Q4 LONG ANSWER 10M ★ High-yield

Describe the structural organisation of proteins from primary to quaternary level. Add a note on denaturation.

Biochemistry · 1st Professional — Pre-Clinical

Definition

Proteins = polymers of amino acids joined by peptide bonds, folded into a specific three-dimensional shape that determines their function.

The Peptide Bond

–COOH of one amino acid + –NH2 of the next → Loss of water (condensation) → peptide (amide) bond

Partial double bond character from resonance → it is rigid and planar, and cannot rotate
Almost always in the trans configuration — except before proline
Rotation is possible only about the φ (phi) and ψ (psi) bonds either side of the α-carbon; the permitted angles are shown on a Ramachandran plot
Uncharged but polar; participates in hydrogen bonding

Levels of Organisation

Level	Definition	Bonds involved
Primary	The sequence of amino acids, N-terminal to C-terminal	Peptide bonds (covalent); disulphide bonds
Secondary	Local regular folding of the backbone	Hydrogen bonds
Tertiary	The overall three-dimensional shape of one polypeptide	Hydrophobic interactions (chief), hydrogen, ionic, van der Waals, disulphide
Quaternary	Arrangement of two or more subunits	Same non-covalent forces; no peptide bonds between subunits

Primary structure

Determined entirely by the gene
It dictates all higher levels — a protein folds spontaneously into its native shape, as Anfinsen showed
Insulin was the first protein sequenced (Sanger, 1955)
A single change can be catastrophic — sickle cell disease is one amino acid substitution

Secondary Structure

Type	Features	Examples
α-Helix	Right-handed; 3.6 residues per turn; pitch 5.4 Å; H-bond between C=O of residue n and N–H of n+4; side chains project outward	Keratin, myoglobin, haemoglobin
β-Pleated sheet	Extended chains side by side; parallel or antiparallel; H-bonds between strands	Silk fibroin, immunoglobulins, amyloid
β-Bend (turn)	Reverses chain direction; often contains glycine and proline	Globular proteins
Triple helix	Three left-handed chains wound into a right-handed superhelix	Collagen

CLINICAL PEARL

Proline is the helix breaker. Its nitrogen is locked in a ring, so it has no N–H to donate a hydrogen bond and its rigid structure kinks the chain. Glycine also destabilises the helix, being too flexible. Both are therefore common at bends.

Tertiary and Quaternary Structure

Hydrophobic interactions are the chief driving force — non-polar side chains turn inward, away from water, while polar ones face outward
Disulphide bonds between cysteine residues are the only covalent bonds stabilising tertiary structure; abundant in secreted proteins such as insulin and immunoglobulins
Chaperones assist folding and prevent aggregation
Domains — independently folding functional units
Quaternary structure allows co-operativity and allosteric regulation — haemoglobin (α2β2), lactate dehydrogenase (tetramer), creatine kinase (dimer)

Denaturation

Denaturation = loss of the native three-dimensional structure, and therefore of biological activity, without breaking peptide bonds — the primary structure is preserved.

Agent	Mechanism
Heat	Disrupts hydrogen bonds and hydrophobic interactions
Extremes of pH	Alters ionisation, breaking ionic bonds
Urea, guanidine hydrochloride	Disrupt hydrogen bonds
Organic solvents, detergents (SDS)	Disrupt hydrophobic interactions
Heavy metals	Bind sulphydryl groups
β-Mercaptoethanol	Reduces disulphide bonds
Mechanical — whipping, ultraviolet light	Physical disruption

Effects — loss of biological activity, decreased solubility, increased viscosity, greater susceptibility to proteases
Sometimes reversible (renaturation) if mild, which is itself the proof that primary structure determines conformation

Forces Stabilising Protein Structure

Bond	Nature	Strength	Between
Peptide	Covalent	Strongest	Backbone amino acids
Disulphide	Covalent	Strong	Two cysteine residues
Hydrogen	Non-covalent	Weak individually, numerous	C=O and N–H, or polar side chains
Ionic (salt bridge)	Non-covalent	Moderate	Oppositely charged side chains
Hydrophobic	Non-covalent	The chief force in folding	Non-polar side chains
Van der Waals	Non-covalent	Very weak	All closely packed atoms

Only the peptide and disulphide bonds are covalent — all higher structure depends on weak forces acting in large numbers, which is exactly why proteins are so easily denatured

Applied Aspects

Sickle cell disease — Glu→Val at position 6 of the β chain creates a hydrophobic patch, so deoxyhaemoglobin polymerises. A primary structure change causing a quaternary structure disease
Prion diseases — Creutzfeldt–Jakob disease and kuru; a normal α-helical protein refolds into a β-sheet form that templates the same change in others. Disease from misfolding alone, with no change in sequence
Amyloidosis and Alzheimer disease — β-sheet aggregates deposited in tissue; stain with Congo red giving apple-green birefringence
Denaturation in practice — sterilisation by heat, disinfection by alcohol, and the use of egg white or milk as a first-aid antidote in heavy metal poisoning
Osteogenesis imperfecta — a glycine substitution in collagen disrupts the triple helix, since only glycine is small enough for the interior position

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