Learning new tricks from an old cycle: the TCA cycle in cyanobacteria, algae and plants
Zhang, Shuyi; Bryant, Donald A.
Abstract Cyanobacteria, algae, and plants absorb and convert sunlight into chemical potential energy by oxygenic photosynthesis. However, because of the rotation of Earth, sunlight is only available during about one half of the diel cycle. Photosynthetic organisms must therefore store energy-rich compounds during the day to provide the fuel to support growth or maintenance energy production at night. In the presence of oxygen, energy-producing respiration occurs in most organisms, which allows the oxidation of energy-rich substrates for the production of proton-motive force for ATP synthesis and other biochemical work. Together with glycolysis and the oxidative pentose phosphate pathway, the tricarboxylic acid (TCA) cycle is one of the three most important pathways of central carbohydrate metabolism. It has long been accepted that a complete TCA cycle is important if not essential for respiratory energy production in the dark for eukaryotes. However, because biomass production is a more central concern for cyanobacteria, the TCA mostly operates as a branched pathway for the production of two essential precursor metabolites, 2-oxoglutarate and oxaloacetate. It was long believed that cyanobacteria had an incomplete TCA cycle due to the absence of 2-oxoglutarate dehydrogenase (OGDH). However, our recent studies demonstrated that the TCA cycle in most cyanobacteria is completed in manner distinct from the classical TCA cycle through the action of two alternative enzymes, 2-oxoglutarate decarboxylase (2-OGDC) and succinic semialdehyde dehydrogenase (SSADH). This review describes current research progress in understanding the functional similarities and differences among the TCA cycles in cyanobacteria, algae, and plants. Moreover, the possible significance of non-traditional TCA cycles in a broader regulatory and evolutionary context is considered.