Tertiary + quaternary structure

disulphide bridge

side chain of one cysteine forms a crosslink with the side chain of another to form a covalent bond - makes protein more resistant to denaturation, called a single cystine molecule

quaternary structure

proteins formed from more than one polypeptide chain

what holds quaternary proteins together

electrostatic, hydrogen, van der waals and disulphide bonds

dimer

a compound whose molecules are composed of two identical/similar monomers

homodimer

2 copies of same polypeptide that come together and join

heterodimer

different polypeptides join together

globular proteins

protein chains arranged in compact domains, usually active components of cellular machinery

fibrous proteins

protein chains are arranged into fibres, structural role

3 types of fibrous proteins

coiled-coil, beta sheets, triple helix

coiled coil eg. keratin structure

7 amino acid repeat - abcdefg, forms alpha helix. a and d are hydrophobic and lie on same side of helix. 2 alpha keratin helices twist around each other to form a coiled coil. dimers line up with others to form staggered antiparallel tetramer - builds up to microfibrils

beta sheets eg fibroins

6 amino acid repeat - forms antiparallel beta sheet. every alternate is glycine so these always project from one side of the sheet - sheets can stack into layers - very strong as different bonds have different directions and all have to be broken

triple helix eg collagens

1/3 are glycine, 15-30% are proline/hydroxyproline - sequence repeats gly-pro-hyp

can't form an alpha helix so forms looe helix instead, around 3 residues per turn

3 polypeptides wind around each other - triple helix

form interchain H bonds. trimers associate to form strong fibres