Improving the Balance Sheet

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Science  27 Apr 2007:
Vol. 316, Issue 5824, pp. 516
DOI: 10.1126/science.316.5824.516b

Plants incorporate (fix) CO2 into hexoses (sugars) by coupling it to the five-carbon compound ribulose-1,5-bisphosphate in a reaction that is catalyzed by the enzyme rubisco. Unfortunately, a competitive and apparently unavoidable reaction, which is also catalyzed by rubisco (see Tcherkez et al. for more on this abominably perplexing phenomenon), uses O2 as a substrate and generates one molecule each of glyoxylate and glycerate (instead of two equivalents of glycerate). Glyoxylate is then converted—via subsequent reactions in the peroxisome and mitochondrion—into glycerate, but in doing so one-quarter of the already fixed carbon atoms are lost as CO2 with the concomitant debiting of already fixed nitrogen atoms in the form of ammonia. Increasing the local concentration of CO2 relative to O2 is an evolutionary achievement found in C4 plants (such as corn), and efforts to introduce a CO2-concentrating module into C3 plants (such as rice) have been pursued.

Kebeish et al. describe a means of reducing the material cost of carbon-atom recovery from glyoxylate. They have engineered the targeting of three bacterial enzymes to the chloroplast in Arabidopsis. The result is that when two molecules of glyoxylate are converted into one of glycerate, the CO2 that is liberated is not lost, but is recaptured by rubisco; the consequences are a decrease in photorespiration, an increase in photosynthesis, and more biomass (leaves and roots) produced. — GJC

Proc. Natl. Acad. Sci. U.S.A. 103, 7246 (2006); Nat. Biotechnol. 25, 10.1038/nbt1299 (2007).

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