Editing Collagen helix (section)

==Structure==
Glycine, proline, and hydroxyproline must be in their designated positions with the correct configuration. For example, hydroxyproline in the Y position increases the thermal stability of the triple helix, but not when it is located in the X position.<ref>{{cite journal | last1=Berisio | first1=Rita | last2=Vitagliano | first2=Luigi | last3=Mazzarella | first3=Lelio | last4=Zagari | first4=Adriana | title=Crystal structure of the collagen triple helix model [(Pro-Pro-Gly)(10)](3) | journal=Protein Science | volume=11 | issue=2 | date=2002 | issn=0961-8368 | pmid=11790836 | pmc=2373432 | doi=10.1110/ps.32602 | pages=262–270}}</ref> The thermal stabilization is also hindered when the hydroxyl group has the wrong configuration. Due to the high abundance of glycine and proline contents, collagen fails to form a regular α-helix and β-sheet structure. Three left-handed helical strands twist to form a right-handed triple helix.<ref>{{cite journal | last=Bella | first=Jordi | title=Collagen structure: new tricks from a very old dog | journal=Biochemical Journal | volume=473 | issue=8 | date=2016-04-15 | issn=0264-6021 | doi=10.1042/BJ20151169 | pages=1001–1025| pmid=27060106 }}</ref> A collagen triple helix has 3.3 residues per turn.<ref>{{cite book |title=Harpers Illustrated Biochemistry |edition=30th }}</ref>

Each of the three chains is stabilized by the [[steric repulsion]] due to the [[pyrrolidine]] rings of [[proline]] and [[hydroxyproline]] [[residue (biochemistry)|residue]]s. The pyrrolidine rings keep out of each other's way when the [[polypeptide]] chain assumes this extended [[Helix|helical]] form, which is much more open than the tightly coiled form of the [[alpha helix]].
The three chains are [[hydrogen bond]]ed to each other. The [[Hydrogen bonding#Bonding|hydrogen bond donors]] are the [[peptide]] NH groups of [[glycine]] residues. The [[Hydrogen bonding#Bonding|hydrogen bond acceptors]] are the CO groups of residues on the other chains. The OH group of [[hydroxyproline]] does not participate in hydrogen bonding but stabilises the trans isomer of proline by stereoelectronic effects, therefore stabilizing the entire triple helix.

The rise of the collagen helix ([[superhelix]]) is 2.9 Å (0.29&nbsp;nm) per residue. The center of the collagen triple helix is very small and hydrophobic, and every third residue of the helix must have contact with the center.<ref name="BB">{{cite journal | last1=Brodsky | first1=Barbara | last2=Thiagarajan | first2=Geetha | last3=Madhan | first3=Balaraman | last4=Kar | first4=Karunakar | title=Triple-helical peptides: An approach to collagen conformation, stability, and self-association | journal=Biopolymers | volume=89 | issue=5 | date=2008 | issn=0006-3525 | doi=10.1002/bip.20958 | pages=345–353| pmid=18275087 }}</ref> Due to the very tiny and tight space at the center, only the small hydrogen of the glycine side chain is capable of interacting with the center.<ref name="BB" /> This contact is impossible even when a slightly bigger amino acid residue is present other than glycine.