Protein Secondary Structure Section G
TURNS AND BENDS IN THE POLYPEPTIDE BACKBONE
Proline (Pro) breaks up secondary structures like alpha-helix and beta-sheet. Because Pro can not bend, Pro is often found at the ends of Alpha Helix and Beta Sheet strands. Thus, the third type of Secondary Structure is actually formed by the absence of the other two types.
Figure 16. Positions of Pro in Relation to Alpha-Helix and Beta Sheet Secondary Structures
Places where the polypeptide backbone bends so that the protein can fold back on itself to form the compact structure also have hydrogen bonds in some cases. These H-bonds occur only between the 1st and 4th amino acid residue of the Reverse Turn and no other H-bonds are formed.
Figure 17. Models of Specific Types of Turns formed in Protein Bends.
(From Voet/Biochemistry ©1990 John Wiley)
©Wilbur H. Campbell, 1995; wcampbel@mtu.edu