Photosynthetic carbon reduction cycle

FIGURE 20-4 The three stages of C02 assimilation in photosynthetic organisms. Stoichiometries of three key intermediates (numbers in parentheses) reveal the fate of carbon atoms entering and leaving the cycle. As shown here, three C02 are fixed for the net synthesis of one molecule of glyceraldehyde 3-phosphate. This cycle is the photosynthetic carbon reduction cycle, or the Calvin cycle. 

Calvin M. The photosynthetic carbon reduction cycle. J. Chem 40. Soc. 1956 1895-1915. 

Discovered sedoheptulose and ribulose phosphates in the intermediates of the photosynthetic carbon reductive cycle. Discovered and identified... 

Photosynthetic Carbon Reduction Cycle (Calvin Cycle)... 

Quantitatively, the most important enzyme of the photosynthetic carbon reduction cycle is rubisco, which represents around 50% of soluble leaf protein. This has a major disadvantage in that inhibition of a large amount of leaf protein could require logistically difficult and probably prohibitively expensive herbicide levels. 

He contributed to discoveries of the photosynthetic carbon reduction cycle. In addition, the opal glass method for the spectral characterization of chloroplasts was firmly established during this stay at Berkeley. Invited by Stacy French, he spent 1955-56 at the Carnegie Foundation Research Laboratories, Stanford, California studying the greening of etiolated... 

The other two enzymes of this part of the photosynthetic carbon reduction cycle respond differently to illumination. Ribulose-phosphate kinase increases in activity by about 66%, but only after about 12 hours of illumination. Ribose-phosphate isomerase increases only by 50% after about 18 hours. These changes in activity do not occur in chloramphenicol-treated leaves. 

The Effect of Illumination of Etiolated Maize on the Activity of Some Enzymes of the Photosynthetic Carbon Reduction Cycle... 

To summarize these observations First, the enzymes of this segment of the photosynthetic carbon reduction cycle are not controlled co-ordinately a single inductive step does not appear to affect the activity (and presumably the production) of these three enzymes in the same way. Second, the kinds of control mechanisms which appear to occur here are (1) a direct and prompt effect of illumination, reflected in the rapid increase in ribulose-diphosphate carboxylase activity, and (2) a more indirect kind of control, possibly involving induction of the other enzymes (e.g., ribose-phosphate isomerase) by small molecules produced in photosynthesis. Unfortunately, it has not been possible to demonstrate an increase in the level of either the isomerase or kinase by administration of glucose and some other carbohydrates to etiolated leaves in darkness. 

Subsequent research has shown that the pentose phosphate so produced is further metabolized in reactions utilizing the enzymes transaldolase and transketolase and involving as intermediates pentose, heptulose, tetrose and triose (5, 7, 4 and 3 carbon sugars) phosphates which are also involved in the photosynthetic carbon reduction cycle. A version of the pentose phosphate pathway is shown in Fig. 4.7. 

Evans MCW, Buchanan BB, Arnon DI. 1966. A new ferredoxin-dependent carbon reduction cycle in a photosynthetic bacterium. Proc Natl Acad Sci USA 55 928-34. 

Elstner EF, Staffer C, Heupel A (1975) Determination of superoxide free radical ion and hydrogen peroxide as products of photosynthetic oxygen reduction. Z Naturforsch 30c 53-57 Emmel T, Sand W, Konig WA, Bock E (1986) Evidence for the existence of a sulphur oxygenase in Sulfolobus brierleyi. J Gen Microbiol 132 3415-3420 Ensign SA, Hyman MR, Arp DJ (1993) In vitro activation of ammonia monooxygenase from Nitrosomonas europaea by copper. J Bacteriol 175 1971-1980 Erickson RH, Hooper AB (1972) Preliminary characterization of variant CO-binding heme protein from Nitrosomonas. Biochim Biophys Acta 275 231-244 Erickson RH, Hooper AB, Terry KR (1972) Solubilization and purification of cytochrome a, from Nitrosomonas. Biochim Biophys Acta 283 155-166 Evans MCW, Buchanan BB, Amon DI (1966) A new ferredoxin-dependent carbon reduction cycle in a photosynthetic bacterium. Proc Natl Acad Sci USA 55 928-934 Falk JE (1964) Porpyrins and metalloporphyrins. Elsevier, Amsterdam... 

The (CH20) is the general formula for a carbohydrate. It was then assumed that the energy stored in the carbohydrate was used in other chemical reactions to synthesize all the other plant materials (proteins, lipids, fats, and so on). It is now clear that amino acids, for example, are immediate products of the photosynthetic reduction of carbon dioxide, and that carbohydrate need not be synthesized first. This is not to minimize the importance of the photosynthesis of carbohydrate, but only to note that many other types of compounds are produced photosynthetically. The overall mechanism and many of the details of the carbon reduction cycle, CO2 to carbohydrate, were worked out by Melvin Calvin and his colleagues, for which he received the Nobel Prize. 

The synthetic reactions requiring electrons and ATP are not limited to the initial reduction of the inorganic oxides. Many secondary photosynthetic pathways in the chloroplast convert the products of the primary carbon reduction cycle plus ammonium and sulfhydryl to a host of secondary products. Among these are carbohydrates, fats, proteins, nucleic acids, various coenzymes, and many other substances needed both for the growth and activity of the chloroplasts and for export to other parts of the cell or organism. 

Now let us turn our attention to the reductive and synthetic reactions of photosynthesis. The basic carbon reduction cycle by which carbon dioxide is reduced to sugar phosphate involves at least twelve intermediate compounds. Some of these substances are found in very small concentrations. Many similar compounds are also present in the photosynthetic cell. In some cases they are closely linked by metabolism to the intermediates in the carbon reduction cycle. In order to understand the mechanism of photosynthetic carbon reduction, one must know the identity of the in-... 

Calvin cycle, reductive pentose phosphate cycle, photosyuthetlc carbon reduction cycle a series of 13 enzyme-catalysed reactions, occurring in the chloro-plast stroma in plants or the cytoplasm in photosyn-thetic bacteria, which are organized into a cycle, the purpose of which is to convert CO2 into carbohydrate using the reduced pyridine nucleotide (NADPH in plants, NADH in photosynthetic bacteria) and ATP generated in the Ught phase of photosynthesis (see Photosynthesis). The cycle was di vered by Melvin Calvin, research that earned him the Nobel Prize for... 

The fundamental basis of photosynthetic carbon metabolism is the incorporation of carbon dioxide by ribulose-bisphosphate carboxylase (rubisco). This leads to the synthesis of three-carbon sugars which are either exported from the chloroplast or metabolized to regenerate the acceptor ribulose bisphosphate. Rubisco is a bifunctional enzyme in that, in parallel to carboxylation, it catalyzes an oxygenation reaction that leads to phospho-glycolate. This is the starting point for photorespiratory metabolism, which will be discussed below (Section 1.6.2). In C4 plants, the conventional C3 pattern of the photosynthetic carbon reduction Calvin cycle is confined to the bundle sheath cells. The surrounding mesophyll cells act as an ancillary carbon dioxide pump, fixing carbon dioxide via phosphoenolpyruvate carboxylase into C4 acids. These are transported to the bundle sheath for decarboxylation.In this way, photorespiration is limited because of the elevated carbon dioxide levels. 

Illtimination of green leaves by intense light results in transient changes of the fluorescence yield. The complex kinetics of fluorescence induction reflects the gradual activation of photosynthetic apparatus (1-3) quick reduction of Q - primary electron acceptor of Photosystem 2, its reoxidation by P700 via electron transport chain, accmulation of plastoquinone pool in reduced state, the beginning of function of Photosystem 1 acceptor part and then the photosynthetic carbon cycle. 

These are involved in a wide range of electron-transfer processes and in certain oxidation-reduction enzymes, whose function is central to such important processes as the nitrogen cycle, photosynthesis, electron transfer in mitochondria and carbon dioxide fixation. The iron-sulfur proteins display a wide range of redox potentials, from +350 mV in photosynthetic bacteria to —600 mV in chloroplasts. 

---