that are usually nearly parallel or antiparallel to each other.
Three-helix bundles
Three-helix bundles are among the smallest and fastest known cooperatively folding structural domains.
villin headpiece domain is only 36 amino acids long and is a common subject of study in molecular dynamics simulations because its microsecond-scale folding time is within the timescales accessible to simulation.[2][3] The 40-residue HIV accessory protein has a very similar fold and has also been the subject of extensive study.[4] There is no general sequence motif associated with three-helix bundles, so they cannot necessarily be predicted from sequence alone. Three-helix bundles often occur in actin-binding proteins and in DNA-binding proteins
.
Four-helix bundles
Four-helix bundles typically consist of four helices packed in a
The specific topology of the helices is dependent on the protein – helices that are adjacent in sequence are often
human growth hormone, cytokine,[5] and Lac repressor C-terminal. The four-helix bundle fold has proven an attractive target for de novo protein design, with numerous de novo four-helix bundle proteins having been successfully designed by rational[6] and by combinatorial[7]
methods. Although sequence is not conserved among four-helix bundles, sequence patterns tend to mirror those of coiled-coil structures in which every fourth and seventh residue is hydrophobic.