Calvin cycle evolution

W. Martin and C. Schnarrenberger. 1997. The evolution of the Calvin cycle from prokaryotic to eukaryotic chromosomes A case study of functional redundancy in ancient pathways through endosymbiosis Curr. Genet. 32 1-18. (PubMed)... 

Since conserved strnctures with a very long lifetime have a great advantage in the evolution, a biological system is used over and over again. The result is cyclic reactions where reactants are fed in along a pathway and products are given away. There are several examples in Natnre, for example, the citric acid cycle and the Calvin cycle. 

It has been suggested that the Hatch and Slack pathway is a modification of the Calvin cycle of advantage to plants growing in dense stands of tropical vegetation where the carbon dioxide concentration may be reduced to a very low level. The reduction of atmospheric CO2 concentration which has occurred since the evolution of photosynthetic reactions may also have contributed to the selection of this reaction sequence. The discovery of the Hatch and Slack pathway, although it is not yet fully authenticated, has demonstrated the possible existence of photosynthetic reactions other than the conventional Calvin cycle and suggests that other and as yet undiscovered variations on the photosynthetic theme may exist. 

According to this reaction scheme the absorption of four pairs of quanta (8 in all) results in the transfer of four electrons from water to NADP, the concomitant synthesis of two molecules of ATP and the evolution of one molecule of oxygen. Qualitatively then the requirements of the carbon reduction cycle are fulfilled. However, only one molecule of ATP is produced per NADP reduced whereas the carbon reduction cycle in its present form requires 3 molecules of ATP per 2 molecules of NADP reduced. Non-cyclic photophosphorylation, as the above mechanism of ATP synthesis is termed, cannot quantitatively satisfy the needs of the carbon reduction cycle. However, by the addition of suitable co-factors isolated chloroplast preparations can be induced to synthesise ATP without the transfer of electrons from water to NADP. The electron path is short-circuited and the electron ejected from chlorophyll P,oo in system I returns eventually to chlorophyll Ptoo- The return route, shown by a dotted line in Fig. 5.10, involves the synthesis of ATP. In this process, usually termed cyclic photophosphorylation, the only measurable product is ATP, oxygen is not evolved and NADP is not reduced. Cyclic photophosphorylation may balance the ATP-NADP stoichiometry for the operation of the Calvin cycle and could also supply ATP for other purposes such as the synthesis of polysaccharides. 

---