Editing Vitamin B6 (section)

==Functions==
PLP is involved in many aspects of macronutrient metabolism, [[neurotransmitter]] synthesis, [[histamine]] synthesis, [[hemoglobin]] synthesis and function, and [[gene expression]]. PLP generally serves as a [[coenzyme]] (cofactor) for many reactions including [[decarboxylation]], [[transamination]], [[racemization]], [[elimination reaction|elimination]], [[single-displacement reaction|replacement]], and beta-group interconversion.<ref name=lpi/><ref name=PKIN2020B6 /><ref name="Combs">{{cite book|url=https://books.google.com/books?id=1CMHiWum0Y4C|title=The Vitamins: Fundamental Aspects in Nutrition and Health|vauthors=Combs GF|publisher=Elsevier Academic Press|year=2007|isbn=978-0-8121-0661-9|edition=3rd|location=San Diego|pages=320–324|lccn=2007026776|oclc=150255807|access-date=April 20, 2018|archive-date=December 31, 2023|archive-url=https://web.archive.org/web/20231231072054/https://books.google.com/books?id=1CMHiWum0Y4C|url-status=live}}</ref>

===Amino acid metabolism===
# [[Transaminase]]s break down amino acids with PLP as a cofactor. The proper activity of these enzymes is crucial for the process of moving [[amine]] groups from one amino acid to another. To function as a transaminase coenzyme, PLP bound to a [[lysine]] of the enzyme then binds to a free amino acid via formation of a [[Schiff's base]]. The process then dissociates the amine group from the amino acid, releasing a [[keto acid]], then transfers the amine group to a different keto acid to create a new amino acid.<ref name=PKIN2020B6 />
# [[Serine racemase]] which synthesizes the neuromodulator [[D-serine]] from its [[enantiomer]] is a PLP-dependent enzyme.
# PLP is a coenzyme needed for the proper function of the enzymes [[cystathionine synthase]] and [[cystathionase]]. These enzymes catalyze reactions in the catabolism of [[methionine]]. Part of this pathway (the reaction catalyzed by [[cystathionase]]) also produces [[cysteine]].
# [[Selenomethionine]] is the primary dietary form of [[selenium]]. PLP is needed as a cofactor for the enzymes that allow selenium to be used from the dietary form. PLP also plays a cofactor role in releasing selenium from selenohomocysteine to produce [[hydrogen selenide]], which can then be used to incorporate selenium into [[selenoprotein]]s.
# PLP is required for the conversion of [[tryptophan]] to [[Niacin (substance)|niacin]], so low vitamin B<sub>6</sub> status impairs this conversion.<ref name="Combs"/>

===Neurotransmitters===
PLP is a cofactor in the biosynthesis of five important [[neurotransmitters]]: [[serotonin]], [[dopamine]], [[epinephrine]], [[norepinephrine]], and [[gamma-aminobutyric acid]].<ref name="Parra2018"/>

===Glucose metabolism===
PLP is a required coenzyme of [[glycogen phosphorylase]], the enzyme necessary for [[glycogenolysis]]. [[Glycogen]] serves as a carbohydrate storage molecule, primarily found in muscle, liver and brain. Its breakdown frees up glucose for energy.<ref name="Parra2018"/> PLP also catalyzes transamination reactions that are essential for providing amino acids as a substrate for [[gluconeogenesis]], the biosynthesis of glucose.<ref name="Combs"/>

===Lipid metabolism===
PLP is an essential component of enzymes that facilitate the biosynthesis of [[sphingolipid]]s.<ref name="Combs"/> Particularly, the synthesis of [[ceramide]] requires PLP. In this reaction, serine is decarboxylated and combined with [[palmitoyl-CoA]] to form [[sphinganine]], which is combined with a fatty [[acyl-CoA]] to form dihydroceramide. This compound is then [[Dihydroceramide desaturase|further desaturated]] to form ceramide. In addition, the breakdown of sphingolipids is also dependent on vitamin B<sub>6</sub> because [[sphingosine-1-phosphate lyase]], the enzyme responsible for breaking down [[sphingosine-1-phosphate]], is also PLP-dependent.

===Hemoglobin synthesis and function===
PLP aids in the synthesis of [[hemoglobin]], by serving as a coenzyme for the enzyme [[aminolevulinic acid synthase]].<ref name="Parra2018"/> It also binds to two sites on hemoglobin to enhance the oxygen binding of hemoglobin.<ref name="Combs"/>

===Gene expression===
PLP has been implicated in increasing or decreasing the expression of certain [[gene]]s. Increased intracellular levels of the vitamin lead to a decrease in the [[transcription (genetics)|transcription]] of [[glucocorticoid]]s. Vitamin B<sub>6</sub> deficiency leads to the increased [[gene expression]] of [[albumin]] [[mRNA]]. Also, PLP influences expression of [[glycoprotein]] IIb by interacting with various [[transcription factor]]s; the result is inhibition of [[platelet]] aggregation.<ref name="Combs"/>

===In plants===
Plant synthesis of vitamin B<sub>6</sub> contributes to protection from sunlight. [[Ultraviolet#Subtypes|Ultraviolet-B radiation]] (UV-B) from sunlight stimulates plant growth, but in high amounts can increase production of tissue-damaging [[reactive oxygen species]] (ROS), i.e., [[oxidants]]. Using ''[[Arabidopsis thaliana]]'' (common name: thale cress), researchers demonstrated that UV-B exposure increased pyridoxine biosynthesis, but in a mutant variety, pyridoxine biosynthesis capacity was not [[Enzyme induction and inhibition|inducible]], and as a consequence, ROS levels, [[lipid peroxidation]], and cell proteins associated with tissue damage were all elevated.<ref name="Havaux2009">{{cite journal |vauthors=Havaux M, Ksas B, Szewczyk A, Rumeau D, Franck F, Caffarri S, Triantaphylidès C |title=Vitamin B6 deficient plants display increased sensitivity to high light and photo-oxidative stress |journal=BMC Plant Biol |volume=9 |issue= 1|pages=130 |date=November 2009 |pmid=19903353 |pmc=2777905 |doi=10.1186/1471-2229-9-130 |url= |doi-access=free |bibcode=2009BMCPB...9..130H }}</ref><ref>{{cite journal |vauthors=Ristilä M, Strid H, Eriksson LA, Strid A, Sävenstrand H |title=The role of the pyridoxine (vitamin B6) biosynthesis enzyme PDX1 in ultraviolet-B radiation responses in plants |journal=Plant Physiol Biochem |volume=49 |issue=3 |pages=284–92 |date=March 2011 |pmid=21288732 |doi=10.1016/j.plaphy.2011.01.003 |url=https://oru.diva-portal.org/smash/get/diva2:137027/FULLTEXT01 |access-date=March 20, 2024 |archive-date=January 6, 2024 |archive-url=https://web.archive.org/web/20240106034824/http://oru.diva-portal.org/smash/get/diva2:137027/FULLTEXT01 |url-status=live }}</ref><ref>{{cite journal |vauthors=Czégény G, Kőrösi L, Strid A, Hideg E |title=Multiple roles for Vitamin B<sub>6</sub> in plant acclimation to UV-B |journal=Scientific Reports |volume=9 |issue= 1|pages=1259 |date=February 2019 |pmid= 30718682|doi=10.1038/s41598-018-38053-w |url=|pmc=6361899 |bibcode=2019NatSR...9.1259C }}</ref> Biosynthesis of [[chlorophyll]] depends on aminolevulinic acid synthase, a PLP-dependent enzyme that uses [[succinyl-CoA]] and [[glycine]] to generate [[aminolevulinic acid]], a chlorophyll precursor.<ref name="Parra2018"/> In addition, plant mutants with severely limited capacity to synthesize vitamin B<sub>6</sub> have stunted root growth, because synthesis of [[plant hormone]]s such as [[auxin]] require the vitamin as an enzyme cofactor.<ref name="Parra2018"/>