Degradation of potent Rubisco inhibitor by selective sugar phosphatase

Abstract

Ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) catalyses the conversion of atmospheric carbon dioxide into organic compounds in photosynthetic organisms. Alongside carboxylating the five-carbon sugar ribulose-1,5-bisphosphate (RuBP)(1-3), Rubisco produces a small amount of xylulose-1,5-bisphosphate (XuBP), a potent inhibitor of Rubisco(4). The AAA+ protein Rubisco activase removes XuBP from the active site of Rubisco in an ATP-dependent process(5,6). However, free XuBP rapidly rebinds to Rubisco, perpetuating its inhibitory effect. Here, we combine biochemical and structural analyses to show that the CbbY protein of the photosynthetic bacterium Rhodobacter sphaeroides and Arabidopsis thaliana is a highly selective XuBP phosphatase. We also show that CbbY converts XuBP to the non-inhibitory compound xylulose-5-phosphate, which is recycled back to RuBP. We solve the crystal structures of CbbY from R. sphaeroides and A. thaliana, and through mutational analysis show that the cap domain of the protein confers the selectivity for XuBP over RuBP. Finally, in vitro experiments with CbbY from R. sphaeroides reveal that CbbY cooperates with Rubisco activase to prevent a detrimental build-up of XuBP at the Rubisco active site. We suggest that CbbY, which is conserved in algae and plants, is an important component of the cellular machinery that has evolved to deal with the shortcomings of the ancient enzyme Rubisco.

A conserved phosphatase destroys toxic glycolytic side products in mammals and yeast

“…If the latter is not degraded by the metabolite repair enzyme L-2-hydroxyglutarate dehydrogenase, it accumulates in tissues to concentrations in the millimolar range, thereby causing a neurological disease known as L-2-hydroxyglutaric aciduria in humans 2,6,7 . This example also illustrates that, even at very low production rates, some side products may accumulate to high concentrations if they are not eliminated by dedicated enzymes 2,8 .…”

Redesigning photosynthesis to sustainably meet global food and bioenergy demand

“…The three enzymes needed in the proposed de novo 2PG salvage pathway are as follows: kinase (step 1), reductase (step 2), and aldolase (step 3). Xylulose-1,5-bisphosphate is a stromal metabolite that is converted to the C3 cycle intermediate xylulose-5-phosphate by the cbbY protein (63). The capacity of the cbbY may not be sufficient to sustain the flux and may need to be up-regulated.…”

Regulation of Ribulose-1,5-bisphosphate Carboxylase/Oxygenase (Rubisco) Activase

“…Some of these are regulatory by nature, such as nocturnal inhibition (1) or inhibition of decarbamylated Rubisco by its substrate ribulose 1,5-bisphosphate (RuBP) (2), whereas others appear to be intrinsic to turnover in an oxygenic atmosphere (3,4). Regardless, the release of competitive inhibitors is substantially accelerated by the motor protein Rca, with subsequent degradation of some inhibitors by their specific phosphatases (5,6). A comprehensive understanding of all coregulatory mechanisms that enhance or diminish biological carbon fixation is critical in predictive modeling of net photosynthesis and biological pathway engineering to improve plant performance (7)(8)(9).…”