Stealth reactions driving carbon fixation

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Science  02 Feb 2018:
Vol. 359, Issue 6375, pp. 517-518
DOI: 10.1126/science.aar6329

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Organisms live in an interconnected dynamic web in which they make, degrade, and interconvert compounds containing carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur. The carbon cycle involves the oxidation of organic compounds to produce CO2 by heterotrophic organisms and the incorporation (“fixation”) of CO2 from the environment into living tissue by autotrophic organisms. Heterotrophic organisms (most animals) obtain the energy for life by conserving the energy obtained by oxidizing organic molecules to CO2 in the form of reducing equivalents (electrons) and adenosine triphosphate (ATP). Autotrophic plants, bacteria, and archaea fix CO2 by a process in which the energy of electrons and ATP is used to produce biomolecules, such as sugars, amino acids, and lipids, thereby replenishing these essential organic molecules in the ecosystem. Cumulatively, autotrophy occurs on the huge scale of 7 × 1016 g of carbon fixed annually (1). Six CO2 fixation pathways differing in their ATP requirements are known to exist. Fixing CO2 using the least ATP possible is key for anaerobes because their metabolism generates much less ATP than does growth on oxygen. The reductive tricarboxylic acid (rTCA) cycle is one of the most evolutionarily ancient and least ATP-demanding autotrophic pathways. On pages 563 and 559 of this issue, Mall et al. (2) and Nunoura et al. (3), respectively, uncover an unexpected ATP-conserving mechanism, and Pachiadaki et al. (4) report a surprising source of reducing equivalents in the rTCA cycle.