Editing Alpha helix (section)

=== Membrane spanning ===
α-Helices are also the most common protein structure element that crosses biological membranes ([[transmembrane protein]]),<ref>Branden & Tooze, chapter 12.</ref> presumably because the helical structure can satisfy all backbone hydrogen-bonds internally, leaving no polar groups exposed to the membrane if the sidechains are hydrophobic.  Proteins are sometimes anchored by a single membrane-spanning helix, sometimes by a pair, and sometimes by a helix bundle, most classically consisting of seven helices arranged up-and-down in a ring such as for [[rhodopsin]]s (see image at right) and other [[G protein–coupled receptor]]s (GPCRs). The structural stability between pairs of α-Helical transmembrane domains rely on conserved membrane interhelical packing motifs, for example, the Glycine-xxx-Glycine (or small-xxx-small) motif.<ref>{{cite journal | vauthors = Nash A, Notman R, Dixon AM | title = De novo design of transmembrane helix–helix interactions and measurement of stability in a biological membrane | journal = Biochimica et Biophysica Acta (BBA) - Biomembranes | volume = 1848 | issue = 5 | pages = 1248–57 | date = 2015 | pmid = 25732028 | doi = 10.1016/j.bbamem.2015.02.020 | doi-access = free }}</ref>