Rubisco light activation

Another regulatory mechanism involves the nocturnal inhibitor 2-carboxyarabinitol 1-phosphate, a naturally occurring transition-state analog (see Box 6-3) with a structure similar to that of the /3-keto acid intermediate of the rubisco reaction (Fig. 20-7 see also Fig. 20-20). This compound, synthesized in the dark in some plants, is a potent inhibitor of carbamoylated ru-bisco. It is either broken down when light returns or is expelled by rubisco activase, activating the rubisco. 

Rubisco Is Activated by Light-Driven Changes in Proton and Magnesium Ion Concentrations... 

Light-induced control via the ferredoxin/ thioredoxin system, Rubisco is activated by CO2 (Chapter 13) and by fructose 6-P and is inhibited by fructose 1,6-P2 (Fig. 23-36), whose accumulation is a signal to turn off the carboxylase. Conversely, fructose... 

TABLE 1. Total light activity, and dark inhibition of rubisco in leaf extracts of rice and soybean cultivars. Samples of 3-4 rice or soybean trifoliate leaves (third leaf) were taken 20 days after planting. 

Substrate RuBP binds much more tightly to the inactive E form of rubisco (An = 20 nM) than to the active ECM form (A, for RuBP = 20 ixM). Thus, RuBP is also a potent inhibitor of rubisco activity. Release of RuBP from the active site of rubisco is mediated by rubisco activase. Rubisco activase is a regulatory protein it binds to A-form rubisco and, in an ATP-dependent reaction, promotes the release of RuBP. Rubisco then becomes activated by carbamylation and Mg binding. Rubisco activase itself is activated in an indirect manner by light. Thus, light is the ultimate activator of rubisco. 

As discussed in Section 22.7, illumination of chloroplasts leads to light-driven pumping of protons into the thylakoid lumen, which causes pH changes in both the stroma and the thylakoid lumen (Figure 22.27). The stromal pH rises, typically to pH 8. Because rubisco and rubisco activase are more active at pH 8, COg fixation is activated as stromal pH rises. Fructose-1,6-bisphosphatase, ribulose-5-phosphate kinase, and glyceraldehyde-3-phosphate dehydrogenase all have alkaline pH optima. Thus, their activities increase as a result of the light-induced pH increase in the stroma. 

The reductive assimilation of C02 requires a lot of ATP and NADPH, and their stromal concentrations increase when chloroplasts are illuminated (Fig. 20-17). The light-induced transport of protons across the thylakoid membrane (Chapter 19) also increases the stromal pH from about 7 to about 8, and it is accompanied by a flow of Mg2+ from the thylakoid compartment into the stroma, raising the [Mg2+] from 1 to 3 ira to 3 to 6 ulm. Several stromal enzymes have evolved to take advantage of these light-induced conditions, which signal the availability of ATP and NADPH the enzymes are more active in an alkaline environment and at high [Mg2+], For example, activation of rubisco by formation of the... 

Cells C02 and light conditions Chi content [mg (kg-FW) ] RubisCO activity [pmol-CO, (h mg-Chl)-1] Apparent C02 fixation rate [pmol-CO, (h mg-Chl) ] Ref. 

Phaseolus leaves in the light was significantly more active than from darkened leaves, despite optimal in vitro activation of the enzyme with CO2 and Mg. Several studies have shown that phosphorylated compounds can be effective inhibitors of rubisco in vitro. The results with Phaseolus, however, are the first to document the importance in vivo of a compound which is light-modulated and present in sufficient amounts to reduce dark enzyme activity to close to zero [22],... 

N. Zhang and A.R. Portis Jr. 1999. Mechanism of light regulation of rubisco A specific role for the larger rubisco activase isoform involving reductive activation by thioredoxin-f Proc. Natl. Acad. Sci. USA 96 9438-9443. (PubMed) (Full Text in PMC)... 

Concurrently to the assimilation in leaves there is always CO2 production in respiration and photorespiration (the first occurs all the time and involves the mitochondria, the latter occurs only in the light and involves also the oxygenase activity of Rubisco in the chloroplasts). Any discrimination in the respiration process will be reflected in the isotopic composition of the CO2 produced and mixed into the leaf internal CO2 pool and its effect must, therefore, be subtracted from Ap. The combined effect of the two respiratory components can be subtracted from Equation (24) as (Farquhar and Lloyd, 1993 Farquhar et al., 1982)... 

The genes that code for ribulose- 1,5-bisphosphate carboxylase (rubisco) are found within the chloroplast (the L subunit) and the nucleus (the S subunit). The activation of these genes is mediated by an increase in light intensity (illumination). Phytochrome also appears to play a role in this activating process. Once the S subunit is transported from the cytoplasm into the chloroplast, both subunits assemble to form the L8S8 holoenzyme. A protein called the large subunit-binding protein appears to assist in the assembly of the holoenzyme. When illumination is low, the synthesis of both subunits is rapidly depressed. 

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