The Calvin cycle is what is referred to as the dark reactions in photosynthesis. It is divided into three phases. The first phase is carboxylation, where CO2 reacts with 3 molecules of rubisco to carboxylate ribulose-1,5-bisphosphate to yield 6 molecules of 3-phosphoglycerate. The second phase is reduction of 3-phosphoglyceraldehyde to give glyceraldehyde-3-phosphate. Reduction phase is actually divided into two steps where firstly, ATP phosphorylates 3-phosphoglycerate to give 1,3-bisphosphoglycerate which is then reduced to 6 molecules of glyceraldehyde-3-phosphate using energy from NADPH. One molecule of glyceraldehyde-3-phosphate is converted into triose phosphates which are in turn converted to the carbohydrates sucrose and starch The last phase is the regeneration phase, where the CO2 acceptor ribulose-1,5-bisphosphate is regenerated from the other 5 molecules of glyceraldehyde -3-phosphate.These reactions are catalysed by enzymes which will be looked into in this article and how their regulation of the Calvin cycle impact on photosynthesis
Phase 1: Carboxylation
In a chemical reaction catalysed by ribulose-1,5-bisphosphate carboxylase-oxygenase (rubisco), 3 molecules of CO2 binds to 3 molecules of ribulose-1,5-bisphosphate (RuBP) in the presence of three molecules of water to give 6 molecules of 3-phosphoglyceraldehyde and 6H+.
This initial step is very important as it incorporates CO2 and thus initially many studies focused on this reaction in order to try and enhance photosynthesis. This was initially done by trying to reduce or deactivate the oxygenase activity of rubisco, as CO2 and O2 competes for the same site to bind, and thus, if this is manipulated so that only CO2 can be bound, then there will be more CO2 to reduce to carbohydrates. But these manipulations have not successful as yet.
More studies have been done now focused on manipulating levels of rubisco. The studies showed that rubisco itself regulates photosynthesis by being regulated by levels of CO2, light intensity and also nitrogen levels. In a study by Raines (2003) using rubisco antisense plants with reduced rubisco, it was observed that reducing levels of rubisco in plants under the same conditions that the plant was grown under, does not have any significant effect on photosynthesis. However when a tobacco antisense plant grown under ambient CO2 and moderate light was exposed to saturating light and/or saturating CO2,an increase in rubisco control over photosynthesis was observed .This led to the conclusion that rubisco is regulated by availability of CO2 and by light intensity and thus in turn regulated photosynthesis.
Phase 2: Reduction
Occurs via 2 reactions
a) 6 3-phosphoglyceraldehyde + 6ATP resulting in 6 1,3-bisphosphoglycerate +6ADP
Catalysed by 3-phosphoglycerate kinase
b) 6 1,3-bisphosphoglycerate +6NADPH +H+ producing 6 glyceraldehyde-3-phosphate + 6NADP+ + 6PiCatalysed by GAPDH.
Raines (2003) reported that no effect on photosynthesis was observed when antisense tobacco plants with reduced GAPDH were grown in highlight greenhouse conditions, but some effect was observed when GADPH activity was reduced to 35% below the wild-type plant.
From the 6 molecules of glyceraldehyde-3-phosphate produced in reduction phase, 5 of them goes into the regeneration phase to regenerate the 3 molecules of the CO2 binding RuBP and 1 molecule goes towards carbohydrate synthesis( sugars and other compounds)
Phase 3: Regeneration
This occurs in a lot of reaction steps and each and the enzyme catalysing it are listed below and if studies have been done to see how the particular enzyme regulates photosynthesis, then these studies and their findings are discussed.
a) 2 glyceraldehyde-3-phosphate producing 2 dihydroxyacetone-3-phosphate, catalysed by triose phosphate isomerase
b) Glyceraldehyde-3-phosphate+ dihydroxyacetone-3-phosphate producing Fructose-1,6-bisphosphate, catalysed by aldolase
Aldolase levels in plants have been observed to have significant control on photosynthesis, looking at carbon partitioning (Raines, 2003). Studies have shown that reduced levels of aldolase (in aldolase antisense plants) result in reduced levels of carbon accumulation looking at levels of starch, but was only shown to have an effect on sucrose levels when its activity was reduced to 30% of the wild type levels. This study showed for the first time that, a non-regulate enzyme which catalyses a freely reversible reaction, can have significant effect or control on photosynthetic carbon flux.
c) Fructose-1,6-bisphosphate + H2O producing fructose-6-phosphate +Pi, catalysed by Fructose-1,6-bisphosphate phosphatase (FBPase)
FBPase is a key regulated enzyme and some studies have been done to see if it has any effect on photosynthesis (Raines, 2003). As for GAPDH, it was observed that FBPase does not have a significant effect on photosynthesis in antisense potato plants but some effect was only observed when FBPase activity was reduced to less than 34% of the wild-type.
d) Fructose-6-phosphate + glyceraldehyde-3-phosphate producing erythrose-4-phosphate + xylulose-5-phosphate, catalysed by transketolase
Partitioning of carbon between sucrose and starch is affected by reductions in transketolase. Studies have shown that as light intensity increases, so does the effect of transketolase on carbon partitioning in antisense tobacco plants. The actual observed results were that levels of sucrose decreased as transketolase activity decreases. In regard to starch accumulation, effects were only observed when activity was reduced to below 60% of the wild-type (Raines 2003). Most studies done using antisense plants in the Calvin cycle, showed a trend towards partitioning of carbon towards starch biosynthesis, but these results show carbon partitioning in favour of sucrose instead.
e) Erythrose-4-phosphate + dihydroxyacetone-3-phosphate producing sedoheptulose-1,7-bisphosphate, catalysed by aldolase
f) Sedoheptulose-1,7-bisphosphate + H2O producing seduheptulose-7-phosphate + Pi, catalysed by sedoheptulose-1,7-bisphosphate phosphatase (SBPase).
SBPase is also a key, regulated enzyme and its effect on photosynthesis, have been studied. The studies showed that small decreases in SBPase activity, lead to reduced photosynthetic carbon fixation in SBPase antisense tobacco plants (Raines, 2003).This was shown by observations made that as SBPase activity decreases, so does starch levels and that starch is barely detectable in plants with less than 20% of wild-type SBPase activity.
g) Sedoheptulose-7-phosphate+ glyceraldehyde-3-phosphate producing ribose-5-phosphate + xylulose-5-phosphate, catalysed by transketolase
h) 2 xylulose-5-phosphate producing 2 ribulose-5-phosphate, catalysed by ribose-5-phosphate epimorase
i) Ribose-5-phosphate producing ribulose-5-phosphate, catalysed by ribose-5-phosphate isomerase
Then the last reaction which is catalysed by ribulose-5-phosphate kinase also called phosphoribulokinase PRKase) is:
j) 3 ribulose-5-phosphate + 3ATP producing 3 ribulose-1,5-phosphate +3ADP +3H+
PRKase is also a key, regulated enzyme and like FBPase and GAPDH, has not been observed to have any significant effect on photosynthesis (Raines 2003).It was observed that activity of PRKase have to be reduced to less than 20% than the wild-type plants, in PRKase antisense tobacco plants, before a decrease in photosynthesis can be observed, when the plants were grown in low light or in nitrogen deficient conditions.
The net equation of the Calvin cycle from all the three phases is thus
3CO2 +5H2O +6NADPH +9ATP producing glyceraldehyde-3-phosphate + 6NADP+ + 3H+ + 9ADP +8Pi
The molecule of glyceraldehyde-3-phosphate that goes into the production of carbohydrate is converted via a cascade of reactions which are also catalysed by different enzymes.
Some of the enzymes mentioned above that did not seem to have any regulatory effect on photosynthesis in the Calvin cycle, can have regulatory effect in other pathways of photosynthesis, thus regulating it in a way. The carbon from the Calvin cycle is partitioned inside the cell into either sucrose synthesis, which is the main transportation molecule of sugars in plants or into starch biosynthesis which is the main storage form of carbohydrates is plants. Therefore these two biosynthesis pathways can have a regulatory effect in photosynthesis and thus they can also be looked at in order to see their effect. By genetic manipulation of these pathways the rate of photosynthesis can be regulated by bioengineers like in the case of sugarcane or potato where high photosynthesis rates are needed for sucrose and starch accumulation respectively.
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