Editing Hemicellulose (section)

== Biosynthesis ==
Hemicelluloses are synthesised from sugar nucleotides in the cell's [[Golgi apparatus]].<ref name="Zhu-2019">{{Citation|last1=Zhu|first1=Xiaoyu|title=Cellulose and Hemicellulose Synthesis and Their Regulation in Plant Cells|date=2019|work=Extracellular Sugar-Based Biopolymers Matrices|volume=12|pages=303–353|editor-last=Cohen|editor-first=Ephraim|publisher=Springer International Publishing|language=en|doi=10.1007/978-3-030-12919-4_7|isbn=978-3-030-12918-7|last2=Xin|first2=Xiaoran|last3=Gu|first3=Ying|s2cid=198238237|editor2-last=Merzendorfer|editor2-first=Hans}}</ref> Two models explain their synthesis: 1) a '2 component model' where modification occurs at two transmembrane proteins, and 2) a '1 component model' where modification occurs only at one transmembrane protein. After synthesis, hemicelluloses are transported to the plasma membrane via Golgi vesicles.

Each kind of hemicellulose is biosynthesized by specialized enzymes.<ref name="Zhu-2019" /><ref name="Pauly-2013">{{Cite journal|last1=Pauly|first1=Markus|last2=Gille|first2=Sascha|last3=Liu|first3=Lifeng|last4=Mansoori|first4=Nasim|last5=de Souza|first5=Amancio|last6=Schultink|first6=Alex|last7=Xiong|first7=Guangyan|date=2013|title=Hemicellulose biosynthesis|journal=Planta|language=en|volume=238|issue=4|pages=627–642|doi=10.1007/s00425-013-1921-1|pmid=23801299|bibcode=2013Plant.238..627P |s2cid=17501948|issn=0032-0935}}</ref>

Mannan chain backbones are synthesized by cellulose synthase-like protein family A (CSLA) and possibly enzymes in cellulose synthase-like protein family D (CSLD).<ref name="Zhu-2019" /><ref name="Pauly-2013" /> Mannan synthase, a particular enzyme in CSLA, is responsible for the addition of mannose units to the backbone.<ref name="Zhu-2019" /><ref name="Pauly-2013" /> The galactose side-chains of some mannans are added by galactomannan galactosyltransferase.<ref name="Zhu-2019" /><ref name="Pauly-2013" /> [[Acetylation]] of mannans is mediated by a mannan O-acetyltransferase, however, this enzyme has not been definitively identified.<ref name="Pauly-2013" />

Xyloglucan backbone synthesis is mediated by cellulose synthase-like protein family C (CSLC), particularly [[glucan synthase]], which adds glucose units to the chain.<ref name="Zhu-2019" /><ref name="Pauly-2013" /> Backbone synthesis of xyloglucan is also mediated in some way by [[xylosyltransferase]], but this mechanism is separate to its transferase function and remains unclear.<ref name="Pauly-2013" /> Xylosyltransferase in its transferase function is, however, utilized for the addition of xylose to the side-chain.<ref name="Zhu-2019" /><ref name="Pauly-2013" /> Other enzymes utilized for side-chain synthesis of xyloglucan include [[galactosyltransferase]] (which is responsible for the addition of [galactose and of which two different forms are utilized), [[fucosyltransferase]] (which is responsible for the addition of fucose), and [[acetyltransferase]] (which is responsible for acetylation).<ref name="Zhu-2019" /><ref name="Pauly-2013" />

Xylan backbone synthesis, unlike that of the other hemicelluloses, is not mediated by any cellulose synthase-like proteins.<ref name="Pauly-2013" /> Instead, xylan synthase is responsible for backbone synthesis, facilitating the addition of xylose.<ref name="Pauly-2013" /> Several genes for xylan synthases have been identified.<ref name="Pauly-2013" /> Several other enzymes are utilized for the addition and modification of the side-chain units of xylan, including [[glucuronosyltransferase]] (which adds [glucuronic acid units), xylosyltransferase (which adds additional xylose units), [[arabinosyltransferase]] (which adds arabinose), [[methyltransferase]] (responsible for [[methylation]]), and acetyltransferase] (responsible for acetylation).<ref name="Pauly-2013" /> Given that mixed-linkage glucan is a non-branched homopolymer of glucose, there is no side-chain synthesis, only the addition of glucose to the backbone in two linkages, β1-3 and β1-4.<ref name="Pauly-2013" /> Backbone synthesis is mediated by enzymes in cellulose synthase-like protein families F and H (CSLF and CSLH), specifically glucan synthase.<ref name="Zhu-2019" /><ref name="Pauly-2013" /> Several forms of glucan synthase from CSLF and CSLH have been identified.<ref name="Zhu-2019" /><ref name="Pauly-2013" /> All of them are responsible for addition of glucose to the backbone and all are capable of producing both β1-3 and β1-4 linkages, however, it is unknown how much each specific enzyme contributes to the distribution of β1-3 and β1-4 linkages.<ref name="Zhu-2019" /><ref name="Pauly-2013" />