Editing Transmembrane protein (section)

===Classification by structure===
There are two basic types of transmembrane proteins:<ref name="Xiong2006">{{cite book|author=Jin Xiong|title=Essential bioinformatics|url=https://books.google.com/books?id=AFsu7_goA8kC&pg=PA208|access-date=13 November 2010|year=2006|publisher=Cambridge University Press|isbn=978-0-521-84098-9|pages=208–}}</ref> [[alpha-helix|alpha-helical]] and [[beta barrel]]s. Alpha-helical proteins are present in the inner membranes of bacterial cells or the plasma membrane of eukaryotic cells, and sometimes in the [[bacterial outer membrane]].<ref>alpha-helical proteins in outer membranes include [[Stannin]] and certain [[lipoproteins]], and others</ref> This is the major category of transmembrane proteins. In humans, 27% of all proteins have been estimated to be alpha-helical membrane proteins.<ref>{{cite journal |vauthors=Almén MS, Nordström KJ, Fredriksson R, Schiöth HB |title=Mapping the human membrane proteome: a majority of the human membrane proteins can be classified according to function and evolutionary origin |journal=BMC Biol. |volume=7 |pages=50 |year=2009 |pmid=19678920 |pmc=2739160 |doi=10.1186/1741-7007-7-50 |doi-access=free }}</ref>
Beta-barrel proteins are so far found only in outer membranes of [[gram-negative bacteria]], [[cell wall]]s of [[gram-positive bacteria]], [[Outer mitochondrial membrane|outer membranes]] of [[mitochondria]] and [[chloroplasts]], or can be secreted as [[pore-forming toxin]]s. All beta-barrel transmembrane proteins have simplest up-and-down topology, which may reflect their common evolutionary origin and similar folding mechanism.<ref>{{cite web |title=Types of Transmembrane Proteins |url=https://www.wikidoc.org/index.php/Transmembrane_protein#Thermodynamic_stability_and_folding |website=WikiDoc |access-date=30 January 2025}}</ref>

In addition to the protein domains, there are unusual transmembrane elements formed by peptides. A typical example is [[gramicidin A]], a peptide that forms a dimeric transmembrane β-helix.<ref>{{Cite journal|last1=Nicholson|first1=L. K.|last2=Cross|first2=T. A.|date=1989|title=Gramicidin cation channel: an experimental determination of the right-handed helix sense and verification of .beta.-type hydrogen bonding|journal=Biochemistry|volume=28|issue=24|pages=9379–9385|language=en|doi=10.1021/bi00450a019|pmid=2482072}}</ref> This peptide is secreted by [[gram-positive bacteria]] as an [[Antimicrobial peptides|antibiotic]]. A transmembrane [[Polyproline helix|polyproline-II helix]] has not been reported in natural proteins. Nonetheless, this structure was experimentally observed in specifically designed artificial peptides.<ref>{{Cite journal|last1=Kubyshkin|first1=Vladimir|last2=Grage|first2=Stephan L.|last3=Ulrich|first3=Anne S.|last4=Budisa|first4=Nediljko|date=2019|title=Bilayer thickness determines the alignment of model polyproline helices in lipid membranes|journal=Physical Chemistry Chemical Physics|volume=21|issue=40|pages=22396–22408|language=en|doi=10.1039/c9cp02996f|pmid=31577299|bibcode=2019PCCP...2122396K|doi-access=free}}</ref>