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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