QUATERNARY STRUCTURE

1bbb.pdb-500

figure_02_42a-2
FIBROUS PROTEINS: STRUCTURAL ROLE
COLLAGEN: a structural protein, the most abundant fibrous protein in the human body. Essential component of connective tissues (tendons, bones, skins, teeth)


Has no 3º

GLOBULAR PROTEINS: METABOLIC ROLE
HAEMOGLOBIN : a transport protein which involves the transport of oxygen in blood; found in the RBC of vertebrates
hhemo_rib1-1

The association of 2 or more polypeptide chains into a functional protein molecule

Not all proteins have quaternary structures (e.g. lysozymes and myoglobin are functional proteins which only have structures; one polypeptide chain that is extensively folded)

Determined by: 1º

Each polypeptide is referred to as a subunit, the subunits are held together by hydrogen bonds, ionic bonds, hydrophobic interactions, and disulphide bonds (and peptide bonds).

Proteins with 2 subunits are dimers, other with >2 are oligomers.

Made up of 4 polypeptides, namely 2 α-globin subunits and 2 β-globin subunits

FUNCTION: SOLUBLE IN WATER
STRUCTURE: Each subunit is arranged so that polar/charged, hydrophilic amino acid R-groups are on the external surface while the non-polar, hydrophobic R-groups of amino acids are pointed towards the interior, shielded from the aqueous environment.

In one subunit,

  • A polypeptide chain called globin
  • Prosthetic component called haem group: with a porphyrin ring and an iron ion (Fe2+)
    Fe2+ binds reversibly to oxygen, so 1 haemoglobin molecule can carry up to 4 oxygen molecules at the same time, forming oxyhaemoglobin
    FUNCTION: CARRYING OXYGEN

FUNCTION: CARRYING OXYGEN
STRUCTURE: 4 polypeptide subunits are held together by ionic bonds, hydrophobic interactions and hydrogen bonds (NO DISULPHIDE LINKAGES!)


Hence, the subunits move relative to each other, allowing a change in structure that influences its affinity for oxygen.


  • Binding of 1 oxygen molecule to 1 haemoglobin subunit induces structural change in the other 3 subunits → increase in affinity for oxygen
  • Initial 'hesitant' loading of 1st oxygen molecule results in rapid loading; co-operative binding of oxygen.
  • When one subunit unloads oxygen, the other 3 follow, due to conformational changes that reduce their affinity for oxygen.

ONE collagen molecule (TROPOCOLLAGEN) consists of three helical polypeptide chains wound around one another

In one individual LOOSE HELIX,

  • Each of the 3 helical polypeptide chains contain ~1000 amino acids, forming a loose helix
  • Intramolecular hydrogen bonds form within each helical polypeptide to stabilise it
  • Amino acid sequence: Repeating tripeptide unit of glycine X-Y, where X is proline and Y is hydroxyproline

FUNCTION: TENSILE STRENGTH
In one tropocollagen molecule (triple helix),

  • Tropocollagen molecule forms a compact coil as almost every 3rd amino acid in each polypeptide chain is a glycine, the smallest amino acid → allowing the amino units to fit into the tight spaces of the centre of triple helix
  • Hydrogen bonds also form between adjacent polypeptide chains to increase tensile strength

FUNCTION: INSOLUBILITY IN WATER
STRUCTURE: Amino acid residues in different helices are already extensively involved in intermolecular hydrogen bonding, limiting interaction with water due to occupied hydroxyl groups.

FUNCTION: RIGIDITY

  • Bulky and relatively inflexible **proline and hydroxyproline residues confer rigidity** to the molecule.

COLLAGEN FIBRILS: to increase tensile strength

  • Each tropocollagen molecule cross-links with neighbouring tropocollagen molecules running parallel to it. Cross-linking (covalent bonds between lysine residues) of adjacent tropocollagen molecules result in the formation of collagen fibril
  • Staggered/overlapping arrangement of tropocollagen minimises points of weaknesses along the length of fibrils and contributes to tensile strength

COLLAGEN FIBRES: to increase tensile strength

  • Collagen fibrils → collagen fibres in bundles
  • Banded appearance due to offset/staggered arrangement of tropocollagen

Lengths of polypeptides and sequences can vary slightly; yet the protein is still functional

Length of polypeptide and sequence are always identical between 2 samples, or else protein might not be functional