Green catalytic converter

A key component of the Kyoto Protocol — the international agreement that aims to reduce greenhouse-gas concentrations — is the control of carbon emissions into the atmosphere. This will require industries to reclaim and recycle the carbon dioxide they currently discard. Elsewhere in this issue (see Nature 432, 779–782; 2004), Jörg Schwender and colleagues show that plants are way ahead of us in this respect.

Oil is the major storage product in most seeds, but the standard biochemical route for its synthesis was thought to be quite wasteful. Plants employ a variation of glycolysis, the metabolic pathway also used by animals, which breaks down glucose to produce energy. One of its end products is pyruvate, which can be converted to acetyl-CoA, a precursor for fatty acids and oils. This reaction also produces CO₂. For every two carbon atoms that are made into oil, at least one should be lost as gas.

Schwender et al. found that oil production in seeds of Brassica napus (oilseed rape or canola; shown here) was nowhere near as inefficient. Using radioactive labelling, they measured the ratio of carbon used to carbon lost as almost three to one. Closer investigation showed that these savings are achieved by a previously unsuspected function of probably the most abundant protein in the world, Rubisco.

Rubisco catalyses a critical reaction in photosynthesis. It combines CO₂ with the sugar ribulose 1,5-bisphosphate to produce two molecules of phosphoglyceric acid as part of a complicated series of chemical reactions known as the Calvin cycle. It turns out that rape seeds have high levels of Rubisco, but no detectable Calvin-cycle activity. Instead, the enzyme's supply of ribulose 1,5-bisphosphate is synthesized afresh from fructose and glyceraldehydes, allowing it to soak up the CO₂ produced from making acetyl-CoA. The resulting pyruvate is then used to make more oil.

Not all seeds can reclaim CO₂ in this way; sunflower seeds, for example, are much less efficient in their oil production. The difference is that growing Brassica seeds are green. Despite being enclosed in a pod, enough light filters through to be captured by the pigment chlorophyll, supplying the energy needed for the synthesis of ribulose. In seeds, at least, it is only the greens that recycle.