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QUATERNARY STRUCTURE
4º
1bbb.pdb-500 (figure_02_42a-2
FIBROUS PROTEINS…
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
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
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GLOBULAR PROTEINS: METABOLIC ROLE
HAEMOGLOBIN : a transport protein which involves the transport of oxygen in blood; found in the RBC of vertebrates
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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)
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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.
Length of polypeptide and sequence are always identical between 2 samples, or else protein might not be functional