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I have been reading about C4 carbon fixation in which CO2 is captured by phosphoenolpyruvate (PEP) to make oxaloacetate.
Are there known pathways in plants that use substantial amounts of this oxaloacetate for anabolism, i.e. without decarboxylation and subsequent use of the CO2 by RuBisCO?
My searches keep getting absorbed by the huge amount of info on conventional photosynthesis.
Q. Are there known pathways in plants that use substantial amounts of this oxaloacetate for anabolism?
A. No on two counts:
One can check this for oneself using the KEGG facility, starting with the page for oxaloacetate. At first sight the list of pathways involving oxaloacetate would appear promising in this respect, including:
| Map | Description |
|---|---|
| map00999 | Biosynthesis of various plant secondary metabolites |
| map01060 | Biosynthesis of plant secondary metabolites |
| map01061 | Biosynthesis of phenylpropanoids |
| map01062 | Biosynthesis of terpenoids and steroids |
| map01063 | Biosynthesis of alkaloids derived from shikimate pathway |
| map01064 | Biosynthesis of alkaloids derived from ornithine, lysine and nicotinic acid |
| map01065 | Biosynthesis of alkaloids derived from histidine and purine |
| map01066 | Biosynthesis of alkaloids derived from terpenoid and polyketide |
| map01070 | Biosynthesis of plant hormones |
However, when one goes to the respective map one finds that oxaloacetate appears either as an incidental component of the tricarboxylic acid cycle, or as a precursor to aspartate (by transamination) e.g.in map01064:
This is supported by the lack of any such mention in the introduction of a recent (2020) paper in Nature Scientific Reports regarding phosphoenolpyruvate carboxylase, the enzyme that catalyses the conversion of phosphoenolpyruvate to oxaloacetate. (Such introductions commonly emphasise the ‘importance’ of the subject of a study.)
Phosphoenolpyruvate carboxylase (PEPc) is a carbon dioxide fixing enzyme that in an irreversible manner and in the presence of Mg2+, converts phosphoenolpyruvate and bicarbonate into oxaloacetate and inorganic phosphorus. It is present in bacteria (including cyanobacteria), algae, fungi and plants. PEPc has been demonstrated to be involved in atmospheric CO2 fixation and storing carbon in cell vacuoles, play an anapleurotic role, supply energy for symbiotic bacteria, produce energy, abiotic stress acclimation, seed formation, and in the development and cell expansion.
Why not?
In general, all most organisms draw off intermediates of the the tricarboxylic acid cycle as synthetic intermediates. It is true that these have to be replenished if the cycle is to continue turning (oxaloacetate condensing with acetyl-CoA, above), but this can be done at several points:
Furthermore, this use of simple intermediates for more complex ones had evolved long before C4 plants came on the scene — indeed the problem had already been solved in C3 plants.
Oxalate Production?
Although it is arguable whether this would be anabolism, @Ryan raised the interesting question of whether oxaloacetate produced in this way might be used for the production of oxalate, which accumulates as crystals in the leaves of certain plants.
The general question of the production of oxalate is discussed in the following review: Annu. Rev. Plant Biol. 2005. 56:41–71, from which it appears that this is not the case. The actual pathway of production appears to be subject to some disagreement, with most reports tending to the view that ascorbic acid is the precursor but others suggesting that the glycolate pathway operates in certain species.
I do not think this is too surprising if one considers that PEP carboxylase and oxalate formation each occur in (quite different) specialized cells. (And this is a general consideration regarding C4 plant metabolism: it has evolved in a specialized structures for a specific purpose, and is regulated according to the diurnal requirements of the plant.)
