Mitocondria

Inferences from the regulation of the TCA

cycle in non-plant systems and future

perspectives

As alluded to above, a further mode of regulation is that

played by compartmentation whilst studies in plants have

provided a fairly comprehensive view as to how this is

controlled at the organellar level as yet little is known

there in plants at finer sub-compartment resolution. This

is in sharp contrast to mammalian and microbial systems.

Indeed the coining of the term ‘metabolon’ to define

supermolecular complexes composed of sequential metabolic

enzymes was first used in the context of the rat TCA

cycle [71]. In such complexes the active domains of

enzymes are structurally connected in metabolon resulting

metabolite channeling between the enzymes [72], a

mechanistic process for the direct delivery of a reaction

intermediate from the active site of one enzyme to the

other(s) without prior dissociation into the bulk solvent.

Metabolite channeling offers a means for high metabolic

rates with low bulk concentrations of intermediates and

extremely rapid responses to changes in the metabolic

status of the cell through reversible assembly and disassembly

of all or some of the constituent parts [73]. These

features fit very well to those required for TCA cycle

regulation and as stated above the TCA cycle metabolon

was uncovered in mammals and yeast in extensive studies

in the 70s and 90s, whilst interactions of enzymes were

recently reported also in bacteria [74]. However, whilst

metabolons have been described for the Calvin–Benson

cycle, glycolysis and specific pathways of specialized

metabolism in plants [75,76], to date there is no molecular

evidence showing an interaction of plant TCA cycle

enzymes even in large scale interactome studies [

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