If an enzyme name is shown in bold, there is experimental evidence for this enzymatic activity.
Locations of Mapped Genes:
Synonyms: glyoxylate bypass, glyoxylate shunt
|Superclasses:||Generation of Precursor Metabolites and Energy|
Pathway Summary from MetaCyc:
The glyoxylate cycle is a sequence of anaplerotic reactions (reactions that form metabolic intermediates for biosynthesis) that enables an organism to use substrates that enter central carbon metabolism at the level of acetyl-CoA as the sole carbon source. Such substrates include fatty acids, alcohols, and esters (often the products of fermentation), as well as waxes, alkenes, and methylated compounds. The pathway does not occur in vertebrates, but it is found in plants and certain bacteria, fungi, and invertebrates.
The pathway is essentially a modified version of the TCA cycle I (prokaryotic) that bypasses those steps in the cycle that lead to a loss of CO2. Acetyl-CoA enters the cycle at two steps, but no carbon escapes it in the form of CO2.
The glyoxylate cycle uses a two-step bypass. One key enzyme, isocitrate lyase (EC 22.214.171.124), converts D-threo-isocitrate to form succinate and glyoxylate. A second key enzyme, malate synthase (EC 126.96.36.199), condenses glyoxylate and a second molecule of acetyl-CoA to form (S)-malate. The subsequent oxidation of malate regenerates the initial acetyl-CoA acceptor molecule of the TCA cycle, oxaloacetate. Thus, the succinate that was formed by isocitrate lyase (EC 188.8.131.52) can be withdrawn from the cycle and used for cell carbon biosynthesis.
The pathway was originally discovered in bacteria [Kornberg57], but later was found to operate in some eukaryotic organisms as well. In plants the cycle is invovled in the metabolism of storage oils during germination of seeds [Brownleader97]. The cycle also operates in developing eggs of nematodes, where it converts triacylglycerols to carbohydrates [Patel78].
In Escherichia coli the pathway is active when growth on 2 carbon compounds requires conservation of 4 carbon TCA intermediates. Two acetyl-CoA are taken up per turn. The glyoxylate cycle is repressed during growth on glucose and induced by growth on acetate [Cortay89, Walsh84, LaPorte84, Nimmo84a].
Superpathways: superpathway of glyoxylate bypass and TCA
Pathway Evidence Glyph:
Brownleader97: Brownleader, M.D., Harborne, J.B., Dey, P.M. (1997). "Carbohydrate metabolism: primary metabolism of monosaccharides." Plant Biochemistry, Eds Dey & Harborne, Academic Press, Harcourt Brace & Co, Publishers, London.
Cortay89: Cortay JC, Bleicher F, Duclos B, Cenatiempo Y, Gautier C, Prato JL, Cozzone AJ (1989). "Utilization of acetate in Escherichia coli: structural organization and differential expression of the ace operon." Biochimie 71(9-10):1043-1049. PMID: 2512996
Nimmo84a: Nimmo GA, Nimmo HG (1984). "The regulatory properties of isocitrate dehydrogenase kinase and isocitrate dehydrogenase phosphatase from Escherichia coli ML308 and the roles of these activities in the control of isocitrate dehydrogenase." Eur J Biochem 1984;141(2);409-14. PMID: 6329757
Walsh84: Walsh K, Koshland DE (1984). "Determination of flux through the branch point of two metabolic cycles. The tricarboxylic acid cycle and the glyoxylate shunt." J Biol Chem 1984;259(15);9646-54. PMID: 6378912
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