Editing Chemotaxis (section)

== Chemoattractants and chemorepellents ==
Chemoattractants and chemorepellents are [[inorganic]] or [[Organic compound|organic]] substances possessing chemotaxis-inducer effect in motile cells. These chemotactic [[ligand]]s create chemical concentration gradients that organisms, prokaryotic and eukaryotic, move toward or away from, respectively.<ref name=":0">{{cite journal | vauthors = Xu F, Bierman R, Healy F, Nguyen H |title=A multi-scale model of Escherichia coli chemotaxis from intracellular signaling pathway to motility and nutrient uptake in nutrient gradient and isotropic fluid environments|doi = 10.1016/j.camwa.2015.12.019 |journal=Computers & Mathematics with Applications|volume=71| issue = 11 |pages=2466–2478 | year=2016| doi-access = free }}</ref>
[[File:Chtx-AttrRep-en.png|right|250x250px|float]]
Effects of chemoattractants are elicited via chemoreceptors such as [[methyl-accepting chemotaxis protein]]s (MCP).<ref name="twenty">{{cite journal | vauthors = Szurmant H, Ordal GW | title = Diversity in chemotaxis mechanisms among the bacteria and archaea | journal = Microbiology and Molecular Biology Reviews | volume = 68 | issue = 2 | pages = 301–19 | date = June 2004 | pmid = 15187186 | pmc = 419924 | doi = 10.1128/MMBR.68.2.301-319.2004 }}</ref> MCPs in E.coli include Tar, Tsr, Trg and Tap.<ref name="four">{{cite journal | vauthors = Yamamoto K, Macnab RM, Imae Y | title = Repellent response functions of the Trg and Tap chemoreceptors of Escherichia coli | journal = Journal of Bacteriology | volume = 172 | issue = 1 | pages = 383–8 | date = January 1990 | pmid = 2403544 | pmc = 208443 | doi = 10.1128/jb.172.1.383-388.1990 }}</ref> Chemoattracttants to Trg include [[ribose]] and [[galactose]] with [[phenol]] as a chemorepellent. Tap and Tsr recognize [[dipeptide]]s and [[serine]] as chemoattractants, respectively.<ref name="four"/>

Chemoattractants or chemorepellents bind MCPs at its extracellular domain; an intracellular signaling domain relays the changes in concentration of these chemotactic ligands to downstream proteins like that of CheA which then relays this signal to flagellar motors via phosphorylated CheY (CheY-P).<ref name="twenty" /> CheY-P can then control flagellar rotation influencing the direction of cell motility.<ref name="twenty"/>

For ''E.coli'', ''[[S. meliloti]]'', and ''[[Rhodobacter sphaeroides|R. spheroides]],'' the binding of chemoattractants to MCPs inhibit CheA and therefore CheY-P activity, resulting in smooth runs, but for ''[[Bacillus subtilis|B. substilis]]'', CheA activity increases.<ref name="twenty"/> Methylation events in ''E.coli'' cause MCPs to have lower affinity to chemoattractants which causes increased activity of CheA and CheY-P resulting in tumbles.<ref name="twenty" /> In this way cells are able to adapt to the immediate chemoattractant concentration and detect further changes to modulate cell motility.<ref name="twenty"/>

Chemoattractants in eukaryotes are well characterized for immune cells. [[Formyl]] [[peptide]]s, such as [[N-Formylmethionine-leucyl-phenylalanine|fMLF]], attract [[White blood cell|leukocytes]] such as [[neutrophil]]s and [[macrophage]]s, causing movement toward infection sites.<ref name="shiffmann">{{cite journal | vauthors = Schiffmann E, Corcoran BA, Wahl SM | title = N-formylmethionyl peptides as chemoattractants for leucocytes | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 72 | issue = 3 | pages = 1059–62 | date = March 1975 | pmid = 1093163 | pmc = 432465 | doi = 10.1073/pnas.72.3.1059 | bibcode = 1975PNAS...72.1059S | doi-access = free }}</ref> Non-acylated methioninyl peptides do not act as chemoattractants to neutrophils and macrophages.<ref name="shiffmann" /> Leukocytes also move toward chemoattractants C5a, a [[Complement system|complement]] component, and [[pathogen]]-specific ligands on bacteria.<ref name="shiffmann" />

Mechanisms concerning chemorepellents are less known than chemoattractants. Although chemorepellents work to confer an avoidance response in organisms, [[Tetrahymena|''Tetrahymena thermophila'']]  adapt to a chemorepellent, [[Netrin-1 peptide]], within 10 minutes of exposure; however, exposure to chemorepellents such as [[Guanosine triphosphate|GTP]], [[PACAP-38]], and [[nociceptin]] show no such adaptations.<ref>{{cite journal | vauthors = Kuruvilla H, Schmidt B, Song S, Bhajjan M, Merical M, Alley C, Griffin C, Yoder D, Hein J, Kohl D, Puffenberger C, Petroff D, Newcomer E, Good K, Heston G, Hurtubise A | title = Netrin-1 Peptide Is a Chemorepellent in Tetrahymena thermophila | journal = International Journal of Peptides | volume = 2016 | pages = 7142868 | date = 2016 | pmid = 27123011 | pmc = 4830718 | doi = 10.1155/2016/7142868 | doi-access = free }}</ref> GTP and [[Adenosine triphosphate|ATP]] are chemorepellents in micro-molar concentrations to both ''Tetrahymena'' and ''[[Paramecium]]''. These organisms avoid these molecules by producing avoiding reactions to re-orient themselves away from the gradient.<ref>{{cite journal | vauthors = Hennessey TM | title = Responses of the ciliates Tetrahymena and Paramecium to external ATP and GTP | language = en | journal = Purinergic Signalling | volume = 1 | issue = 2 | pages = 101–10 | date = June 2005 | pmid = 18404496 | pmc = 2096533 | doi = 10.1007/s11302-005-6213-1 }}</ref>