Glyoxylate cycle 3322454 211394148 2008-05-10T03:21:43Z 143.229.181.41 [[Image:Glyoxylatepath.svg|thumb|right|400px|Overview of the Glyoxylate Cycle]] '''The glyoxylate cycle''' is a [[metabolic pathway]] occurring in [[plant]]s, certain [[vertebrate]]s, and several [[microorganism]]s, such as [[E. coli]] and [[yeast]]. The glyoxylate cycle allows these organisms to use [[fat]]s for the synthesis of [[carbohydrate]]s. This sequence of enzyme-catalyzed chemical conversions is absent in most [[vertebrate]]s, including [[human]]s. == The relationship between citric acid and glyoxylate cycles and gluconeogenesis == [[Fatty acid]]s from [[lipid]]s are commonly used as energy source by vertebrates via degradation by [[beta oxidation]] into acetate molecules. This acetate, bound to the active thiol group of [[coenzyme A]], enters the ''[[citric acid cycle]]'' (TCA cycle) where it is fully [[oxidation|oxidized]] to [[carbon dioxide]]. This pathway thus allows [[cell (biology)|cells]] to obtain [[energy]] from fat. To utilize acetate from fat for biosynthesis of carbohydrates, the glyoxylate cycle, whose initial reactions are identical to the TCA cycle, is used. Cell-wall containing organisms, such as [[plant]]s, [[Fungus|fungi]], and [[bacteria]], during [[growth]] require very large amount of [[carbohydrate]]s for biosynthesis of complex structural [[polysaccharides]], such as [[cellulose]], [[glucan]]s, and [[chitin]]. In these organisms, in the absence of available carbohydrates (for example, in certain microbial environments or during [[seed germination]] in plants), the glyoxylate cycle permits synthesis of glucose from lipids via acetate generated in fatty acid β-oxidation. The glyoxylate cycle bypasses the steps in the citric acid cycle where carbon is lost in the form of CO<sub>2</sub>. The two initial steps of glyoxylate cycle are identical to those in the citric acid cycle: ''acetate → citrate → isocitrate''. In the next step, catalyzed by the first glyoxylate cycle enzyme, [[isocitrate lyase]], isocitrate undergoes cleavage into [[succinate]] and [[glyoxylate]] (the latter gives the cycle its name). Glyoxylate condenses with acetyl-CoA (a step catalyzed by [[malate synthase]]), yielding [[malate]]. Both [[malate]] and [[oxaloacetate]] can be converted into [[phosphoenolpyruvate]], which is the substrate of [[phosphoenolpyruvate carboxykinase]] the first enzyme in [[gluconeogenesis]]. The net result of the glyoxylate cycle is therefore the production of glucose from fatty acids. Succinate generated in the first step can enter into the citric acid cycle to eventually form oxaloacetate. In plants the glyoxylate cycle occurs in special [[peroxisome]]s which are called [[glyoxysome]]s. Vertebrates cannot perform this cycle because they lack its two key [[enzyme]]s, isocitrate lyase and malate synthase. [[Category:Metabolic pathways]] [[cs:Glyoxylátový cyklus]] [[de:Glyoxylat-Zyklus]] [[fr:Cycle du glyoxylate]] [[it:Ciclo del gliossilato]] [[he:מעגל הגליוקסילט]] [[ms:Kitar Glioksilat]] [[ja:グリオキシル酸回路]] [[pl:Cykl glioksylanowy]] [[pt:Ciclo do glioxilato]] [[th:วัฏจักรไกลออกซีเลต]]