Editing Keratin (section)

=== Type I and II keratins ===
The human genome has 54 functional annotated keratin genes, of which 28 are [[type I keratin]]s and 26 are [[type II keratin]]s.<ref>{{cite book | vauthors = Bernot KM, Coulombe PA, Zaher H | chapter = Cytoskeleton &#124; Intermediate Filaments | title = Encyclopedia of Biological Chemistry III | pages = 193–199 | date = 2021 | doi = 10.1016/B978-0-12-819460-7.00037-2 | quote = Type I and type II IFs are part of the keratin (or cytokeratin) family of proteins found in all epithelia. The human genome features 54 functional keratin genes, with 28 type I and 26 type II keratin genes (see Table 1). Type I keratins tend to be smaller and acidic compared to the larger, neutral–basic type II keratins. | isbn = 978-0-12-822040-5 }}</ref> 
[[File:Keratin.jpg|thumb|Keratin (high molecular weight) in [[bile duct]] cell and oval cells of [[horse]] [[liver]].]]

Fibrous keratin molecules supercoil to form a very stable, left-handed [[superhelix|superhelical]] motif to multimerise, forming filaments consisting of multiple copies of the keratin [[monomer]].<ref name="Voet_1998">{{Cite book | vauthors = Voet D, Voet JG, Pratt CW | chapter = Proteins: Three-Dimensional Structure | title = Fundamentals of Biochemistry | pages = 158 | date = 1998 | quote = Fibrous proteins are characterized by a single type of secondary structure: a keratin is a left-handed coil of two a helices | chapter-url = http://biochem118.stanford.edu/Papers/Protein%20Papers/Voet%26Voet%20chapter6.pdf | archive-url = https://web.archive.org/web/20060917080333/http://biochem118.stanford.edu/Papers/Protein%20Papers/Voet%26Voet%20chapter6.pdf | archive-date = 2006-09-17 | url-status = live | publisher = Wiley | isbn = 978-0-471-58650-0 }}</ref>

The major force that keeps the coiled-coil structure is [[Hydrophobicity|hydrophobic interactions]] between [[apolar]] residues along the keratin's helical segments.<ref name="Hanukoglu_2014">{{cite journal | vauthors = Hanukoglu I, Ezra L | title = Proteopedia entry: Coiled-coil structure of keratins: Multimedia in Biochemistry and Molecular Biology Education | journal = Biochemistry and Molecular Biology Education : a Bimonthly Publication of the International Union of Biochemistry and Molecular Biology | volume = 42 | issue = 1 | pages = 93–94 | date = January 2014 | pmid = 24265184 | doi = 10.1002/bmb.20746 | doi-access = free }}</ref>

Limited interior space is the reason why the [[triple helix]] of the (unrelated) structural protein [[collagen]], found in [[skin]], [[cartilage]] and [[bone]], likewise has a high percentage of [[glycine]]. The connective tissue protein [[elastin]] also has a high percentage of both glycine and [[alanine]]. [[Silk]] [[fibroin]], considered a β-keratin, can have these two as 75–80% of the total, with 10–15% [[serine]], with the rest having bulky side groups. The chains are antiparallel, with an alternating C → N orientation.<ref>{{cite web | title = Secondary Protein | url = http://elmhurst.edu/~chm/vchembook/566secprotein.html | publisher = Elmhurst.edu | access-date = 2010-09-23 | url-status = dead | archive-url = https://web.archive.org/web/20100922111144/http://elmhurst.edu/~chm/vchembook/566secprotein.html | archive-date = 2010-09-22 }}</ref> A preponderance of [[amino acid]]s with small, [[chemical reaction|nonreactive]] side groups is characteristic of structural proteins, for which H-bonded close packing is more important than [[chemical specificity]].