Photosynthesis: Rubisco rescue.

The sunlight-driven conversion of carbon dioxide into organic matter fuels nearly all life on Earth. The enzyme Rubisco catalyses the fixation of carbon dioxide in photosynthesis, and thereby serves as the entry point for inorganic carbon into the biosphere. Rubisco limits biomass accumulation when sunlight is plentiful, and has therefore been heavily targeted in bioengineering efforts to boost crop yields1. Central to the success of such strategies is a thorough understanding of the way in which this enzyme is assembled and activated. Writing in this issue of Nature Plants, Bracher and colleagues2 report the structure and function of a sugar phosphatase that contributes to the maintenance of carbon fixation through the highly selective breakdown of an endogenous inhibitor that would otherwise quickly overwhelm Rubisco. Rubisco uses carbon dioxide to carboxylate the five-carbon compound ribulose bisphosphate (RuBP). The first step in this process involves the abstraction of a proton from Rubisco-bound RuBP to yield a highly reactive enediolate intermediate (Fig. 1). If all goes well, this intermediate accepts a molecule of carbon dioxide, setting in motion the synthesis of usable sugars. But it can also accept a molecule of oxygen, initiating the photorespiratory pathway that leads to the loss of fixed carbon. On occasion, the enediolate intermediate can accept a proton, generating xylulose bisphosphate (XuBP). Structurally highly similar to RuBP, XuBP inhibits Rubisco by binding to and ultimately blocking the enzyme’s active sites. XuBP is generated at particularly high rates in C3 plants3, as they lack the ability to concentrate carbon dioxide at the site of fixation. In general, the motor protein Rubisco activase (Rca) triggers the release of tightbinding inhibitors from Rubisco4. Powered by ATP hydrolysis, Rca is thought to alter the conformational state of Rubisco to favour the opening of active sites. In recent years, interest in Rca has grown at a phenomenal rate due to the publication PHOTOSYNTHESIS