The Calvin Cycle

That RuDP was the carbon dioxide acceptor was established in further experiments when the light and dark reactions were separated. 

Light was required for the generation of NADPH needed to reduce phosphoglyceric acid to give glyceraldehyde 3-phosphate. The NADPH was then used to give the three molecules of ATP needed to phospho-rylate ribulose phosphate and the triose phosphates, so completing the Calvin cycle. 

Cantoni, G.L. (1955). Enzymatic Mechanisms and the Biological Significance of Transmethylation Reactions, pp. 233-237, ibid. 

Johnson, P., Scales, B Eds. (1972). Liquid Scintillation Counting. Heyden Son, London. 

Hevesy, G. (1947). Radioactive indicators in turnover studies. Advances in Enzmol. 7, 111-214. 

We have already become familiar with the first step in the fixation and reduction of CO2. The subsequent reaction steps were also discovered by Calvin s group using the technique outlined above in connection with 3-phosphoglyceraldehyde. Let us consider the complete sequence of reactions, including the first step once again (Fig. 36). 

present in the form of HCO3—, is fixed in the acceptor, ribulose-1,5-diphosphate, by means of the enzyme carboxydismutase. An intermediate with 6 C atoms is formed, the identity of which is still unknown. This substance is unstable. It decomposes into two molecules of 3-phosphoglyceric acid. The latter is then reduced to 3-phosphoglyceraldehyde by means of the ATP and NADPH + H+ formed in the primary processes. 3-Phosphoglyceraldehyde exists in equilibrium with its isomer, dihydroxy acetone phosphate. The equilibrium is controlled by the enzyme triose phosphate isomerase. 3-Phosphoglyceralde-hyde and dihydroxy acetone phosphate ar referred to collectively as triose phosphate. 

At triose phosphate the pathway branches in one direction two molecules of triose phosphate (one each of 3-phosphoglyceraldehyde and dihydroxy acetone phosphate) combine to form a molecule of fructose-1,6-diphosphate. The reaction mechanism is a so-called aldol condensation, and, accordingly, the enzyme controlling the step is called aldolase. A phosphatase can then cleave a phosphate residue from fructose-1,6-diphosphate. Fructose-6-phosphate is formed which can be converted into other sugars. In this way, the CO2 of the air is utilized for the synthesis of carbohydrates. 

In the other direction, triose phosphates participate in the regeneration of the CO2 acceptor ribulose-1,5-diphosphate. A balance sheet presents the following picture. In a complicated sequence of interconversions, which we will not consider in detail here, 3 molecules of triose phosphate and one molecule of fructose-6-phosphate, thus, a total of 15 C atoms, are put in. Three molecules-of ribulose-5-phosphate, i.e. 15 C atoms again, are put out. Ribulose-5-phosphate is then converted into 

Let us now return to the experiments of Calvin. Starting from and returning to ribulose-1,5-diphosphate we find a cycle which provides a supply of (a) carbohydrates, e.g. fructose-6-phosphate, and (b) the CO2 acceptor, ribulose-1,5-diphosphate. This cycle is known as the Calvin cycle, 

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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... 

Each number in parentheses represents the number of carbon atoms in a compound, and the number preceding the parentheses indicates the stoichiometry of the reaction. Thus, 6(1), or 6 COg, condense with 6(5) or 6 RuBP to give 12 3-phosphoglycerates. These 12(3)s are then rearranged in the Calvin cycle to form one hexose, 1 (6), and regenerate the six 5-carbon (RuBP) acceptors. 

The Calvin cycle enzymes serve three important ends ... 

Most of the enzymes mediating the reactions of the Calvin cycle also participate in either glycolysis (Chapter 19) or the pentose phosphate pathway (Chapter 23). The aim of the Calvin scheme is to account for hexose formation from 3-phosphoglycerate. In the course of this metabolic sequence, the NADPH and ATP produced in the light reactions are consumed, as indicated earlier in Equation (22.3). 

Balancing the Calvin Cycle Reactions To Account for Net Hexose Synthesis... 

Compartmentation of these reactions to prevent photorespiration involves the interaction of two cell types, mescrphyll cells and bundle sheath cells. The meso-phyll cells take up COg at the leaf surface, where Og is abundant, and use it to carboxylate phosphoenolpyruvate to yield OAA in a reaction catalyzed by PEP carboxylase (Figure 22.30). This four-carbon dicarboxylic acid is then either reduced to malate by an NADPH-specific malate dehydrogenase or transaminated to give aspartate in the mesophyll cells. The 4-C COg carrier (malate or aspartate) then is transported to the bundle sheath cells, where it is decarboxylated to yield COg and a 3-C product. The COg is then fixed into organic carbon by the Calvin cycle localized within the bundle sheath cells, and the 3-C product is returned to the mesophyll cells, where it is reconverted to PEP in preparation to accept another COg (Figure 22.30). Plants that use the C-4 pathway are termed C4 plants, in contrast to those plants with the conventional pathway of COg uptake (C3 plants). 

Write a balanced equation for the synthesis of a glucose molecule from ribulose-l,5-bisphosphate and COg that involves the first three reactions of the Calvin cycle and subsequent conversion of the two glyceraldehyde-3-P molecules into glucose. 

Photosynthesis in green plants occurs in two basic processes. In the dark (the Calvin cycle) carbon dioxide is reduced by a strong reducing agent, the reduced form of nicotinamide adeninedinucleotide phosphate, NADPH2, with the help of energy obtained from the conversion of ATP to ADP ... 

Fig. 6.3 The Calvin cycle or the dark reactions of photosynthesis see Cooper and also Stryer in Further Reading.

Much interest has recently been shown in artificial photosynthesis. Photosynthesis is a system for conversion or accumulation of energy. It is also interesting that some reactions occur simultaneously and continuously. Fujishima et al. [338] pointed out that a photocatalytic system resembles the process of photosynthesis in green plants. They described that there are three important parts of the overall process of photosynthesis (1) oxygen generation by the photolysis of water, (2) photophosphorylation, which accumulates energy, and (3) the Calvin cycle, which takes in and reduces carbon dioxide. The two reactions, reduction of C02 and generation of 02 from water, can occur simultaneously and continuously by a sonophotocatalytic reaction. 

Figure 38, Chapter 3. A bifurcation diagram for the model of the Calvin cycle with product and substrate saturation as global parameters. Left panel Upon variation of substrate and product saturation (as global parameter, set equalfor all irreversible reactions), the stable steady state is confined to a limited region in parameter space. All other parameters fixed to specific values (chosen randomly). Right panel Same as left panel, but with all other parameters sampled from their respective intervals. Shown is the percentage r of unstable models, with darker colors corresponding to a higher percentage of unstable models (see colorbar for numeric values).

Figure 39, Chapter 3. Bifurcation diagrams for the model of the Calvin cycle for selected parameters. All saturation parameters are fixed to specific values, and two parameters are varied. Shown is the number of real parts of eigenvalues larger than zero (color coded), with blank corresponding to the stable region. The stability of the steady state is either lost via a Hopf (HO), or via saddle node (SN) bifurcations, with either two or one eigenvalue crossing the imaginary axis, respectively. Intersections point to complex (quasiperiodic or chaotic) dynamics. See text for details.

Bistability and switching are crucial concepts of cellular regulation [80, 98] and can often be detected using the graphical methods outlined above. See also [98,287] for several illustrative examples. For a number of pathways, transitions between different states were observed, either in silico, in vivo, or both. Examples include the glycolytic pathway [273, 294], the Calvin cycle [113, 125], and models of the human erythrocytes [295, 296],... 

The pathway is depicted in Fig. 35. The Calvin cycle, taking place in the chloroplast stroma of plants, is a primary source of carbon for all organisms and of central importance for a variety of biotechnological applications. The set of reactions, summarized in Table VIII, is adopted from the earlier models of... 

As one of its characteristic features, the Calvin cycle leads to a net synthesis of its intermediates with significant implications for the stability of the cycle. Obviously, the balance between withdrawal of triosephosphates (TP) for biosynthesis and triosephosphates that are required for the recovery of the cycle is crucial. The overall reaction of the Calvin cycle is... 

Figure 36. The Calvin cycle leads to an autocatalytic net synthesis of cycle intermediates. Upon three cycles, one triosephosphate is synthesized for export. The figure is inspired by a depiction of the Calvin cycle given on http //sandwalk.blogspot.com/2007/07/Calvin cycle regeneration.html.

Figure 42. The distribution of the largest real part within the spectrum of eigenvalues for the model of the Calvin cycle described in Section VIII. F. Only a minority of sampled models correspond to a stable steady state. See also Fig. 37 for convergence in dependence of the number of samples.

Figure 44. The correlation coefficient of the saturation parameters with stability, here identified with the largest real part within the spectrum of eigenvalues. Models (Jacobians) of the Calvin cycle are iteratively sampled and the correlation coefficient between each saturation parameter and the largest real part of the eigenvalues are evaluated. Negative values imply a negative correlation, that is, small saturation parameters correspond to a higher probability of instability.

J. Schwender, F. Goffman, J. B. Ohlrogge, and Y. Shachar Hill, Rubisco without the Calvin cycle improves the carbon efficiency of developing green seeds. Nature 432, 779 782 (2004). 

M. G. Poolman, Computer modeling applied to the Calvin cycle. Ph.D. thesis, Oxford Brookes... 

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