.4.1) names hydrogen bonds, ionic bonds and disulfide bridges. AQA mark schemes at A-level also consistently credit hydrophobic interactions — all four must be known:

InteractionBetweenStrengthDisrupted by

Hydrogen bondsPolar R-groupsWeak individually; many presentHeat, pH change

Ionic bondsOppositely charged R-groups (–NH₃⁺ and –COO⁻)ModeratepH change (alters charge)

Disulfide bridgesTwo cysteine R-groups (–SH + –SH → –S–S–)Strong (covalent)Reducing agents

Hydrophobic interactionsNon-polar R-groups clustering away from waterWeak individually; cumulative effectDetergents, non-polar solvents

Exam Tip — Name All Four

AQA mark schemes award marks for each named interaction. Never say "bonds hold the tertiary structure" without naming them. For full marks, state all four: hydrogen bonds, ionic bonds, disulfide bridges, hydrophobic interactions.

Quaternary Structure

DEFINITION

Quaternary structure is the association of two or more polypeptide chains (subunits) into a functional protein. It may also involve non-protein prosthetic groups.

Haemoglobin — The Classic Example

4 polypeptide subunits: 2 α-chains and 2 β-chains

Each subunit has a haem prosthetic group containing an iron (Fe²⁺) ion

Fe²⁺ binds oxygen reversibly → haemoglobin can carry O₂

Quaternary structure enables cooperative binding of O₂ (binding of one O₂ increases affinity for the next)

Collagen — Another Example

Three polypeptide chains wound into a triple helix

Hydrogen bonds and covalent cross-links between chains give enormous tensile strength

Every third amino acid is glycine (smallest R-group — fits in the centre of the helix)

Not all proteins have quaternary structure

Quaternary structure only applies to proteins made of two or more polypeptide chains. A protein consisting of a single polypeptide (like myoglobin) has only primary, secondary and tertiary structure. Never state that all proteins have quaternary structure.

Four Levels — Summary

LevelDescriptionBonds involvedExample

PrimarySequence of amino acids in chainPeptide bonds (covalent)Any polypeptide sequence

SecondaryLocal folding — α-helix or β-pleated sheetHydrogen bonds (between C=O and N–H of backbone)α-helix in keratin

TertiaryOverall 3D shape of single polypeptideH-bonds, ionic bonds, disulfide bridges, hydrophobic interactions (between R-groups)Enzyme active site shape

QuaternaryTwo or more polypeptides + any prosthetic groupsSame as tertiary (between subunits)Haemoglobin (4 subunits + 4 haem groups)

Hierarchy Rule

Each level of structure depends on the level below it. Primary sequence → determines secondary folding → determines tertiary shape → determines quaternary assembly. A change at primary level propagates up through all subsequent levels, changing protein function.

Structure Determines Function

AQA SPEC (3.1.4.1)

The relationship between primary, secondary, tertiary and quaternary structure, and protein function. Students should be able to relate the structure of proteins to properties of proteins named throughout the specification.

1

Enzymes: Specific tertiary structure → specific active site shape → complementary to specific substrate only. Change R-group interactions (denaturation) → active site shape lost → no enzyme–substrate complex → no catalysis.

2

Antibodies: Specific tertiary/quaternary structure → specific antigen-binding site (variable region) → complementary to specific antigen. This specificity arises directly from the amino acid sequence.

3

Haemoglobin: Quaternary structure with haem groups → O₂ binding. Cooperative binding requires all four subunits to interact correctly. Sickle cell mutation changes one amino acid → alters tertiary shape of subunit → haemoglobin forms fibrous aggregates instead of soluble molecules.

4

Collagen: Triple helix structure → high tensile strength → suitable for tendons, bone matrix, skin, blood vessel walls. The repetitive Gly-X-Y sequence is essential for helix formation.

Denaturation

Denaturation is the permanent disruption of the bonds maintaining the tertiary (and quaternary) structure of a protein, resulting in loss of function. Under extreme conditions (high temperature, extreme pH) this is effectively irreversible at physiological conditions.

What is disrupted?

CauseBonds disruptedEffectHigh temperatureHydrogen bonds, ionic bonds (not peptide bonds)Active site shape lost; protein unfoldsExtreme pHIonic bonds (H⁺/OH⁻ alter charges on R-groups); hydrogen bondsActive site shape lost; enzyme inactivatedHeavy metalsIonic bonds, hydrogen bonds; bind to –SH groups disrupting disulfide bridgesProtein structure disrupted

Denaturation ≠ Hydrolysis

Denaturation changes the protein's shape by disrupting non-covalent bonds (H-bonds, ionic bonds) that maintain tertiary structure. The peptide bonds and primary structure are NOT broken by denaturation. Hydrolysis breaks peptide bonds. These are different processes — do not confuse them.

Language Alert

AQA mark schemes require precision: "the active site changes shape" or "the bonds maintaining tertiary structure are disrupted" — not "the protein is destroyed" or "the protein breaks down." The primary structure remains intact; only the shape changes.

The Biuret Test for Proteins

Biuret Test — Biochemical Test (AT f)

1

Add sodium hydroxide solution (NaOH) to the sample (makes the solution alkaline — essential first step)

2

Add a few drops of dilute copper(II) sulfate solution (CuSO₄) — which is blue

3

If protein is present: colour changes from blue → purple/lilac/violet

4

If no protein: solution remains blue

Why purple?

In alkaline conditions, Cu²⁺ ions form a complex with peptide bonds (the –CO–NH– groups). This coordination complex absorbs light and appears purple. The test detects peptide bonds — so dipeptides and polypeptides both give a positive result. Free amino acids do not (no peptide bonds).

Common Error — Biuret Order

NaOH must be added first to create alkaline conditions before the CuSO₄. Reversing the order gives an incorrect result. Also: the positive result is purple/lilac/violet — never say "blue" for protein. Blue = reagent alone = negative result.

DEEP Framework: Extended Answer

Question: "Explain how the tertiary structure of an enzyme is important for its function." [4 marks]

D

Define: Tertiary structure is the precise 3D folding of a single polypeptide chain, maintained by interactions between R-groups — hydrogen bonds, ionic bonds, disulfide bridges and hydrophobic interactions.

E

Explain mechanism: The specific pattern of these R-group interactions folds the polypeptide into a precise 3D shape. This shape includes the active site — a cleft or groove with a specific geometry and pattern of chemical groups. The active site is complementary in shape and charge to the specific substrate molecule.

E

Evidence/Example: When the active site binds the substrate (induced fit model — the active site slightly changes shape to fit around the substrate), enzyme–substrate complexes form, lowering activation energy and catalysing the reaction. If the tertiary structure is disrupted (e.g. by high temperature or extreme pH) the active site shape changes → substrate can no longer bind → enzyme loses catalytic activity.

P

Practical/Quantitative: Enzyme activity can be measured vs temperature/pH — at the optimum, maximum rate; beyond optimum, rate falls to zero as denaturation removes functional active sites. This is measured by recording the rate of product formation (e.g. oxygen production by catalase on hydrogen peroxide).

Practice Question

(a) Describe the general structure of an amino acid and state what determines the differences between the 20 common amino acids. [2 marks]

(b) Distinguish between the bonds responsible for secondary structure and those responsible for tertiary structure in proteins. [3 marks]

(c) A student heats an enzyme solution to 90°C and observes that enzyme activity falls to zero. Explain this result at the molecular level, identifying which bonds are disrupted and what structural change occurs. [3 marks]

D:

(a) An amino acid has a central carbon atom bonded to an amine group (–NH₂), a carboxyl group (–COOH), a hydrogen atom, and a variable R-group (side chain) (1). All 20 common amino acids differ only in their R-group (1).

E:

(b) Secondary structure: hydrogen bonds between the C=O group of one peptide bond and the N–H group of another peptide bond along the backbone (not between R-groups) (1). Tertiary structure: interactions between R-groups, specifically hydrogen bonds, ionic bonds, disulfide bridges (covalent, between two cysteine –SH groups), and hydrophobic interactions (1). Secondary structure uses backbone bonds; tertiary structure uses R-group interactions — these are distinct (1).

E:

(c) High temperature (90°C) increases kinetic energy, providing sufficient energy to disrupt the non-covalent bonds maintaining tertiary structure — particularly hydrogen bonds and ionic bonds between R-groups (1). This causes the polypeptide to unfold — the active site changes shape (1). The active site is no longer complementary to the substrate; enzyme–substrate complexes cannot form and catalytic activity is lost (denaturation) (1). Note: peptide bonds (primary structure) are NOT broken.

Common Exam Errors

Alert 1 — Secondary vs Tertiary H-bonds

Secondary structure H-bonds: between C=O and N–H of the peptide bond backbone. Tertiary structure H-bonds: between R-groups. Students who say "hydrogen bonds between R-groups create the secondary structure" lose marks every year. The distinction is explicit in AQA mark schemes.

Alert 2 — Denaturation language

Never write: "the protein is broken down" or "peptide bonds are broken" when describing denaturation. Write: "the bonds maintaining the tertiary structure (H-bonds, ionic bonds) are disrupted → the active site changes shape → substrate cannot bind → enzyme is denatured." The primary structure (peptide bonds) is not affected by denaturation.

Alert 3 — Not all proteins have quaternary structure

Quaternary structure only applies to proteins made of two or more polypeptide chains. A protein consisting of a single polypeptide (like myoglobin) has only primary, secondary and tertiary structure. Haemoglobin (four chains) has all four levels. Do not state that quaternary structure is a universal property of all proteins.

Alert 4 — Biuret test order and result

Add NaOH first, then CuSO₄. Positive result = purple/lilac (not red, not blue-black). The test detects peptide bonds, not amino acids. Free amino acids give a negative result.

Summary

Key Points

TopicKey Fact

Amino acidCentral C bonded to –NH₂, –COOH, –H, and variable R-group. 20 types, differ only in R-group.

Peptide bond–CO–NH– formed by condensation of –COOH and –NH₂; one H₂O eliminated per bond

PrimaryAmino acid sequence; held by covalent peptide bonds; determined by DNA

Secondaryα-helix or β-pleated sheet; H-bonds between C=O and N–H of backbone

Tertiary3D shape of single polypeptide; R-group interactions: H-bonds, ionic, disulfide bridges, hydrophobic

Quaternary2+ polypeptides ± prosthetic groups; e.g. haemoglobin (4 subunits + 4 haem groups)

DenaturationDisrupts non-covalent bonds (H-bonds, ionic) maintaining tertiary/secondary structure → active site changes shape → loss of function; peptide bonds (primary structure) intact

Biuret testNaOH first → then CuSO₄. Positive = purple/lilac. Detects peptide bonds.

Exam Tip

For any protein structure question: (1) name the level of structure, (2) name the specific bonds involved at that level, (3) link the structure to function. For tertiary structure, always name all four interaction types. For denaturation, always say which bonds are disrupted and what structural change results.