Carbon glucose-6-phosphate

The reaction mechanism is similar to the reaction promoted by phosphohexose isomerase in step (2) of glycolysis (Fig. 14-4). After the triose phosphate isomerase reaction, C-1, C-2, and C-3 of the starting glucose are chemically indistinguishable from C-6, C-5, and C-4, respectively (Fig. 14-6), setting up the efficient metabolism of the entire six-carbon glucose molecule. 

Regeneration of ribulose bisphosphate occurs by the same reactions that occur in the hexose-monophosphate shunt. The hexose monophosphate shunt interconverts 3-, 4-, 5-, 6- and 7-carbon sugar phosphates. In the formation of one molecule of glucose from CO2, carrying out the previous reactions six times is necessary. Making the six molecules of ribulose-bisphosphate required for fixing six COi s occurs by the following sets of reactions ... 

Fig. 2.6. The numbers of ATP molecules produced in glycolysis and in the Krebs cycle. Thirty six ATPs result from the splitting of one glucose molecule. Note that each 6-carbon glucose splits into two 3-carbon molecules, each of which generates its own ATPs. Each NADH from the Krebs cycle yields 3 ATPs. Each FADH, yields 2 ATPs. Each NADH from the cytosol yields only 2 ATPs. Each GTP yields one ATP. DHAP, Dihydroxyacetone phosphate. ...

Overall, the hydrogen stored in NADPH is used to reduce C02 to carbohydrate units (0H2O). This is not a direct reaction because the C02 is first combined with a C5 compound, ribulose diphosphate (RDP), which then spontaneously splits into two identical C3 molecules, phosphoglyceric acid (PGA). Most of the PGA is used to synthesize further RDP but some is reduced by NADPH, using energy supplied by the ATP/ADP system, to give triose phosphate, which in turn is converted into the glucose phosphate from which various carbohydrates are synthesized. This assimilatory path is known as the Calvin cycle and is involved in all autotrophic carbon fixation, whether photosynthetic or chemosynthetic. 

Conversion of Six-Carbon Glucose to Three-Carbon Glyceraldehyde-3-Phosphate... 

Energy investment phase, in which sugar phosphates are synthesized at the expense of ATP conversion to ADP. Six-carbon substrate (glucose) is converted to two three-carbon sugar phosphates... 

Valuable information concerning the carbohydrates formed during steady state photosynthesis has been obtained by using cellulose chro matography in conjunction with radioactive carbon (29-32,57,58,235). During the flrst few seconds of photosynthesis there is concurrent formation of fructose and sedoheptulose phosphate followed by ketopentose phosphate and later by glucose phosphates (32,57). Examination of many plants shows sedoheptulose to be of widespread occurrence (31,192). 

The six-carbon product of the transaldolase reaction enters the cytosolic pool of fructose 6-phosphate and may be utilized in energy production through the glycolytic pathway. Alternatively, it may be isomerized into glucose phosphate and reprocessed through the oxidative phase of the pathway. 

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