Thursday, December 17, 2009

The Calvin Cycle and Photosynthesis

For the National Science Foundation's explanation how the Calvin cycle was established contrary to Calvin et al's finding of the light carboxylation reaction, go to NSFfunding.com's Synopsis site.

On this post, the author Francis K. Fong provides, in Section A, a summary of what today is known as the Calvin cycle; and in Section B, what according to Calvin's original papers - on the Berkeley group's finding from their C-14 tracer studies of the reductive path of carbon in photosynthesis supported by Fong and his coworkers' publications in the permanent literature - is the experimental finding of caron fixation in photosynthesis.

Fig.A. The Calvin cycle: Fig.18 of Calvin's Nobel Lecture, left; Wikipedia's represenation, right.

In Fig.A, the Wikipedia's schematic representation of the Calvin cycle is shown. This and literally hundreds of others shown online and in the print media differ from Calvin et al's published findings in the permanent literature, that, in photosynthesis, the addition of one molecule of CO2 to the 5-carbon RuBP yields one 3-carbon molecule of phosphoglycerate, PGA and the other of phosphoglyceraldehyde, PGL, Fig.B.


A. The Calvin Cycle

The Calvin cycle denotes the photosynthetic dark reactions.   It is NSF's Dark Photosynthesis Funding Standard (DPFS), which the Foundation and the author Franis K. Fong have sought to correct for the past three decades

The Calvin cycle requires that photosynthesis in green plants or algae, the reduction in sunlight of carbon dioxide by water to organic matter:

H2O + CO2 → organic fuel + O2

occurs in the dark.

In photosynthesis, the initial step of carbon fixation is the addition of a carbon dioxide molecule to a 5-C molecule, ribulose 1,5-bisphosphate (RuBP), to yield a 6-C intermediate.

In the Calvin cycle, the splitting of the 6-C intermediate into two molecules of PGA, D-glycerate-3-phosphate is followed by reduction of the PGA by NADPH (the reduced form of NADP+, nicotinamide adenine dinucleotide phosphate) and energy (of the "high-energy phosphoanhydride bonds" of ATP, adenosine triphosphate) from the "chlorophyll light reactions" of the Z scheme. 

The Calvin cycle initiates with the dark CO2 fixation reaction, in which the CO2 is not reduced to the sugar oxidation level.  It is the carboxylation reaction, in which the addition of 1 molecule of CO2 to 1 molecule of the RuBP to form, in the dark, 2 molecules of PGA:

RuBP + CO2 → 2 PGA (1)

The PGA molecules are then reduced, in the dark, by 4 [H] (hydrogen equivalents) from the NADPH to PGL:

2PGA + 4[H] → 2 PGL (2)

Thereafter, PGL undergoes condensation to regenerate the RuBP, and a new cycle begins.

In photosynthesis, the Calvin cycle or the dark reaction cycle is regulated by two chlorophyll light reactions called photosystems in the Z scheme.  The light reactions convert the energy of light as “high energy” phosphoanhydride bonds of ATP, and as reducing power NADP+ to NADPH. This is known as "photophosphorylation."  In the dark reaction pathway, the free energy for cleavage of -P bonds in ATP and the reducing power of NADPH are said to be used to reduce CO2 to form biomass.


B. Photosynthesis: Light and Dark Reactions 

The photophosphorylation reactionns yielding ATP and NADPH are photosynthetic dark reations, the after reactions for converting or storing excess energy from the primary chlorophyll light reactions in photosynthesis. 


In the late 1970's, after Fong and his co-workers demonstrated the in vitro chlorophyll water-splitting and carbon-reduction reactions, the late Dan Arnon, the foremost researcher in photophosphorylation, invited Fong to a weeklong visit at Berkeley.  Arnon had wanted to win the Nobel Prize, but the Nobel Committee did not award the Prize to plant physiologists.  Arnon wanted to see if Fong could collaborate with him on some basis of mutual interest.  For an historical perspective on Fong's research papers, as they relate to the research at LRL - as seen through the eyes of the leading worker in dark photosyntehsis research, of which photophosphorylation is the essential process, refer to Daniel Israel Arnon Papers, BANC MSS 99/315 c, The Bancroft Library, Berkeley. Series 1: Correspondence, 1938-1994—Benson, Andrew A. 1955-1973 (ctn. 1, folder 32); Calvin, Melvin 1957-1987 (ctn.1, folder 76); Fong, Francis K. 1977-1979 (ctn. 2, folder 28); Horecker, Bernard L. 1954-1973 (ctn. 2, folder 81); and Kamen, Martin David 1960-1989 (ctn. 3, folder 2).

For practically the entire duration of the Berkeley group's C-14 tracer studies, from 1952 to 1959, Calvin et al reported in their original papers that carboxylation of the RuBP in the light results in direct reduction of the carbon dioxide to sugar. See, 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; Vishniac, W., Horecker, B.L., and Ochoa, S. (1957) Adv. Enzymol. 19, 1-77; 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.


In Calvin and Pon (1959), the final paper of Calvin's C-14 tracer studies listed above, he concluded that RuBisCO, ribulose-1,5-bisphosphate carboxylase-oxygenase, is possibly not the enzyme for carbon fixation in plants; that, in fact, photosynthesis may occur via a reductive system as yet unknown. By 1961, the "very careful kinetic analysis" by Bassham et al presumably left no room for doubting the correctness of that conclusion. No reasonable grounds existed for Calvin to "reject" reaction (L). Indeed, he inserted in his Nobel Lecture, at Fig.20, a schematic representation of the reductive splitting into one molecule of PGA and the other, PGL, glyceraldehyde phosphate, a 3-C sugar. Calvin's reductive path of carbon is reproduced below, Fig.B(A), where reaction (L) is given by the dashed arrow.

Fig.B. Reductive path of carbon in photosynthesis: Calvin (1961); Fong (1989)


In 1988, Fong and Butcher showed the non-equivalence of the "upper" and "lower" 3-C fragments in the 6-C intermediate. See, Fong, Francis K. and Butcher, Karen A. (1988) Biochem. Biophys. Res. Commun., 150, 399-404.

Based on this showing, Fong et al, in 1989, published the reductive path of carbon in photosynthesis, which is reporduced above in Fig.B(B). F.K. Fong, K.A. Butcher, A. Agostiano, M. Della Monica, M.S. Showell, and J. V. Schloss (1989), "Coupling Between the Light and Dark Reactions of Oxygen Evolution and CO2 Fixation in Photosynthesis: Early Experiments in Photosynthesis Revisited," in Enzymatic and Model Carboxylation and Reduction Reactions for Carbon Dioxide Utilization, Eds. M. Aresta and J.V. Schloss, Series C: Mathematical and Physical Sciences - Vol. 314, NATO ASI Series Asvanced Science Institutes Series, Kluwer Academic Publishers, Dordrect/Boston/London.

For an outline of the patterned activity for making of the Calvin cycle from a news story released 7-4-55 by Purdue instructor Dale W. Margerum, go to NSFfunding.com's Home Page.

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