During carbon fixation, carbon dioxide in the stroma (which enters the chloroplast by diffusion) reacts with a five-carbon sugar called ribulose bisphosphate (RuBP) to form a six-carbon compound. This reaction is catalysed by an enzyme called ribulose bisphosphate carboxylase (large amounts present within the stroma), otherwise known as rubisco. As soon as the six-carbon compound is formed, it splits to form two molecules of glycerate 3-phosphate. Glycerate 3-phosphate is then used in the reduction reactions.
Rubisco activity is modulated in plants by an inhibitor and an activator. The inhibitor 2′-carboxy arabinitol 1-phosphate (2CAIP) accumulates in some plants during darkness and binds to the active site of Rubisco (5, 6). 2CAIP is degraded by a specific phosphatase, which presumably allows Rubisco to function during photosynthesis in the light. Rubisco can be severely inhibited by a range of sugar bisphosphates, including substrate analogues. The enzyme Rubisco activase has the ability to relieve the inhibition caused by sugar bisphosphates (7), possibly by interacting with Rubisco and altering the affinity of the enzyme for bisphosphates.
Changes in the global environment since the beginning of the Industrial Revolution have raised concerns about the impact on natural and agroecosystems. Increasing CO2 levels and corresponding changes in temperature will directly affect the earth's carbon balance by altering photosynthetic carbon fixation. Thus, it is essential to develop an understanding of the mechanistic response of photosynthesis to environmental change. Models of photosynthesis (, ) have been widely adapted to predict how environmental changes will influence photosynthesis and to pinpoint biochemical limitations in the process. These models are based primarily on the kinetics of fully activated Rubisco, and they make a clear distinction between limitations attributable to the enzyme amount and those limiting the regeneration of the substrate, RuBP. The report by Crafts-Brandner and Salvucci () highlights the importance of Rubisco activation as a determinant of photosynthetic performance under conditions associated with a changing global climate. The clear conclusion of these experiments is that Rubisco activation is the major limitation to CO2 fixation during photosynthesis, certainly under high CO2, high temperature, and optimal light.
The paper by Crafts-Brandner and Salvucci () in this issue of PNAS provides evidence that, with plants under heat stress, the activation state of Rubisco and photosynthesis as measured by CO2 exchange is reduced. By duplicating the temperature response in the test tube under controlled conditions, and by using Rubisco and Rubisco activase isolated from tobacco, they were able to ascribe the limitation to a specific biochemical event, the inability of Rubisco activase to keep pace with a faster deactivation of Rubisco. Increased CO2 also decreased the activation state of Rubisco in leaves, and the authors conclude that the response could be explained by a decreased energy charge in the chloroplast that reduced the ATPase activity of Rubisco activase. By calculating photosynthesis on the basis of the kinetics of Rubisco and the amount of active enzyme in the leaf, the authors have shown that, under both high temperature and high CO2, photosynthesis was constrained by the activity of Rubisco activase.
What Biol 1510 students need to remember about C4 is that these plants have added a CO2 concentration mechanism to feed rubisco and the Calvin cycle; the mechanism uses PEP carboxylase to initially make a 4-carbon compound, that then releases CO2 to rubisco in leaf cells that are exposed to little oxygen. While this mechanism reduces the oxygenase activity of rubisco, it has an extra energy cost in the form of another ATP per mole CO2 fixed.
So how can these factors have an effect on the rate of photosynthesis? Lets start off with the light intensity. When the light intensity is poor, there is a shortage of ATP and NADPH, as these are products from the light dependent reactions. Without these products the light independent reactions can't occur as glycerate 3-phosphate cannot be reduced. Therefore a shortage of these products will limit the rate of photosynthesis. When the carbon dioxide concentration is low, the amount of glycerate 3-phosphate produced is limited as carbon dioxide is needed for its production and therefore the rate of photosynthesis is affected. Finally, many enzymes are involved during the process of photosynthesis. At low temperatures these enzymes work slower. At high temperatures the enzymes no longer work effectively. This affects the rate of the reactions in the Calvin cycle and therefore the rate of photosynthesis will be affected.
Following import into chloroplasts and removal of the transit peptide, mature S subunits are assembled with chloroplast-synthesized L subunits to give the active L8S8 Rubisco holoenzyme (21, 22). This assembly process requires the assistance of another chloroplast protein (23) now known as chaperonin 60 (cpn60) (24, 25). In fact, studies on the assembly of Rubisco in chloroplasts and bacteria (23, 26, 27) led to the discovery of the molecular chaperone cpn60 and its role in the correct folding of Rubisco and many other proteins (24, 25, 28). Productive folding of Rubisco requires Mg , ATP hydrolysis, and a smaller cochaperonin molecule (29). Cpn60-mediated folding of Rubisco in bacteria uses a cochaperonin oligomer with 10-kDa subunits (24), but the folding of Rubisco in chloroplasts seems to involve a co-chaperonin oligomer with 21-kDa subunits (30). The reason for this larger cochaperonin in chloroplasts and the mechanistic details of the Rubisco assembly process in plants are currently under investigation.
Plant cells compartmentalize Rubisco in chloroplasts, but the genetic information is shared between chloroplast and nucleus. The rbcL genes are present in chloroplast DNA (13), and their transcription and translation in plastids uses sequences that are similar to those found in prokaryotes (14, 15), to the extent of allowing direct expression when transferred to E. coli (16). The S-subunit genes are located on nuclear chromosomes and have a more complex structural arrangement. The rbcS genes contain introns and are present as small multigene families that are often closely linked (17). Light-induced expression is mediated by both phytochrome and blue light photoreceptors (18), and positive and negative regulatory sequences are located in cis-acting transcriptional control regions. The rbcS promoters also appear to contain nuclear matrix attachment regions (MARs) (19), which may be important for their expression. The highest level of rbcS mRNA is found in leaves, but it is also found in the photosynthetic tissues in stems, petals and pods. S subunits are synthesized on free cytoplasmic polyribosomes as precursor molecules with an ^-terminal transit peptide (20). The S-subunit precursors are imported post-translationally into chloroplasts in a process requiring ATP, and the transit peptide is removed (21, 22).
Although is responsible for the vast bulk of organic carbon on the surface of the Earth, its oxygenase activity can severely reduce photosynthetic efficiency. Some plants have evolved a way to minimize the oxygenase activity of Rubisco.
In most prokaryotes, the organization of the Rubisco S-subunit (rbcS) and L-subunit (rbcL) genes is relatively simple, with both genes usually closely linked and transcribed together. To the 5′ side of the initiator methionine residue of many rbc genes is a sequence similar to a Shine-Dalgarno site for ribosome binding, and sufficiently conserved to allow correct translation when cloned into Escherichia coli (11, 12).
Rubisco has oxygenase activity as well as carboxylase activity; it sometimes fixes O2 to RuBP instead of CO2. The oxygenase activity occurs at low CO2, high O2 conditions, and becomes pronounced at high temperatures. As a result, organic carbon is oxidized, the opposite of photosynthesis, which reduces inorganic carbon to make organic carbon.