Editing Metabolism (section)

===Carbohydrates and glycans===
{{further|Gluconeogenesis|Glyoxylate cycle|Glycogenesis|Glycosylation}}

In carbohydrate anabolism, simple organic acids can be converted into [[monosaccharide]]s such as [[glucose]] and then used to assemble [[polysaccharide]]s such as [[starch]]. The generation of [[glucose]] from compounds like [[pyruvate]], [[lactic acid|lactate]], [[glycerol]], [[glycerate 3-phosphate]] and [[amino acid]]s is called [[gluconeogenesis]]. Gluconeogenesis converts pyruvate to [[glucose-6-phosphate]] through a series of intermediates, many of which are shared with [[glycolysis]].<ref name="Bouché-2004"/> However, this pathway is not simply [[glycolysis]] run in reverse, as several steps are catalyzed by non-glycolytic enzymes. This is important as it allows the formation and breakdown of glucose to be regulated separately, and prevents both pathways from running simultaneously in a [[futile cycle]].<ref>{{cite journal | vauthors = Boiteux A, Hess B | title = Design of glycolysis | journal = Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences | volume = 293 | issue = 1063 | pages = 5–22 | date = June 1981 | pmid = 6115423 | doi = 10.1098/rstb.1981.0056 | doi-access = free | bibcode = 1981RSPTB.293....5B }}</ref><ref>{{cite journal | vauthors = Pilkis SJ, el-Maghrabi MR, Claus TH | title = Fructose-2,6-bisphosphate in control of hepatic gluconeogenesis. From metabolites to molecular genetics | journal = Diabetes Care | volume = 13 | issue = 6 | pages = 582–99 | date = June 1990 | pmid = 2162755 | doi = 10.2337/diacare.13.6.582 | s2cid = 44741368 }}</ref>

Although fat is a common way of storing energy, in [[vertebrate]]s such as humans the [[fatty acid]]s in these stores cannot be converted to glucose through [[gluconeogenesis]] as these organisms cannot convert acetyl-CoA into [[pyruvate]]; plants do, but animals do not, have the necessary enzymatic machinery.<ref name="Ensign-2006">{{cite journal | vauthors = Ensign SA | title = Revisiting the glyoxylate cycle: alternate pathways for microbial acetate assimilation | journal = Molecular Microbiology | volume = 61 | issue = 2 | pages = 274–6 | date = July 2006 | pmid = 16856935 | doi = 10.1111/j.1365-2958.2006.05247.x | s2cid = 39986630 | doi-access = free }}</ref> As a result, after long-term starvation, vertebrates need to produce [[Ketone body|ketone bodies]] from fatty acids to replace glucose in tissues such as the brain that cannot metabolize fatty acids.<ref>{{cite journal | vauthors = Finn PF, Dice JF | title = Proteolytic and lipolytic responses to starvation | journal = Nutrition | volume = 22 | issue = 7–8 | pages = 830–44 | year = 2006 | pmid = 16815497 | doi = 10.1016/j.nut.2006.04.008 }}</ref> In other organisms such as plants and bacteria, this metabolic problem is solved using the [[glyoxylate cycle]], which bypasses the [[decarboxylation]] step in the citric acid cycle and allows the transformation of acetyl-CoA to [[oxaloacetate]], where it can be used for the production of glucose.<ref name="Ensign-2006"/><ref name="Kornberg-1957">{{cite journal | vauthors = Kornberg HL, Krebs HA | title = Synthesis of cell constituents from C2-units by a modified tricarboxylic acid cycle | journal = Nature | volume = 179 | issue = 4568 | pages = 988–91 | date = May 1957 | pmid = 13430766 | doi = 10.1038/179988a0 | s2cid = 40858130 | bibcode = 1957Natur.179..988K }}</ref> Other than fat, glucose is stored in most tissues, as an energy resource available within the tissue through glycogenesis which was usually being used to maintained glucose level in blood.<ref>{{cite journal| vauthors = Evans RD, Heather LC |date=June 2016|title=Metabolic pathways and abnormalities|journal=Surgery (Oxford)|volume=34|issue=6|pages=266–272|doi=10.1016/j.mpsur.2016.03.010|s2cid=87884121 |issn=0263-9319|url=https://ora.ox.ac.uk/objects/uuid:84c0a8e7-38e9-4de2-ba19-9f129a07987a|access-date=28 August 2020|archive-date=31 October 2020|archive-url=https://web.archive.org/web/20201031143458/https://ora.ox.ac.uk/objects/uuid:84c0a8e7-38e9-4de2-ba19-9f129a07987a|url-status=live}}</ref>

Polysaccharides and [[glycan]]s are made by the sequential addition of monosaccharides by [[glycosyltransferase]] from a reactive sugar-phosphate donor such as [[uridine diphosphate glucose]] (UDP-Glc) to an acceptor [[hydroxyl]] group on the growing polysaccharide. As any of the [[hydroxyl]] groups on the ring of the substrate can be acceptors, the polysaccharides produced can have straight or branched structures.<ref>{{cite book | vauthors = Freeze HH, Hart GW, Schnaar RL | chapter=Glycosylation Precursors |date=2015 |url=http://www.ncbi.nlm.nih.gov/books/NBK453043/ |title=Essentials of Glycobiology| veditors = Varki A, Cummings RD, Esko JD, Stanley P |edition=3rd |place=Cold Spring Harbor (NY) |publisher=Cold Spring Harbor Laboratory Press |pmid=28876856 |access-date=2020-07-08 |doi=10.1101/glycobiology.3e.005|doi-broken-date=1 November 2024 |archive-date=24 February 2022|archive-url=https://web.archive.org/web/20220224114901/https://www.ncbi.nlm.nih.gov/books/NBK453043/|url-status=live}}</ref> The polysaccharides produced can have structural or metabolic functions themselves, or be transferred to lipids and proteins by the enzymes [[oligosaccharyltransferase]]s.<ref>{{cite journal | vauthors = Opdenakker G, Rudd PM, Ponting CP, Dwek RA | title = Concepts and principles of glycobiology | journal = FASEB Journal | volume = 7 | issue = 14 | pages = 1330–7 | date = November 1993 | pmid = 8224606 | doi = 10.1096/fasebj.7.14.8224606 | doi-access = free | s2cid = 10388991 }}</ref><ref>{{cite journal | vauthors = McConville MJ, Menon AK | title = Recent developments in the cell biology and biochemistry of glycosylphosphatidylinositol lipids (review) | journal = Molecular Membrane Biology | volume = 17 | issue = 1 | pages = 1–16 | year = 2000 | pmid = 10824734 | doi = 10.1080/096876800294443 | doi-access = free }}</ref>