Phosphatase hexose diphosphate

Efficient utilization of the fructose requires phosphorylation of the glyceraldehyde. Tracer experiments show that the carbon 1 of fructose appears as both carbons 1 and 6 of glucose. This is the result of triose phosphate isomerization followed by (conventional) aldolase condensation to hexose diphosphate. The conversion of fructose diphosphate to glucose-6-phosphate requires a phosphatase and an isomerase, as discussed in the pentose phosphate pathway. 

From that point on, the sjmthesis of glucose is easy, being simply the reverse of glycolysis. Only the phosphorylation reactions of hexose by ATP are not reversible, but phosphate can be spht off the hexose diphosphate by specific phosphatases. Fiudihermore, the synthesis of carbohydrates does not end with free glucose but usually with the polymer glycogen. The polymerization, however, proceeds through phosphorylated intermediates. 

At first two molecules of triose phosphate combine to form hexose diphosphate fructose diphosphate), which is then hydrolyzed by a specific phosphatase to fructose 6-phosphate and phosphate. Hexose phosphate and triose phosphate interact in a transketolase reaction to produce erythrose 4-phosphate and xylulose 5-phosphate, which then rearranges to ribulose 5-phosphate (the first pentose molecule). In a type of aldol condensation, erythrose phosphate and triose phosphate combine to form sedoheptulose diphosphate (C4 - - C3 = C7), which is subsequently dephos-phorylated. The enzyme transketolase then transfers a C2 fragment from heptulose to triose phosphate yielding 2 moles of pentose xylulose 5-phosphate and ribose 5-phosphate) both must be rearranged to ribulose 5-phosphate. Having ribulose 5-phosphate available, the cycle can commence again first, phosphorylation with ATP to the diphosphate, then acceptance of CO2 by the diphosphate, and production of 2 moles of phosphoglyceric acid. 

It had been known from at least the time of Pasteur that the presence of sodium or potassium phosphate aided the progress of a yeast fermentation. Later intensive study showed that a complex group of enzymes (phosphatases and phosphorylases) was responsible for the phosphorylation, dephosphorylation and interconversion of D-glucose 6-phosphate, D-fructose 6-phosphate, D-fructose 1,6-diphosphate and similar substances in various types of cells and muscle tissue. Detailed reviews of the field are available. - A further advance was made in 1936, when Cori and Cori noted that in certain circumstances well-washed frog muscle immersed in a sodium phosphate buffer utilized the inorganic phosphate to produce a new hexose phosphate (the Cori ester). This compound was later shown to be a-D-glucopyranose-l-phosphate and yielded crystalline dipotassium and brucine salts. The Cori ester arose because... 

The phosphoenolpyruvate may then be converted to fructose diphosphate by reversal of steps 10, 9,8, 7 and 5 in the glycolytic sequence shown in Fig. 9.4.This is then converted to fructose-6-phosphate by hexose diphosphatase and then to glucose-6-phosphate by the reverse of step 2 and finally to glucose by glucose-6-phosphatase.The glucose may then eventually be used to provide energy. The energy balance sheet may be prepared as follows ... 

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