Secondary structures are short regions of ordered structure arising from folding of the protein chain. The structures are stabilized by hydrogen bonding within the backbone. The two major types of secondary structure are the alpha-helix and the beta-pleated sheet.

 

Alpha Helix

The alpha helix is a helical structure held together by hydrogen bonds between the backbone N-H and C=O groups. In the structure below, turn on the hydrogen bond display and notice how the hydrogen bonds are formed within the backbone and the sidechains do not participate. Rotate the structure and try different Highlight and Display combinations.

Highlight	Backbone	Sidechains	H-bonds
Display	Backbone Only	Ribbon

 

Beta-Pleated Sheet

The beta-pleated sheet (or beta sheet) is similar to the alpha-helix in that it is held together by hydrogen bonding between groups in the backbone. In the example below, the backbone loops around several times to form the beta-pleated sheet and the strands run anti-parallel (in opposite directions) to each other.

Highlight	Backbone	Sidechains	H-bonds
Display	Backbone Only	Ribbon

 

Profilin - An Example Structure

The structure below shows how a single protein chain can contain both alpha helices and beta sheets. The intervening protein sequences connecting different secondary structures are called random coils.

The manner in which all of the secondary structures of a single protein chain are arranged to from the overall structure is called tertiary structure.