Editing Ricin (section)

=== Structure ===
In terms of structure, ricin closely resembles abrin-a, an isomer of [[abrin]]. The [[Protein quaternary structure|quaternary structure]] of ricin is a globular, glycosylated heterodimer of approximately 60–65 [[Dalton (unit)|kDa]].<ref name="pmid4730499"/> Ricin toxin A chain and ricin toxin B chain are of similar molecular weights, approximately 32 kDa and 34 kDa, respectively.[[File:Alignment Abrin Ricin.png|thumb|A comparison of the similar structures of abrin-a (red) and ricin (blue)|alt=|left]]
* '''Ricin toxin A chain''' (RTA) is an ''N''-[[glycoside hydrolase]] composed of 267 amino acids.<ref name="pmid4730499">{{cite journal | vauthors = Olsnes S, Pihl A | title = Different biological properties of the two constituent peptide chains of ricin, a toxic protein inhibiting protein synthesis | journal = Biochemistry | volume = 12 | issue = 16 | pages = 3121–3126 | date = July 1973 | pmid = 4730499 | doi = 10.1021/bi00740a028 }}</ref> It has three structural domains with approximately 50% of the [[polypeptide]] arranged into [[alpha-helix|alpha-helices]] and [[beta-sheet]]s.<ref name="pmid7990130">{{cite journal | vauthors = Weston SA, Tucker AD, Thatcher DR, Derbyshire DJ, Pauptit RA | title = X-ray structure of recombinant ricin A-chain at 1.8 A resolution | journal = Journal of Molecular Biology | volume = 244 | issue = 4 | pages = 410–422 | date = December 1994 | pmid = 7990130 | doi = 10.1006/jmbi.1994.1739 }}</ref> The three domains form a pronounced cleft that is the active site of RTA.
* '''Ricin toxin B chain''' (RTB) is a [[lectin]] composed of 262 amino acids that is able to bind terminal [[galactose]] residues on cell surfaces.<ref name="pmid1717462">{{cite journal | vauthors = Wales R, Richardson PT, Roberts LM, Woodland HR, Lord JM | title = Mutational analysis of the galactose binding ability of recombinant ricin B chain | journal = The Journal of Biological Chemistry | volume = 266 | issue = 29 | pages = 19172–19179 | date = October 1991 | pmid = 1717462 | doi = 10.1016/S0021-9258(18)54978-4 | doi-access = free }}</ref> RTB forms a bilobal, barbell-like structure lacking [[alpha helix|alpha-helices]] or [[beta sheet|beta-sheets]] where individual lobes contain three [[protein domain|subdomains]]. At least one of these three subdomains in each homologous lobe possesses a sugar-binding pocket that gives RTB its functional character.
While other plants contain the protein chains found in ricin, both protein chains must be present to produce toxic effects. For example, plants that contain only protein chain A, such as [[barley]], are not toxic because without the link to protein chain B, protein chain A cannot enter the cell and do damage to ribosomes.<ref name="Harkup-2015">{{Cite book|title = A is For Arsenic: The poisons of Agatha Christie| author = [[Kathryn Harkup]] |publisher = Bloomsbury Sigma|year = 2015|isbn = 978-1-4729-1130-8|location = London|pages = [https://archive.org/details/isforarsenicpois0000hark/page/222 222–236]|url = https://archive.org/details/isforarsenicpois0000hark/page/222}}</ref>