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'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's Home Page.

Thursday, December 10, 2009

The Fong-Butcher Model : Photoreductive Carboxylation in Photosynthesis

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

In 1995-96, along the lines of a molecular mechanism the author Francis K. Fong proposed for chlorophyll photosynthesis in plants, Nick Winograd demonstrated(1) the conversion of sunlight into electricity, a photogalvanic (photovoltaic) effect. Fong proposed the two hydration states of the chlorophyll; Winograd showed experimentally that chlorohpyll a dihydrate was the photoreactive species. In 1978-79, Fong and Galloway demonstrated that the photogalvanic conversion by Winograd was in vitro a result of the chlorophyll water splitting reaction.(2) In 1979, in CO2-saturated solutions, Fong and co-workers showed that this water splitting reaction resulted in photoreduction of carbon to organic fuels.(3) In the course of this activity Diestler and Fong invoked the Born-Oppenheimer adiabatic approximation in formulating a nonequilibrium theory of chemical rate processes in condensed media. Here, the photoactivation of a "reactive complex" results in either deactivation through non-radiative multiphon transitions to the ground state or yield the primary photochemical products.

On completion of the Fong-Butcher pathway: from left, Angela Agostiano, Karen Butcher, the auther and Margareta Fong
Appropriately, this theory was used to understand the chlorophyll photosynthetic reaction in green plants based on Calvin et el's finding of the light carboxylation reaction, leading to the noncyclic light reaction described by Fong and Butcher,(13) a schematical representation of which is shown below.

The Fong-Butcher pathway
Significantly, the non-cyclic photoreductive carboxylation reaction in photosynthesis was peer-reviewed for publication by Bernie Horecker, who first demonstrated the in vitro RuBP carboxylation reaction,(7) on which was based the Calvin cycle.  The Fong-Butcher model provides a detailed mechanism for the asymmetric reductive carboxylation system in photosyntehsis reported by Calvin and his co-workers. The 5-carbon ribulose bisphosphate (RuBP) picks up a CO2 molecule to make the 6-carbon adduct, which splits into two non-equivalent PGA (phosphyglycerate) fragments. A non-cyclic pathway is suggested for the direct biosynthesis of sucrose from the 3-PGA freed from C-3, C-4 and C-5 of the six-carbon adduct. Concomitant to the appearance of sucrose as the principal product, an Mg2+-bound 3-PGA molecule from C-1, C-2 and C-2' of the C6 intermediate is generated and enters into a cyclic path to regenerates the RuBP.

There exists a steady-state reaction cycle, which mediates the noncyclic path of succrose synthesis and oxygen evolution.  This cycle resembles the Calvin cycle, except that, instead of the two molecules of PGA, the two 3-carbon fragments from the splitting of the 6-carbon CO2-RuBP adduct are not the same, one being PGA and the other, PGL.  The steady-state rection cycle exists only in the light.  On transition from the conditions of light to darkness, it along with its reaction intermediates decay exponnentially to zero.

As long ago as 1845, Mayer recognized the role of sunlight as a source of energy for photosyntehsis, the conversion of carbon dioxide and water to organic matter. See, Lundegarth, H.: "On Oxidation of Cytochrome f by Light," Physiol. Plantarum, 7, 375-382 (954).

For von Baeyer's original paper on photosynthesis, see, Baeyer, A.: "Ueber die Wasserentziehung urd ihre Bedeutung fur das Pflanzenleben und die Gahrung,"" Ber. deut. chem. Ges., 3, 63-75 (1870). Reviewed in Photosynthesis and Related Processes, I, E. I. Rabinowitch, New York.
Interscience Publishers, p. 246 (1945).

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