QUATERNARY STRUCTURE
4º
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
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 3º 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