Glyoxylate Cycle

Abstract

Some organisms possess enzymes that allow the net conversion of acetyl‐CoA to succinate. This is accomplished by a special pathway, the glyoxylate cycle, which may be regarded as an anaplerotic (replenishing) pathway.

Keywords: acetyl‐CoA; succinate; glyoxysome; isocitrate lyase; malate synthase; metabolism; enzyme

Figure 1.

The central role of acetyl‐CoA in overall metabolism. The citric acid cycle, which is an amphibolic pathway, is shown in green. Catabolic pathways are in blue. ATP‐requiring steps or pathways are in red. In eukaryotes, different pathways are confined to different organelles (see Table ).

Figure 2.

The glyoxylate cycle (red) and its relationship with the citric acid cycle (blue).

Figure 3.

Oxidation steps occurring within plant glyoxysomes. FADH2 is reoxidized with the generation of H2O2, which is handled by catalase. Possible mechanisms for the reoxidation of NADH are shown (a–c) and are discussed in the text.

Figure 4.

The glyoxylate pathway within plants. The classical view of the glyoxylate cycle is shown at the top. Glyoxysomal NADH is reoxidized through some kind of redox system. More recent views on the glyoxylate pathway within plants are shown below. Reoxidation of NADH occurs in the mitochondrial oxidative phosphorylation pathway. This involves a shuttle system and transport of various metabolites. The exact location of aconitase (glyoxysome or cytoplasm?) is still a matter of discussion.

Figure 5.

Bifurcation at the level of isocitrate in bacteria. Enzyme activities of isocitrate dehydrogenase (ICDH) and possibly also isocitrate lyase (ICL), which in bacteria occur in the same compartment, are regulated through phosphorylation.

Figure 6.

Sequence homologies among isocitrate lyases and malate synthases from plants, fungi, bacteria and invertebrates. Sequence data can be retrieved from GenBank, using the following accession numbers. ICLases. From plants: M83534 (Arabidopsis thaliana); L08482 (Brassica napus); Z35499 (Cucumis sativus); D78256 (Cucurbita maxima); L02329 (Glycine max); X52136 (Gossypium hirsutum); U18678 (Lycopersicon esculentum); M17145 (Ricinus communis). From fungi: X62696 (Aspergillus nidulans); D00703 (Candida tropicalis); JC6182 (Coprinus cinereus); X62697 (Neurospora crassa); X65554 (Saccharomyces cerevisiae); X72848 (Yarrowia lipolytica). From bacteria: U18765 (Chlamydomonas reinhardtii); M22621 (Escherichia coli); Z29367 (Rhodococcus fascians). MSases. From plants: J04468 (Brassica napus); X15425 (Cucumis sativus); X56948 (Cucurbita maxima); L01629 (Glycine max); X52305 (Gossypium hirsutum); X78852 (Raphanus sativus); X52806 (Ricinus communis); L35914 (Zea mays). From fungi: X56671 (Aspergillus nidulans); D13415 (Candida tropicalis); P21360 (Hansenula polymorpha); X56672 (Neurospora crassa); X64407 (Saccharomyces cerevisiae). From bacteria: X12431 (Escherichia coli); U63518 (Streptomyces arenae). The invertebrate bifunctional enzyme (ICLase + MSase): U23159 (Caenorhabditis elegans). Boxes correspond to highly conserved regions; the nine fully conserved tryptophan residues in MSase are indicated with a blue diamond. In ICLase sequences a capital (respectively lower case) letter indicates a conserved amino acid in 16 (respectively 9) out of 18 ICLase sequences. In MSase sequences a capital (respectively lower case) letter indicates a conserved amino acid in 14 (respectively 8) out of 16 MSase sequences. Amino acids known to be involved in ICLase in substrate‐binding and/or catalysis are indicated with a red line. In the bifunctional invertebrate enzyme, V8 protease digestion occurs as indicated by green triangles.

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Beeckmans, Sonia(Apr 2001) Glyoxylate Cycle. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1038/npg.els.0001370]