Protein folding and purification

Hydrophobic effect

• hydrophobic regions cluster together to minimise interactions with water - entropic effect

• water molecules for H bonds only with themselves, not with hydrophobic molecules

• adapted to reduce the size of the hydrophobic-water interface so fold into structures with nonpolar cores

Hydrogen bonding

• electrostatic attraction between lone pair of electrons on elec. neg. atom and H bonded to elec. neg. atom

• drive formation of 2* structure with NH and CO backbone

• drive formation of 3* structure with sidechains

Van der Waals interactions (ind. dip-dip)

• random movement generates transient electric dipole 

• many weak interactions produce sig. cum. stability 

• facilitates dense packing of protein hydrophobic cores

Ionic interactions

• oppositely charged a.a. sidechains held in proximity will form electrostatic interactions

Disulphide bonds

• very strong covalent bond

• not every sulphur forms disulphide bond, often many combinations but only one produces required structure

How can protein folding be aided?

• once part of folded structure has formed, may increase likelihood of subsequent favourable interactions (cascade of lowest energy conformation)

• chaperone proteins in cells increase likelihood of favourable interactions

• leads to native (completely folded) structure