Editing Beta sheet (section)

==Parallel β-helices==
[[File:1QRE L3betaHelix.jpg|thumb|right| End-view of a 3-sided, left handed β-helix ({{PDB|1QRE}})]]
A [[beta helix|β-helix]] is formed from repeating structural units consisting of two or three short β-strands linked by short loops. These units "stack" atop one another in a helical fashion so that successive repetitions of the same strand hydrogen-bond with each other in a parallel orientation. See the [[beta helix|β-helix]] article for further information.

In lefthanded β-helices, the strands themselves are quite straight and untwisted; the resulting helical surfaces are nearly flat, forming a regular [[triangular prism]] shape, as shown for the 1QRE archaeal carbonic anhydrase at right. Other examples are the lipid A synthesis enzyme [[LpxA]] and insect antifreeze proteins with a regular array of Thr sidechains on one face that mimic the structure of ice.<ref>{{cite journal | vauthors = Liou YC, Tocilj A, Davies PL, Jia Z | title = Mimicry of ice structure by surface hydroxyls and water of a beta-helix antifreeze protein | journal = Nature | volume = 406 | issue = 6793 | pages = 322–4 | date = July 2000 | pmid = 10917536 | doi = 10.1038/35018604 | bibcode = 2000Natur.406..322L | s2cid = 4385352 }}</ref>

[[File:Pectate lyase R3betaHelix.jpg|thumb|right| End-view of a 3-sided, right-handed β-helix ({{PDB|2PEC}})]]
Righthanded β-helices, typified by the [[pectate lyase]] enzyme shown at left or [[Phage P22 tailspike protein|P22 phage tailspike protein]], have a less regular cross-section, longer and indented on one of the sides; of the three linker loops, one is consistently just two residues long and the others are variable, often elaborated to form a binding or active site.<ref>{{cite book | title =Introduction to Protein Structure | vauthors = Branden C, Tooze J |year=1999 |publisher=Garland |location=New York |isbn=0-8153-2305-0 |pages=20–32 }}</ref> <br>
A two-sided β-helix (right-handed) is found in some bacterial [[metalloprotease]]s; its two loops are each six residues long and bind stabilizing calcium ions to maintain the integrity of the structure, using the backbone and the Asp side chain oxygens of a GGXGXD sequence motif.<ref>{{cite journal | vauthors = Baumann U, Wu S, Flaherty KM, McKay DB | title = Three-dimensional structure of the alkaline protease of Pseudomonas aeruginosa: a two-domain protein with a calcium binding parallel beta roll motif | journal = The EMBO Journal | volume = 12 | issue = 9 | pages = 3357–64 | date = September 1993 | doi = 10.1002/j.1460-2075.1993.tb06009.x | pmid = 8253063 | pmc = 413609 }}</ref> This fold is called a β-roll in the [[Structural Classification of Proteins database|SCOP classification]].