Subsystem: Glyoxylate bypass
This subsystem's description is:
Many substrates enter the central carbon metabolism on the level of acetyl-coenzyme A (CoA). These substrates include: acetate, fatty acids, waxes, alcohols, alkanes, polyhydroxyalkanoates, etc. In aerobic and facultative anaerobic bacteria OXIDATION of acetyl-CoA proceeds via the citric acid cycle. For the ASSIMILATION of acetyl-CoA the citric acid cycle is modified bypassing the two decarboxylation steps: isocitrate is cleaved into succinate and glyoxylate, which condenses with a second molecule of acetyl-CoA to form malate, resulting in the net fixation of two molecules of acetyl-CoA to one molecule of malate (Kornberg and Krebs, 1957; Ensign, 2006). The malate replenishes oxaloacetate, leaving one succinate and NADH as the products. The cells incorporate the succinate into biomass by first oxidizing it to oxaloaceate from which PEP is synthesized. The two key enzymes of the so-called glyoxylate cycle are isocitrate lyase and malate synthase (Dixon and Kornberg, 1959).
It is clear that glyoxylate cycle can not be the only solution for acetyl-CoA assimilation, because several organisms capable of growth with C2 compounds as sole carbon source, lack isocitrate lyase (ICL) activity, or show labeling pattern after growth on acetate inconsistent with the operation of the glyoxylate cycle (Erb et al., 2007 and refs therein). Several putative pathways for acetate assimilation by such isocitrate-lyase-negative organisms have been proposed (Meister et al., 2005 and refs therein), but none have been elucidated completely. For example -
(1) for purple nonsulfur bacteria, e.g. Rhodobacter sphaeroides and Rhodospirillum rubrum (Ivanovsky et al., 1997),
(2) for methylotroph Methylobacterium extorquens (Korotkova et al., 2002),
(3) for streptomycetes (Liu and Reynolds, 1999).
An exciting and elegant work is under way in the labs of Brigit E. Alber and G. Fuchs – the ethylmalonyl-CoA pathway of C2 assimilation that they proposed, although far from being completely understood, promises to explain metabolic transformations during growth on acetate in many (if not all) ICL-negative organisms (Alber et al., 2006; ). It is encoded separately - see SS:"Ethylmalonyl-CoA pathway of C2 assimilation"
For more information, please check out the description and the additional notes tabs, below
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