state that globular proteins are generally soluble and have physiological roles and fibrous proteins are generally insoluble and have structural roles

Proteins – Cambridge IGCSE / A‑Level Biology

Learning Objective (AO1)

State that globular proteins are generally soluble and have physiological roles, whereas fibrous proteins are generally insoluble and have structural roles.

1. Amino‑acid structure & peptide bond

  • General formula: NH₂–CH(R)–COOH where R is the side‑chain that distinguishes the 20 standard amino acids.

  • In a protein the carboxyl group of one amino‑acid reacts with the amino group of the next, releasing H₂O (condensation) to form a peptide bond (–CO–NH–).

  • Repeating peptide bonds give a polypeptide chain – the primary structure of a protein.

2. Four levels of protein structure (AO1)

LevelDefinitionTypical example
PrimaryLinear sequence of amino‑acids linked by peptide bonds.Insulin (single polypeptide chain)
SecondaryRegular folding of the chain into α‑helix or β‑sheet stabilised by hydrogen bonds.α‑helix in keratin; β‑sheet in silk fibroin
TertiaryThree‑dimensional shape of a single polypeptide, stabilised by hydrophobic interactions, H‑bonds, ionic bonds and disulphide bridges.Myoglobin (oxygen‑binding protein)
QuaternaryAssembly of two or more polypeptide subunits into a functional protein.Haemoglobin – classic quaternary protein (α₂β₂ tetramer); Immunoglobulins (antibodies) – also quaternary, linking to Topic 11 (Immunity)

3. Intra‑molecular interactions (AO2)

  • Hydrophobic interactions – drive non‑polar side‑chains to the interior; crucial for the overall folding of globular proteins (tertiary level).

  • Hydrogen bonds – stabilise α‑helices and β‑sheets (secondary structure) and also contribute to tertiary/quaternary stability.

  • Ionic (electrostatic) bonds – attraction between oppositely charged side‑chains; important in tertiary and quaternary packing.

  • Disulphide bridges (S–S bonds) – covalent links between cysteine residues; give extra rigidity to tertiary structure and lock quaternary subunits together (e.g., secreted enzymes, antibodies).

InteractionPredominant structural level(s) it stabilisesTypical example
Hydrophobic interactionsTertiary (core formation) – also contributes to quaternary packingMyoglobin interior
Hydrogen bondsSecondary (α‑helix, β‑sheet); also tertiary/quaternaryα‑helix in keratin
Ionic bondsTertiary and Quaternary (surface charge complementarity)Salt bridges in haemoglobin
Disulphide bridgesTertiary and Quaternary (covalent locking)IgG antibodies, insulin

4. Globular proteins (soluble, physiological roles)

Compact, roughly spherical molecules. Hydrophobic residues are buried inside, while polar residues line the surface, making them generally soluble in aqueous media. Their mobility allows a wide range of physiological functions.

  • Enzymes – e.g., DNA polymerase (catalysis)

  • Transport & storage – haemoglobin (O₂ transport), albumin (osmotic balance)

  • Regulation & signalling – insulin, growth hormone

  • Immune defence – antibodies (IgG)

Structure–function example: Haemoglobin

Quaternary structure: tetramer (α₂β₂). Each subunit contains a heme group with an iron atom that binds one O₂ molecule. The soluble, globular nature lets haemoglobin circulate in blood and rapidly pick up/release O₂ – a direct link to Topic 9 (Gas exchange) and Topic 10 (Transport in animals).

5. Fibrous (structural) proteins (insoluble, mechanical roles)

Long, repetitive polypeptide chains that assemble into extended filaments or sheets. Side‑chains are often non‑polar, producing strong intermolecular forces and generally insoluble in water. Their rigidity provides mechanical strength.

  • Collagen – triple‑helix of three polypeptide strands; gives tensile strength to skin, bone, tendons.

  • Keratin – α‑helical coiled‑coil; forms hair, nails, outer skin layer.

  • Elastin – cross‑linked polypeptides; provides elasticity in arteries and lungs.

  • Fibrin – forms a mesh in blood clots.

Structure–function example: Collagen

Primary structure: repeating Gly‑X‑Y pattern (X and Y often Pro or hydroxy‑Pro). Three such chains wind into a right‑handed triple helix, stabilised by hydrogen bonds. The tightly packed, insoluble fibres resist stretching, explaining collagen’s role in connective tissue (Topic 4 – Structure & support).

6. Comparison of globular and fibrous proteins

FeatureGlobular proteinsFibrous proteins
ShapeCompact, roughly sphericalExtended filamentous or sheet‑like
SolubilityGenerally soluble in waterGenerally insoluble in water
Primary functionPhysiological – catalysis, transport, regulation, immunityStructural – support, protection, elasticity
Dominant secondary structureMixture of α‑helices, β‑sheets and random coilsPredominantly α‑helices (keratin) or β‑sheet ribbons (silk fibroin)
Typical examplesHaemoglobin, enzymes, antibodies, insulinCollagen, keratin, elastin, fibrin

7. Links to other syllabus topics (cross‑referencing)

  • Haemoglobin – Topic 9 (Gas exchange) & Topic 10 (Transport in animals).

  • Antibodies – Topic 11 (Immunity).

  • Collagen – Topic 4 (Structure & support of organisms).

  • Enzymes – Topic 6 (Enzymes and metabolism).

8. Practical / experimental component (AO3)

Simple solubility test – compare a globular protein (egg‑white albumin) with a fibrous protein (raw chicken tendon containing collagen).

  1. Weigh 0.5 g of each sample.

  2. Add 10 mL of distilled water, stir for 2 min, and allow to stand for 5 min.

  3. Observe: albumin forms a clear solution (soluble); tendon remains cloudy/undissolved (insoluble).

  4. Record observations and relate them to the protein’s structural class.

This activity reinforces the link between structure (solubility) and function, and provides evidence for AO3 (practical skills and data interpretation).

9. Key points to remember (AO1–AO2)

  1. Solubility is a quick indicator of protein class: soluble → globular; insoluble → fibrous.

  2. Globular proteins are mobile in cytosol or plasma, enabling catalytic, transport, regulatory and immune functions.

  3. Fibrous proteins form rigid, insoluble fibres that give mechanical strength, protection or elasticity.

  4. Both classes are built from the same 20 amino acids; differences arise from sequence patterns, secondary‑structure propensity and higher‑order folding.

  5. Understanding the four intra‑molecular forces explains why the two classes adopt such contrasting shapes.

Suggested diagram: a soluble globular protein (e.g., enzyme) shown as a compact sphere next to an insoluble fibrous protein (e.g., collagen fibril), with labels for solubility, dominant secondary structure and functional role.