Editing Alpha helix (section)

== Functional roles ==
[[Image:Coiled-coil TF Max on DNA.jpg|thumb|left|200px|Leucine zipper coiled-coil helices & [[Basic helix-loop-helix|DNA-binding helices]]: transcription factor [[MAX (gene)|Max]] ([[Protein Data Bank|PDB]] file 1HLO)]]
[[Image:1gzm opm.png|thumb|right|200px|Bovine rhodopsin ([[Protein Data Bank|PDB]] file 1GZM), with a bundle of seven helices crossing the membrane (membrane surfaces marked by horizontal lines)]]

=== DNA binding ===
α-Helices have particular significance in [[DNA]] binding motifs, including [[helix-turn-helix]] motifs, [[leucine zipper]] motifs and [[zinc finger]] motifs. This is because of the convenient structural fact that the diameter of an α-helix is about {{cvt|12|Å|nm}} including an average set of sidechains, about the same as the width of the major groove in B-form [[DNA]], and also because [[coiled-coil]] (or leucine zipper) dimers of helices can readily position a pair of interaction surfaces to contact the sort of symmetrical repeat common in double-helical DNA.<ref>Branden & Tooze, chapter 10</ref>  An example of both aspects is the [[transcription factor]] Max (see image at left), which uses a helical coiled coil to dimerize, positioning another pair of helices for interaction in two successive turns of the DNA major groove.

=== 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>

=== Mechanical properties ===
α-Helices under axial tensile deformation, a characteristic loading condition that appears in many alpha-helix-rich filaments and tissues, results in a characteristic three-phase behavior of stiff-soft-stiff tangent modulus.<ref>{{cite journal | vauthors = Ackbarow T, Chen X, Keten S, Buehler MJ | title = Hierarchies, multiple energy barriers, and robustness govern the fracture mechanics of alpha-helical and beta-sheet protein domains | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 104 | issue = 42 | pages = 16410–5 | date = October 2007 | pmid = 17925444 | pmc = 2034213 | doi = 10.1073/pnas.0705759104 | bibcode = 2007PNAS..10416410A | doi-access = free }}</ref> Phase I corresponds to the small-deformation regime during which the helix is stretched homogeneously, followed by phase II, in which alpha-helical turns break mediated by the rupture of groups of H-bonds. Phase III is typically associated with large-deformation covalent bond stretching.