Closing the Cycle

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Science  16 Dec 2011:
Vol. 334, Issue 6062, pp. 1508-1509
DOI: 10.1126/science.1215655

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Cyanobacteria have played a central role in the evolution of life on Earth, both by producing oxygen as a photosynthetic byproduct and by generating organic carbon, the major ecological energy commodity. Biochemical studies in the late 1960s implied that these bacteria have an unusual metabolism: Their tricarboxylic acid (TCA) cycle appeared to be incomplete, with a key enzyme, 2-oxoglutarate dehydrogenase, missing (1, 2). Aerobic organisms use the TCA cycle to generate reductant for adenosine triphosphate (ATP) synthesis and to create three of the precursors for the biosynthesis of all cellular compounds. It is thus of central importance to cellular metabolism. An incomplete TCA cycle has been used to explain various aspects of the physiology of cyanobacteria. On page 1551 of this issue, Zhang and Bryant (3) show that in most cyanobacteria, two other enzymes perform the function of the missing 2-oxoglutarate dehydrogenase, thereby closing the TCA cycle (see the figure).