Card Set Information
Biological Molecules - Proteins
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 (-NH
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.
: disulfide bridges, ionic bonds, H bonds, hydrophilic/hydrophobic interactions between R groups
Tertiary structure gives proteins their specific 3D configuration
2 or more
: 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 (-NH
) & (-COOH) groups.
R groups containing COO
attracted to R groups with NH
Formed at suitable pH but can be broken if pH changes
4. Hydrophobic/hydrophilic interactions
Weak bonds formed when
R groups project out and interact with water while
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
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 Fe
which binds to a molecule of oxygen.
Thus each haemoglobin molecule can bind 4 O
- 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.