1
$\begingroup$

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.

$\endgroup$
3
  • $\begingroup$ I assume you are aware of the position of OAA in the TCA cycle and the glyoxylate pathway. Anything TCA cycle intermediates can do, additional OAA can do. But it wouldn’t because the whole point in C4 plants is to build up carbon dioxide. I don’t see what you’re getting at. $\endgroup$
    – David
    Commented May 14, 2023 at 18:13
  • $\begingroup$ I see that any oxaloacetate produced in excess of other requirements can go on to be used for CO2 transport for C4 photosynthesis. I’m trying to wrap my mind around the fact that CO2 capture by Rubisco seems inefficient, but there must be a reason (unknown to me) why CO2 captured earlier as oxaloacetate is not used for anabolic reactions. $\endgroup$ Commented May 15, 2023 at 11:07
  • $\begingroup$ I have posted an answer which I hope addresses your question. I haven't actually discussed the inefficiency of Rubisco. It is topic that most of us find strange when first encountered — "Surely Nature could have evolved the enzyme to get round the problem". It may be chemically unsurmountable, or evolution does not require perfection, just something that works. Be that as it may, the phenomenon is undisputable, but the explanation is the subject of a different, speculative question (which may already have been asked). $\endgroup$
    – David
    Commented May 22, 2023 at 16:45

1 Answer 1

1
$\begingroup$

Q. Are there known pathways in plants that use substantial amounts of this oxaloacetate for anabolism?

A. No on two counts:

  1. This oxaloacetate” is used in C4 plants for the specific purpose of creating a high concentration of carbon dioxide in the environment of RuBisCO after conversion to malate and aspartate and transport of the latter to the bundle-sheath cells — cells that have evolved in C4 plants for the purpose.
  2. Oxaloacetate is not involved in any any anabolic pathways specific to plants, although aspartate (that may be derived from it) may be an early intermediate in certain pathways.

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: OAA as a precursor to nicotine

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:

Intermediate entry points to TCA cycle

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.

Oxaloacetate acetylhydrolase

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.)

$\endgroup$
2
  • $\begingroup$ nice answer, I'm wondering about the accumulation of oxalates in plants, though. Wouldn't those qualify as anabolic products of the TCA cycle intermediate? $\endgroup$
    – Ryan
    Commented May 24, 2023 at 4:58
  • 1
    $\begingroup$ @Ryan — Finally got round to answering your question. $\endgroup$
    – David
    Commented Jun 2, 2023 at 17:17

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .