Reactions Calvin-Benson cycle

The set of reactions that transforms 3-phosphoglycerate into hexose is named the Calvin-Benson cycle (often referred to simply as the Calvin cycle) for its discoverers. The reaction series is indeed cyclic because not only must carbohydrate appear as an end product, but the 5-carbon acceptor, RuBP, must be regenerated to provide for continual COg fixation. Balanced equations that schematically represent this situation are... 

FIGURE 22.23 The Calvin-Benson cycle of reactions. The number associated with the arrow at each step indicates the number of molecules reacting in a turn of the cycle that produces one molecule of glucose. Reactions are numbered as in Table 22.1. 

All the O atoms evolved as Og come from water none comes from carbon dioxide. But 12 O atoms are evolved as 6 Og, and only 6 O atoms appear as 6 HgO in the equation. Also, 6 COg have 12 O atoms, yet there are only 6 O atoms in CgHigOg. How can yon account for these discrepancies Hint Consider the partial reactions of photosynthesis ATP synthesis, NADP reduction, photolysis of water, and the overall reaction for hexose synthesis in the Calvin-Benson cycle.)... 

Fig. 3-5 The Calvin-Benson cycle. This is the series of reactions by which CO2 is fixed into organic material.

The light independent reactions take place in the stroma with the help of ATP and NADPH. In a process called the Calvin-Benson cycle, or carbon fixation, carbon dioxide from the atmosphere is captured and converted into carbohydrates [135]. The reaction is catalyzed by the enzyme RuBisCO (ribulose-1,5-biphosphate... 

A quantitatively much more important pathway of C02 fixation is the reductive pentose phosphate pathway (ribulose bisphosphate cycle or Calvin-Benson cycle Fig. 17-14). This sequence of reactions, which takes place in the chloroplasts of green plants and also in many chemiautotrophic bacteria, is essentially a way of reversing the oxidative pentose phosphate pathway (Fig. 17-8). The latter accomplishes the complete oxidation of glucose or of glucose 1-phosphate by NADP+ (Eq. 17-48) ... 

The overall reaction of carbon dioxide reduction in the Calvin-Benson cycle (Fig. 17-14) becomes... 

Figure 17-14 (A) The reductive carboxylation system used in reductive pentose phosphate pathway (Calvin-Benson cycle). The essential reactions of this system are enclosed within the dashed box. Typical subsequent reactions follow. The phosphatase action completes the phosphorylation-dephosphorylation cycle. (B) The reductive pentose phosphate cycle arranged to show the combining of three C02 molecules to form one molecule of triose phosphate. Abbreviations are RCS, reductive carboxylation system (from above) A, aldolase, Pase, specific phosphatase and TK, transketolase.

The Calvin-Benson cycle and the pentose phosphate pathway (Eq. 17-12) have many features in common but run in opposite directions. Since the synthesis of glucose from C02 requires energy, the energy expenditure for the two processes will obviously differ. Describe the points in each pathway where a Gibbs energy difference is used to drive the reaction in the desired direction. 

The key reaction of the Calvin-Benson cycle of C02 reduction is the carboxylation of ribulose bisphos-phate to form two molecules of 3-phosphoglycerate (Eq. 13-48). The properties of ribulose bisphosphate carboxylase (rubisco, Figs. 13-10 to 13-12), which catalyzes this reaction, are discussed in Chapter 13. It... 

The overall effect is to transport C02 from the mesophyll cells into the bundle sheath cells along with two reducing equivalents, which appear as NADPH following the action of the malic enzyme. The C02, the NADPH, and additional NADPH generated in the chloroplasts of the bundle sheath cells are then used in the Calvin-Benson cycle reactions to synthesize 3-phospho-glycerate and other materials. Of the C02 used in the bundle sheath cells, it is estimated that 85% comes via the C4 cycle and only 15% enters by direct diffusion. The advantage to the cell is a higher C02 tension, less competition with 02, and a marked reduction in photorespiration. 

Calvin cycle (aka Calvin-Benson Cycle or Carbon Fixation) Series of biochemical, enzyme-mediated reactions during which atmospheric carbon dioxide is reduced and incorporated into organic molecules, eventually some of this forms sugars. In eukaryotes, this occurs in the stroma of the chloroplast. 

PGA (phosphoglycerate) A three-carbon molecule formed when carbon dioxide is added to ribulose biphosphate (RuBP) during the dark reaction of photosynthesis (Calvin, or Calvin-Benson Cycle). PGA is converted to PGAL, using ATP and NADPH. 

The isotopic fingerprinting of photosynthesis. Photosynthesis is currently an important means of fixing inorganic carbon and is used by plants, algae, bacteria, and Archaea. A particularly important enzyme (catalyst) in this process is Rubisco, the official shortened name for a molecule with a much more intricate systematic name. The reactions which take place in this process operate according to the Calvin-Benson Cycle and lead to isotopic fractionations of up to 30%o. 

P in high concentrations turns on the Calvin-Benson cycle. Like the reactions of gluconeogenesis (Chapter... 

In the light-independent or dark reactions the enzyme RuBisCO (ribulose-1,5-bisphosphate carboxylase oxygenase) consumes CO2 from the atmosphere and in a complex process called the Calvin-Benson cycle releases three-carbon sugars that are subsequently combined to form glucose. 

Transketolase (TK) is involved in anaerobic carbohydrate metabolisms such as the nonoxidative phase of the pentose phosphate pathway. In plants and photosynthetic bacteria, TK is involved in the Calvin-Benson cycle. TK catalyses the transfer of a 2-carbon dihydroxyethyl group from a ketose phosphate (donor substrate such as D-xylulose 5-phosphate) to the Cl position of an aldose phosphate (acceptor substrate such as o-ribose 5-phosphate) (Figure 4.3) (Schneider and Lindqvist 1998). The first product is an aldose phosphate released from the donor (such as glyceraldehyde 3-phosphate) and the second is a ketose phosphate (such as sedoheptulose 7-phosphate), in which the 2-carbon fragment is attached to the acceptor. Examples of the substrates and the products mentioned above are for the first reaction of the pentose phosphate pathway. In the second reaction of the same pathway, the acceptor is D-ery-throse 4-phosphate and the second product is o-fructose 6-phosphate. A snapshot X-ray crystallographic study revealed that an ot-carbanion/enamine a,p-dihydroxyethyl ThDP is formed as a key intermediate (Fiedler et al. 2002). Then, a nucleophilic attack of the a-carbanion intermediate on the acceptor substrate occurs. 

The NADPH and ATP generated are used within the chloroplast to fix carbon dioxide (CO2) and reduce it to sugar. This sequence of reactions is called the Calvin-Benson-Bassham cycle and this cycle results in the production of the phosphorylated 3-carbon sugar glyceraldehyde-3-phosphate, also called triose. This triose sugar is exported out of the chloroplast and in this course we will refer to it as CH2O which is the chemical composition of sugar when normalized to 1 carbon atom. 

AH the sugar (CH2O) needs to be formed by photosynthesis in the chloroplast. In the Calvin—Benson—Bassham cycle CO2 is reduced to the level of sugar (CH2O) according to the following reaction and stoichiometry ... 

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