Biological Molecules - Protein Structure Flashcards
What are amino acids?
- The monomer of polypeptides
- 100 amino acids identified, 20 occur naturally in proteins
- The same 20 amino acids occur in all living organisms, providing indirect evidence for evolution
- All amino acids have the same general structure: the only difference between each one is the nature of the R group
Why are R groups important?
- They can attract / repel each other
- They can also form bonds (disulphide bridges, hydrogen and ionic bonds) with other amino acid R groups in the polypeptide, causing it to fold
Where are hydrophilic R groups found?
- On the outside of the protein
Where are hydrophobic R groups found?
- On the inside of the protein (away from water)
What happens during the primary structure of a protein and what bonding occurs?
- The sequence of amino acids in a polypeptide chain
- Peptide bonds
What happens during the secondary structure of a protein and what bonding occurs?
- Folding of the polypeptide chain into α-helix or β-pleated sheets
- Hydrogen bonds form between NH (group of one amino acid) and C=O (group of another)
What is the difference between α-helix and β-pleated sheets?
- α-helix chains coil in a spiral shape
- β-pleated sheet chains run parallel to themself
What happens during the tertiary structure of a protein and what bonding occurs?
- Further 3D folding of the polypeptide chain
- disulphide bridges, ionic bonds are more hydrogen bonds
What happens during the quaternary structure of a protein and what bonding occurs?
- More than 1 polypeptide and can sometimes contain prosthetic groups (e.g. Haem)
- Disulphide bridges, ionic bonds and more hydrogen bonds
What are the features of fibrous proteins?
- Little or no tertiary structure
- Long parallel polypeptide chains
- Cross linkages at intervals forming long fibres or sheets
- Usually insoluble
- Many have structural roles
- E.g. keratin and collagen
What are the features of globular proteins?
- Have complex tertiary and sometimes quaternary structures
- Folded into spherical (globular) shapes
- Usually soluble as there are hydrophobic side chains in the centre of the structure
- Roles in metabolic reactions
- E.g. enzymes and haemoglobin
Explain how a change in the primary structure of a globular protein may result in a different three-dimensional structure (3)
- Sequence of amino acids changes
- Tertiary structure changes/folds in a different way
- Different ionic, hydrogen and disulphide bonds form in different place
Explain why the quaternary structure of collagen makes it a suitable molecule for a tendon (2)
- It has 3 polypeptide chains wound together to form a strong rope like structure
- Held together with a regular pattern of many H bonds/cross bridges
- Has strength in the direction of pull of the tendon
Suggest how the cross-linkages between the amino acids of polypeptide chains increase the strength and stability of a collagen fibre (2)
- Regular pattern of bonds / cross bridges prevents the individual polypeptide chains from sliding past one another
- They gain strength because they act as a single unit
Explain why the arrangement of collagen molecules is necessary for the efficient functioning of tendons (2)
- The junctions, between adjacent collagen, are points of weakness
- If they all occurred at the same point in a fibre, this would be a major weak point at which the fibre might break
