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- consist of 1 or more polypeptides folded into globular/fibrous form
- A polypeptide is a linear polymer chain of amino acids bonded together by peptide bonds
FORMATION OF PEPTIDE BOND
- One amino acid loses a hydroxyl (-OH) group from (-COOH) group while the other loses a H atom from (-NH2) group
- C atom of the first amino acid can thus bond with the N atom of the second, forming a peptide bond
STRUCTURE OF PROTEINS
- Simple linear strand of amino acids held together by peptide bonds in the polypeptide
- Based on the different arrangements of the 20 fundamental amino acids.
- Amino acid sequence det the biological function, furthest levels of organisation and hence different properties of proteins.
- Thus, a change in 1 amino acid may completely alter shape and properties of the polypeptide
- Polypeptide chain that is coiled up and held together by many intra-molecular H bonds
- H atom of NH group of 1 amino acid H-bonded to O atom of C=O group of another amino acid 4 places ahead
- H bond if the result of e- sharing in the NH group, leaving H atom slightly +ve and the C=O group, leaving O atom slightly -ve, so they attract each other.
2. beta-pleated sheet
- 1 polypeptide chain consisting of parallel adj chains running in either opp or same direction
- Chains joined by H bonds between H atom of NH group of 1 amino acid and O atom of C=O group of another amino acid in adj chain
- Structure gives protein stability, high tensile strength & flexibility
- One polypeptide chain further coiled and extensively folded to form compact 3D globular structure.
- Bonds: disulfide bridges, ionic bonds, H bonds, hydrophilic/hydrophobic interactions between R groups
- Tertiary structure gives proteins their specific 3D configuration
- Aggregation of 2 or more extensively coiled polypeptide chains
- Bonds: disulfide linkages, ionic bonds, H bonds, hydrophobic/hydrophilic interactions between R groups
TYPES OF BONDING & INTERACTIONS
1. Peptide bonds
- Strong covalent bonds formed between adj amino acids
- Strongest bonds in polypeptide chains
2. Disulfide bonds
- Strong covalent bond from from oxidation of sulphydryl (-SH) groups of 2 neighbouring cysteines' R groupds
- Broken by reducing agents (e.g. urea)
3. Ionic bonds
- Strong bonds formed between ionised/charged (-NH2) & (-COOH) groups.
- R groups containing COO- attracted to R groups with NH3+
- Formed at suitable pH but can be broken if pH changes
4. Hydrophobic/hydrophilic interactions
Weak bonds formed when hydrophilic R groups project out and interact with water while hydrophobic R groups shielded inside protein interact with non-polar R groups of amino acids
5. H bonds
- H atoms in NH groups or OH group form bond with O atoms in OH groups
- H bonds formed between R groups in tertiary structure
- Numerous; stabilise the structure
GLOBULAR VS FIBROUS
- 2 alpha chains
- 2 beta chains
- Polypeptide chain first coiled into alpha-helices, then folded into spherical globular protein held by H bonds and ionic bonds
- Hydrophobic side-chains point inwards to maintain 3D shape. Hydrophilic side-chains point outwards to maintain solubility
- Each chain carries prosthetic haem group containing Fe2+ which binds to a molecule of oxygen.
- Thus each haemoglobin molecule can bind 4 O2 molecules
- Fibrous protein with quaternary but no tertiary structure.
- Found in skin, tendons, cartilage, bones, teeth etc.
- Tropocollagen consists of 3 polypeptide chains wound around each other to give triple helix
- Each polypeptide chain is a loosely wound left-handed helix that wind around 2 others and twist into a triple helix, linked by H bonds
- Glycine at every 3rd amino acid allows close packing to form tight coil for compact structure
- Covalent bonds formed between the lysines in adj chains. Cross-links hold tropocollagen molecules side by side, forming fibrils.
- In fibrils, tropocollagens lie parallel with staggered ends, overlapping aids in forming strong fibres.
FORMATION & ASSEMBLY OF COLLAGEN FIBRE
- Synthesis of collagen polypeptide occus on the ribosome on RER
- Collagen threaded into RER lumen where loose winding of polypeptide and hydroxylation of selected prolines & lysines occur
- ER vesicles carrying polypeptide bud off ER, travel to GA where glycosylation of selected hydroxylysines occur
- Further assembly of 3 collgen polypeptides forms tropocollagen. Glycine molecule at every 3rd AA allows close packing of collagen molecule, which buds off GA, forming secretory vesicle
- Secretory vesicle moves towards and fuses with CSM
- Secreted into extracellular matrix
- Polypeptides cleaved to remove N & C terminals
- Self assembly of tropocollagen into fibrils via cross-linking between lysine residues and overlaps of staggered ends
- Aggregation of collagen fibrils form fibres.