Turns and loops

Turns

• serve to change the direction of the polypeptide chain, so if chain is going north, then a U-turn will change direction to south
  
• involves 4 residues, h-bond between carbonyl of residue i and amide of residue i+3
• type I is the most common
• turns are mostly found in the surface, so residues are usually charged and polar
• type I' and II' are mirror image conformations and are rare because of steric hindrance

Loops

• longer than turns, between 6 and 16 residues long
• term used to describe 2 stranded beta-sheet
  
• beta-hairpin (loop) like turns, generally polar, flexible and are often found in active sites, bin