Light-independent reactions

Overview of the Calvin cycle and carbon fixation

The Calvin cycle or Calvin–Benson cycle or Reductive Pentose Phosphate cycle is a series of biochemical redox reactions that take place in the stroma of chloroplasts in photosynthetic organisms. It was discovered by Melvin Calvin, James Bassham, and Andrew Benson at the University of California, Berkeley by using the radioactive isotope carbon-14. It is one of the light-independent (dark) reactions, used for carbon fixation.

Overview

INTRODUCTION

For the reasons to become apparent, this edit - done in italics - leaves untouched in its entirety the original article on the Calvin cycle. The Calvin cycle, or Benson-Calvin cycle, or Bassham-Benson-Calvin cycle in photosynthesis, or the photosynthetic carbon cycle of dark reactions in photosynthesis, does not exist. The purported experimental verification of the Calvin cycle originates from a single news story published in the July 4, 1955, issue of Chem. & Eng. News, contrary to the entire body of original papers by Calvin et al on the Berkeley group's C-14 tracer studies.

OMITTED ORIGINAL PAPERS BY MELVIN CALVIN AND FRANCIS K. FONG IN THE PERMANENT LITERATURE

This, the original Calvin cycle Wikipedia webpage, has for its first reference, the 1950 paper by Bassham, Benson and Calvin, in which Bassham, Benson and Calvin proposed the photosynthetic carbon cycle, later known as the Calvin cycle. The second reference skips over 56 years of research published in the permanenet literature, to the monograph (not a research paper in a peer-reviewed journal) by Campbell, Williamson and Heyden (2006).

Unfortunately, omitted from this Wikipedia article are the original papers by the Calvin group at Berkeley: Calvin,M. and Massini,P. (1952) Experientia 8, 445-484; Wilson,A.T. and Calvin,M. (1955) J. Am. Chem. Soc. 77, 5948-5957; Bassham, J.A., Shibata, K., Steenberg, K., Bourdon, J. and Calvin, M. (1956) J. Am. Chem. Soc., 78, 4120-4124; Bassham, J.A. and Calvin, M. (1957) "The Path of Carbon in Photosynthesis," Prentice-Hall, Inc., Englewood Cliffs, N.J.; Calvin, M. and Pon, N.G. (1959) J. Cellular Comp. Physiol., 54, Suppl. 1, 51-74; and Bassham, J.A. and Kirk, M. (1960) Biochim. Biophys. Acta, 43, 447-464.

In these omitted papers, in refuting Bassham, Benson and Calvin (1950), Calvin et al reported that photosynthetic carbon fixation occurs in a noncyclic path, directly in the light, via a reductive system as yet unknown. In short, the Calvin cycle does not exist in photosynthesis.

In 1988, Francis K. Fong and his group at Purdue and NSFfrunding.com provided a molecular mechanism for the reductive carbon metabolic path reported by Calvin et al. See, Fong, Francis K. and Butcher, Karen A. (1988) Biochem. Biophys. Res. Commun., 150, 399-404.

THE FONG-BUTCHER MODEL OF PHOTOREDUCTIVE CARBON PATH IN PHOTOSYNTHESIS IN SUPPORT OF THE ORIGINAL PAPERS BY CALVIN ET AL

Fong and Butcher (1988) showed that carbon fixation in photosynthesis is regulated in two kinetically coupled pathways involving a specialized pair of non-equivalent, enzyme-bound glycerate-3-P (3-PGA) molecules obtained from ribulose 1,5-bisphosphate (RuBP) carboxylation in the light. A non-cyclic pathway is suggested for the direct biosynthesis of sucrose from the ["lower"] 3-PGA obtained from C-3, C-4 and C-5 of the six-carbon carboxylation adduct. Concomitant to the appearance of sucrose as the principal product, the Mg2+-bound ["upper"] 3-PGA molecule formed from C-1, C-2 and C-2' of the C6 intermediate is released and subsequently reduced, with the intervention of the NADPH and ATP - in a Calvin-cycle-like metabolic path - in regenerating the RuBP. It is proposed that the nocturnal inhibitor, 2-carboxyarabinitol-1-phosphate (1-PCA) is obtained from a condensation of 3-PGA and glyceraldehyde.

On transition from conditions of light to the dark, the metabolic pools for the entire photosynthetic apparatus decay exponentially to zero, consistent with Calvin et al's observation that the Calvin cycle does not exist in photosynthesis.

For the reasons given above, in support of those reported by Calvin, Fong and their co-workers - that the Calvin cycle in photosynthesis, i.e., the photosynthetic carbon cycle of dark reactions, does not exist - it is not possible to edit this, the original webpage on the Calvin cycle.

For a schematic representation of Fong and Butcher (1988) on the photoreductive carbon path in photosynthesis, see, Fig. 2 of NSFfunding.com's synopsis site, The Calvin Cycle Website.

THE ORIGINAL, UNEDITED VERSION OF THIS "OVERVIEW," WHICH IS CONTRARY TO THE ORIGINAL PAPERS IN THE PERMANENT LITERATURE BY CALVIN, FONG AND THEIR C0-WORKERS, IS SHOWN AS FOLLOWS.

During photosynthesis, light energy is used in generating chemical free energy, stored in glucose. The light-independent Calvin cycle, also known (erroneously) as the "dark reaction" or "dark stage," uses the energy from short-lived electronically-excited carriers to convert carbon dioxide and water into organic compounds that can be used by the organism (and by animals that feed on it). This set of reactions is also called carbon fixation. The key enzyme of the cycle is called RuBisCO. In the following biochemical equations, the chemical species (phosphates and carboxylic acids) exist in equilibria among their various ionized states as governed by the pH.

The enzymes in the Calvin cycle are functionally equivalent to many enzymes used in other metabolic pathways such as gluconeogenesis and the pentose phosphate pathway, but they are to be found in the chloroplast stroma instead of the cell cytoplasm, separating the reactions. They are activated in the light (which is why the name "dark reaction" is misleading), and also by products of the light-dependent reaction. These regulatory functions prevent the Calvin cycle from being respired to carbon dioxide. Energy (in the form of ATP) would be wasted in carrying out these reactions that have no net productivity.

The sum of reactions in the Calvin cycle is the following:

3 CO₂ + 6 NADPH + 5 H₂O + 9 ATP → glyceraldehyde-3-phosphate (G3P) + 2 H⁺ + 6 NADP⁺ + 9 ADP + 8 P_i

or

3 CO₂ + 6 C₂₁H₂₉N₇O₁₇P₃ + 5 H₂O + 9 C₁₀H₁₆N₅O₁₃P₃ → C₃H₅O₃-PO₃^2- + 2 H⁺ + 6 NADP⁺ + 9 C₁₀H₁₅N₅O₁₀P₂ + 8 P_i

Hexose (six-carbon) sugars are not a product of the Calvin cycle. Although many texts list a product of photosynthesis as C₆H₁₂O₆, this is mainly a convenience to counter the equation of respiration, where six-carbon sugars are oxidized in mitochondria. The carbohydrate products of the Calvin cycle are three-carbon sugar phosphate molecules, or "triose phosphates," namely, glyceraldehyde-3-phosphate (G3P).

Steps of the Calvin cycle

1. The enzyme RuBisCO catalyses the carboxylation of ribulose-1,5-bisphosphate, RuBP, a 5-carbon compound, by carbon dioxide (a total of 6 carbons) in a two-step reaction. The product of the first step is enediol-enzyme complex that can capture CO₂ or O₂. Thus, enediol-enzyme complex is the real carboxylase/oxygenase. The CO₂ that is captured by enediol in second step produces a six-carbon intermediate initially that immediately splits in half, forming two molecules of 3-phosphoglycerate, or PGA, a 3-carbon compound (also: 3-phosphoglycerate, 3-phosphoglyceric acid, 3PGA).
2. The enzyme phosphoglycerate kinase catalyses the phosphorylation of 3PGA by ATP (which was produced in the light-dependent stage). 1,3-Bisphosphoglycerate (glycerate-1,3-bisphosphate) and ADP are the products. (However, note that two PGAs are produced for every CO₂ that enters the cycle, so this step utilizes two ATP per CO₂ fixed.)
3. The enzyme G3P dehydrogenase catalyses the reduction of 1,3BPGA by NADPH (which is another product of the light-dependent stage). Glyceraldehyde 3-phosphate (also G3P, GP, TP, PGAL) is produced, and the NADPH itself was oxidized and becomes NADP⁺. Again, two NADPH are utilized per CO₂ fixed.

(Simplified versions of the Calvin cycle integrate the remaining steps, except for the last one, into one general step - the regeneration of RuBP. Also, one G3P would exit here.)

1. Triose phosphate isomerase converts all of the G3P reversibly into dihydroxyacetone phosphate (DHAP), also a 3-carbon molecule.
2. Aldolase and fructose-1,6-bisphosphatase convert a G3P and a DHAP into fructose 6-phosphate (6C). A phosphate ion is lost into solution.
3. Then fixation of another CO₂ generates two more G3P.
4. F6P has two carbons removed by transketolase, giving erythrose-4-phosphate. The two carbons on transketolase are added to a G3P, giving the ketose xylulose-5-phosphate (Xu5P).
5. E4P and a DHAP (formed from one of the G3P from the second CO₂ fixation) are converted into sedoheptulose-1,7-bisphosphate (7C) by aldolase enzyme.
6. Sedoheptulose-1,7-bisphosphatase (one of only three enzymes of the Calvin cycle that are unique to plants) cleaves sedoheptulose-1,7-bisphosphate into sedoheptulose-7-phosphate, releasing an inorganic phosphate ion into solution.
7. Fixation of a third CO₂ generates two more G3P. The ketose S7P has two carbons removed by transketolase, giving ribose-5-phosphate (R5P), and the two carbons remaining on transketolase are transferred to one of the G3P, giving another Xu5P. This leaves one G3P as the product of fixation of 3 CO₂, with generation of three pentoses that can be converted to Ru5P.
8. R5P is converted into ribulose-5-phosphate (Ru5P, RuP) by phosphopentose isomerase. Xu5P is converted into RuP by phosphopentose epimerase.
9. Finally, phosphoribulokinase (another plant-unique enzyme of the pathway) phosphorylates RuP into RuBP, ribulose-1,5-bisphosphate, completing the Calvin cycle. This requires the input of one ATP.

Thus, of 6 G3P produced, three RuBP (5C) are made, totaling 15 carbons, with only one available for subsequent conversion to hexose. This required 9 ATPs and 6 NADPH per 3 CO₂.

RuBisCO also reacts competitively with O₂ instead of CO₂ in photorespiration. The rate of photorespiration is higher at high temperatures. Photorespiration turns RuBP into 3PGA and 2-phosphoglycolate, a 2-carbon molecule that can be converted via glycolate and glyoxalate to glycine. Via the glycine cleavage system and tetrahydrofolate, two glycines are converted into serine +CO₂. Serine can be converted back to 3-phosphoglycerate. Thus, only 3 of 4 carbons from two phosphoglycolates can be converted back to 3PGA. It can be seen that photorespiration has very negative consequences for the plant, because, rather than fixing CO₂, this process leads to loss of CO₂. C4 carbon fixation evolved to circumvent photorespiration, but can occur only in certain plants native to very warm or tropical climates, for example, corn.

Products of the Calvin cycle

The immediate products of one turn of the Calvin cycle are 2 glyceraldehyde-3-phosphate (G3P) molecules, 3 ADP, and 2 NADP⁺ (ADP and NADP⁺ are regenerated in the Light-dependent reactions). Each G3P molecule is composed of 3 carbons. In order for the Calvin cycle to continue, RuBP (ribulose 1,5-bisphosphate) must be regenerated. So, 5/6 carbon from the 2 G3P molecules are used for this purpose. Therefore, there is only 1 net carbon produced to play with for each turn. To create 1 surplus, G3P requires 3 carbons, and therefore 3 turns of the Calvin cycle. To make one glucose molecule (which can be created from 2 G3P molecules) would require 6 turns of the Calvin cycle. Surplus G3P can also be used to form other carbohydrates such as starch, sucrose, and cellulose, depending on what the plant needs.

See also

• Light-independent reaction
• Light-dependent reactions
• Citric Acid Cycle
• Photorespiration
• C₄ carbon fixation
• Nitrogen Fixation

References

• Bassham, J.A. (2003). Mapping the carbon reduction cycle: a personal retrospective. Photosynthesis Research, volume 76, pages 25–52 (see: Template:Entrez Pubmed).Mario Otmman (1998)

• Diwan, Joyce J. (2005). Photosynthetic Dark Reaction at [2]

• Calvin, Melvin | Article | World Book Advanced at [3]