Carbon reduction cycle

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. 

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. 

Bassham JA. Mapping the carbon reduction cycle a personal perspective. Photosynth. Res. 2002 76 35-52. 

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

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 incorporation of COz into carbohydrate by eukaryotic photosynthesizing organisms, a process that occurs within chloroplast stroma, is often referred to as the Calvin cycle. Because the reactions of the Calvin cycle can occur without light if sufficient ATP and NADPH are supplied, they have often been called the dark reactions. The name dark reactions is somewhat misleading, however. The Calvin cycle reactions typically occur only when the plant is illuminated, because ATP and NADPH are produced by the light reactions. Therefore light-independent reactions is a more appropriate term. Because of the types of reactions that occur in the Calvin cycle, it is also referred to as the reductive pentose phosphate cycle (RPP cycle) and the photo synthetic carbon reduction cycle (PCR cycle). 

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. 

Photosynthetic Carbon Reduction Cycle (Calvin Cycle)... 

The recovery of assimilation rate included both an increase in the efficiency of the carboxylation processes and the level of carbon reduction cycling. This was probably due to increasing reductant production as the light reactions recovered, which would both induce carboxylation enzymes and allow greater levels of carbon cycling. 

The inhibition of 0app at loc in the light is probably a result of several factors. In addition to inhibition of PSII, the reactions of the carbon reduction cycle may be inhibited at low temperature in the light (6). 

Another enzyme of the carbon reduction cycle activates PGA, converting it to phosphoryl-3-phosphoglyceric acid (IV). This acid anhydride can then be reduced in a subsequent enzymatic step mediated by triose phosphate dehydrogenase. For its reducing agent, this enzyme uses nicotinamide adenine dinucleotide phosphate (NADPH) and thereby converts the carboxylic acid to... 

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-... 

FiU. 6. The carbon reduction cycle of photosynthesis. Solid arrows indicate reactions of the carbon reduction cycle as formulated by Calvin and coworkers. Dashed line represents hypothetical reductive carboxylation reaction discussed in text. Open arrows indicate start of some of the biosynthetic paths leading from intermediate compounds of the cycle. Asterisks indicate approximate relative degree of labeling after a few seconds of photosynthesis. They reflect the results of degradation studies by various workers, as discussed in the text. 

The complete photossnnthetic carbon reduction cycle is shown in detail in Fig. 6. A somewhat idealized distribution of label found experimentally following a short period of photosynthesis with C 02 is indicated by the asterisks. 

Under conditions of steady photosynthesis, the concentrations of the intermediates of the carbon reduction cycle remain constant. Suppose that n molecules of CO2 enter the cycle by the carboxyla-tion reaction. Then n atoms of carbon incorporated into organic compounds are taken from the cycle by secondary reactions. These reactions utilize cycle intermediates as a starting point for the synthesis of various end products. 

PGA and the sugar phosphates clearly account for most of the found in individual compounds following a few seconds of photosynthesis with C 02. Nonetheless, one might ask whether or not other important pathways of CO2 reduction not involving these compounds have been overlooked. For example, there might be a pathway from CO2 to sucrose which does not include the intermediate compounds of the carbon reduction cycle. If so, this path would have to include substances which are so small in concentration as not to be seen, or which are so unstable as not to be isolated by the methods of paper chromatography. 

The fatty acids thus photosynthesized are then esterified with glycerol or glycerol phosphate formed directly from triose phosphate of the carbon reduction cycle [Eq. (27)]. Galactolipids... 

Buchanan (1953) reported that sucrose is photosynthesized from fructose-6-phosphate of the carbon reduction cycle. One molecule of fructose-6-phosphate is converted to glucose-6-phosphate, which reacts with uridine triphosphate (UTP) to form uridine diphosphoglucose (UDPG). UDPG is always labeled with during short periods of photosynthesis with The UTP was... 

A study of the kinetics of the labeling of alanine show that its rate of labeling reaches a maximum as soon as the intermediates of the carbon reduction cycle are saturated with C . Since no secondary products of carbon photosynthesis such as sucrose are approaching saturation at this time (3-5 minutes), it appears that alanine is formed directly from intermediates of the cycle. Presumably, alanine is formed from PGA by the transamination of pyruvic acid derived from phosphoenolpyruvic acid which in turn is derived from PGA [Eq. (28)]. 

Other amino acids such as serine and aspartic acid are presumably photosynthesized from intermediate keto acids derived from the carbon reduction cycle. In each case, transamination by glutamic acid is probably the major synthetic reaction. 

As for the carbon reduction cycle, probably the most important unanswered questions have to do with the mechanisms of the carboxylation reaction and other steps in the cycle. Although all of the biochemical evidence from isolated enzyme systems suggests that the carboxylation reaction is a nonreductive carboxylation of ribulose diphosphate leading to the formation of two molecules of PGA, kinetic evidence with whole cells indicates the possibility of a reductive carboxylation leading to the formation of one molecule of PGA and one molecule of triose phosphate. If this reductive carboxylation does occur, it may be that electrons are somehow conveyed directly from the light reaction to the carbon reduction cycle (Bassham, 1964). If there is such a difference between the... 

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... 

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