The mechanism of Rubisco activase: Insights from studies of the properties and structure of the enzyme

Abstract

Rubisco, the primary carboxylating enzyme in photosynthesis, must be activated to catalyze CO2 fixation. The concept of an 'activase', a specific protein for activating Rubisco, was first introduced in 1985 based largely on biochemical and genetic studies of a high CO2-requiring mutant of Arabidopsis (Salvucci et al. (1985) Photosynth Res 7: 193-201). Over the past ten years, details about the occurrence, structure, and properties of Rubisco activase have been elucidated. However, the mechanism of action of Rubisco activase remains elusive. This review discusses the need for and function of Rubisco activase and summarizes information about the properties and structure of Rubisco activase. The information is evaluated in the context of the mechanism of Rubisco activase.

Modulation of Rubisco Activity during the Diurnal Phases of the Crassulacean Acid Metabolism Plant Kalanchoëdaigremontiana

“…The increase in carbamylation over phase II and the first part of phase III was atypically slow compared with C 3 plants (Portis, 1992(Portis, , 1995Salvucci and Ogren, 1996). The slow carbamylation of Rubisco was mirrored by a gradual rise in total activity of Rubisco and may be indicative of very slow removal of a daytime inhibitor (Parry et al, 1997), which again was presumably dependent on Rubisco activase.…”

“…The presence of tightbinding sugar phosphate inhibitors, including CA1P, may also be significant in the regulation of Rubisco activity (Keys et al, 1995;Parry et al, 1997). CA1P binds tightly to carbamylated sites in low light and at night in a large number of species (Kobza and Seeman, 1989;Holbrook et al, 1994) and removal of this ligand is also facilitated by Rubisco activase (Portis, 1992(Portis, , 1995Wang and Portis, 1992;Salvucci and Ogren, 1996;Hammond et al, 1998).…”

Rubisco activase constrains the photosynthetic potential of leaves at high temperature and CO 2

“…Deactivation can involve the spontaneous loss of the carbamate and formation of the RubiscoRuBP complex or formation of an inactive quaternary complex consisting of carbamylated Rubisco containing bound metal and a tightly bound sugar phosphate, the latter either formed at the active site or produced naturally by metabolism (22,25). Likewise, activation of Rubisco can involve either dissociation of RuBP from decarbamylated sites and spontaneous carbamylation of the sites by CO 2 or release of a tightly bound sugar phosphate from sites that are already carbamylated (22,24,25). Regardless of the carbamylation status, a decrease in activation state in response to environmental conditions indicates that the equilibrium has shifted toward deactivation and less of the sites are catalytically competent.…”

“…Carbamylated sites also form dead-end complexes, not with RuBP, but with other sugar phosphates including isomers and epimers of RuBP that are produced at the active site by misprotenation of RuBP (22). To facilitate the removal of sugar phosphates from Rubisco active sites, chloroplasts contain a AAAϩ protein (23) called Rubisco activase (24,25). Activase is an ATPase that loosens the binding of Rubisco for sugar phosphates (26).…”

“…Activase is an ATPase that loosens the binding of Rubisco for sugar phosphates (26). Thus, activase promotes ''activation'' of Rubisco by facilitating dissociation of sugar phosphates from either decarbamylated sites containing RuBP or analogs of RuBP, or carbamylated sites containing analogs of RuBP or the reaction intermediate (24,25). Because of this important function, activase ultimately determines the proportion of Rubisco active sites that are catalytically competent.…”