Editing Pyridoxal phosphate (section)

==Role as a coenzyme==
PLP acts as a coenzyme in all [[transamination]] reactions, and in certain [[decarboxylation]], [[deamination]], and [[racemization]] reactions of [[amino acid]]s.<ref name=Dolphin>{{cite book | vauthors = Dolphin D, Poulson R, Avramovic O | chapter = Vitamin B6: Pyridoxal Phosphate | volume = 1, Part B | title = Coenzymes and Cofactors | publisher = Wiley Interscience | location = New York | date = 1986 | isbn = 978-0471097853 | chapter-url = http://www.daviddolphin.com/books/book15-volume-preface.pdf | access-date = 2015-01-04 | archive-date = 2016-03-04 | archive-url = https://web.archive.org/web/20160304093445/http://www.daviddolphin.com/books/book15-volume-preface.pdf | url-status = live }}</ref>  The aldehyde group of PLP forms a [[Schiff base|Schiff-base]] linkage (internal [[aldimine]]) with the ε-amino group of a specific lysine group of the [[aminotransferase]] enzyme. The α-amino group of the amino acid substrate displaces the ε-amino group of the active-site lysine residue in a process known as transaldimination. The resulting external aldimine can lose a proton, carbon dioxide, or an amino acid sidechain to become a quinonoid intermediate, which in turn can act as a nucleophile in several reaction pathways.

In transamination, after deprotonation the quinonoid intermediate accepts a proton at a different position to become a [[ketimine]]. The resulting ketimine is hydrolysed so that the amino group remains on the complex.<ref name="pmid15581583">{{cite journal | vauthors = Toney MD | title = Reaction specificity in pyridoxal phosphate enzymes | journal = Archives of Biochemistry and Biophysics | volume = 433 | issue = 1 | pages = 279–87 | date = January 2005 | pmid = 15581583 | doi = 10.1016/j.abb.2004.09.037 }}</ref> In addition, PLP is used by aminotransferases (or transaminases) that act upon unusual sugars such as [[perosamine]] and [[desosamine]].<ref name="samuel">{{cite journal | vauthors = Samuel G, Reeves P | title = Biosynthesis of O-antigens: genes and pathways involved in nucleotide sugar precursor synthesis and O-antigen assembly | journal = Carbohydrate Research | volume = 338 | issue = 23 | pages = 2503–19 | date = November 2003 | pmid = 14670712 | doi = 10.1016/j.carres.2003.07.009 }}</ref> In these reactions, the PLP reacts with [[glutamate]], which transfers its alpha-amino group to PLP to make pyridoxamine phosphate (PMP).  PMP then transfers its nitrogen to the sugar, making an [[amino sugar]].

PLP is also involved in various beta-elimination reactions such as the reactions carried out by [[serine dehydratase]] and [[ColD|GDP-4-keto-6-deoxymannose-3-dehydratase (ColD)]].<ref name="samuel"/>

It is also active in the condensation reaction in [[heme]] synthesis.

PLP plays a role in the conversion of [[levodopa]] into [[dopamine]], facilitates the conversion of the excitatory neurotransmitter glutamate to the inhibitory neurotransmitter [[GABA]], and allows [[S-adenosyl methionine|SAM]] to be decarboxylated to form [[propylamine]], which is a precursor to polyamines.