A supersecondary structure is a compact three-dimensional protein structure of several adjacent elements of a secondary structure that is smaller than a protein domain or a subunit. Supersecondary structures can act as nucleations in the process of protein folding.
Examples
Helix supersecondary structures
thumb|Image of a helix hairpin
Helix hairpin
A helix hairpin, also known as an alpha-alpha hairpin, is composed of two antiparallel alpha helices connected by a loop of two or more residues. True to its name, it resembles a hairpin. A longer loop has a greater number of possible conformations. If short strands connect the helices, then the individual helices will pack together through their hydrophobic residues. The function of a helix hairpin is unknown; however, a four helix bundle is composed of two helix hairpins, which have important ligand binding sites.
Helix corner
A helix corner, also called an alpha-alpha corner, has two alpha helices almost at right angles to each other connected by a short 'loop'. This loop is formed from a hydrophobic residue. The function of a helix corner is unknown.
Two residue loops are called beta turns or reverse turns. Type I' and Type II' reverse turns occur most frequently because they have less steric hindrance than Type I and Type II turns. The function of beta hairpins is unknown.
Beta corner
A beta hairpin has two antiparallel beta strands that are at about a 90 degree angle to each other. It is formed by a beta hairpin changing direction with one strand having a glycine residue and the other strand having a beta bulge. Beta corners have no known function.
Rossman fold
Rossman folds, named after Michael Rossman, consist of 3 beta strands and 2 helices in an alternating fashion: beta strand, helix, beta strand, helix, beta strand. This motif tends to reverse the direction of the chain within a protein. Rossman folds have an important biological function in binding nucleotides such as NAD within most dehydrogenases.