Abstract
Ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), the key enzyme of the Calvin-Benson-Bassham cycle of photosynthesis, requires conformational repair by Rubisco activase for efficient function. Rubisco mediates the fixation of atmospheric CO2 by catalyzing the carboxylation of the five-carbon sugar ribulose-1,5-bisphosphate (RuBP). It is a remarkably inefficient enzyme, and efforts to increase crop yields by bioengineering Rubisco remain unsuccessful. This is due in part to the complex cellular machinery required for Rubisco biogenesis and metabolic maintenance. To function, Rubisco must undergo an activation process that involves carboxylation of an active site lysine by a non-substrate CO2 molecule and binding of a Mg2+ ion. Premature binding of the substrate RuBP results in an inactive enzyme. Moreover, Rubisco can also be inhibited by a range of sugar phosphates, some of which are “misfire” products of its multistep catalytic reaction. The release of the inhibitory sugar molecule is mediated by the AAA+ protein Rubisco activase (Rca), which couples hydrolysis of ATP to the structural remodeling of Rubisco. Rca enzymes are found in the vast majority of photosynthetic organisms, from bacteria to higher plants. They share a canonical AAA+ domain architecture and form six-membered ring complexes but are diverse in sequence and mechanism, suggesting their convergent evolution. In this review, we discuss recent advances in understanding the structure and function of this important group of client-specific AAA+ proteins.
Highlights
Ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) is the central enzyme of the CalvinBenson-Bassham (CBB) cycle of photosynthesis (Figure 1A)
Binding to inhibited Rubisco stimulates the ATPase activity of RsRca ∼4-fold (Mueller-Cajar et al, 2011), in a manner dependent on both the RbcL C-terminus and the top surface of the RsRca hexamer. These findings suggest that RsRca docks onto Rubisco with its top surface and the pore loops transiently pull the C-terminal tail of RbcL into the central pore, to facilitate opening of the active site pocket and release the inhibitory sugar phosphate (Figure 3E)
Based on recent insights into the structural and functional diversity of Rubisco activases, these proteins represent an important paradigm to understanding how the AAA+ module can be adapted to the repair of a specific enzyme
Summary
Ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) is the central enzyme of the CalvinBenson-Bassham (CBB) cycle of photosynthesis (Figure 1A). Rubisco catalyzes the carboxylation of one molecule of ribulose-1,5-bisphospate (RuBP) and produces two molecules of 3phosphoglycerate (3PG), which are used for the synthesis of sugars, starch, amino acids, and fatty acids (Miziorko and Lorimer, 1983). The specificity of Rubisco for CO2 is limited and the enzyme can use oxygen as a substrate (Whitney et al, 2011). In this reaction, referred to as photorespiration, Rubisco catalyzes the oxygenation of RuBP, producing only one molecule of 3PG and one molecule of the toxic by-product 2-phosphoglycolate (2P-glycolate) (Figure 1A). Photorespiration has long been regarded as a wasteful process, but recent advances suggest that it
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