Glyceraldehyde- 1,3-bisphosphate

Pyruvate, Citrate, Succinate, Glyceraldehyde-3-phosphate, Fructose-1,6-bisphosphate, 3-PhosphogIyceric acid... 

Fructose-6-phosphate + ATP. fructose-1,6-bisphosphate + ADP + Fructose-1,6-bisphosphate dihydroxyacetoiie-P + glyceraldehyde-3-P Dihydroxyacetoiie-P . glyceraldehyde-3-P ... 

Subsequent action by fructose-l-phosphate aldolase cleaves fructose-l-P in a manner like the fructose bisphosphate aldolase reaction to produce dihy-droxyacetone phosphate and D-glyceraldehyde ... 

Write a balanced equation for the synthesis of a glucose molecule from ribulose-l,5-bisphosphate and COg that involves the first three reactions of the Calvin cycle and subsequent conversion of the two glyceraldehyde-3-P molecules into glucose. 

One of the steps in the biological pathway for carbohydrate metabolism is the conversion of fructose 1,6-bisphosphate into dihydroxyacetone phosphate and glyceraldehyde 3-phosphate. Propose a mechanism for the transformation. 

Fructose 1,6-bisphosphate undergoes ring opening and is cleaved by a retro-aldol reaction into glyceraldehyde 3-phosphate and dihydroxyacetone phosphate (DHAP). 

Figure 29.9 Mechanism of step 4 in Figure 29.7, the cleavage of fructose 1,6-bisphosphate to yield glyceraldehyde 3-phosphate and dihydroxyacetone phosphate.

Glyceraldehyde 3-phosphate and dihydroxyacetone phosphate join together in an aldol reaction to give fructose 1.6-bisphosphate. 

The D-fructose 1,6-bisphosphate aldolase (FruA EC 4.1.2.13) catalyzes in vivo the equilibrium addition of (25) to D-glyceraldehyde 3-phosphate (GA3P, (18)) to give D-fructose 1,6-bisphosphate (26) (Figure 10.14). The equilibrium constant for this reaction of 10 strongly favors synthesis [34]. The enzyme occurs ubiquitously and has been isolated from various prokaryotic and eukaryotic sources, both as class I and class II forms [30]. Typically, class I FruA enzymes are tetrameric, while the class II FruA are dimers. As a rule, the microbial class II aldolases are much more stable in solution (half-lives of several weeks to months) than their mammalian counterparts of class I (few days) [84-86]. 

This reaction is followed by another phosphorylation with ATP catalyzed by the enzyme phosphofructoki-nase (phosphofructokinase-1), forming fructose 1,6-bisphosphate. The phosphofructokinase reaction may be considered to be functionally irreversible under physiologic conditions it is both inducible and subject to allosteric regulation and has a major role in regulating the rate of glycolysis. Fructose 1,6-bisphosphate is cleaved by aldolase (fructose 1,6-bisphosphate aldolase) into two triose phosphates, glyceraldehyde 3-phosphate and dihydroxyacetone phosphate. Glyceraldehyde 3-phosphate and dihydroxyacetone phosphate are inter-converted by the enzyme phosphotriose isomerase. 

Figure 17-2. The pathway of glycolysis. ( ,—P, HOPOj " .inhibition.) At asterisk Carbon atoms 1-3 of fructose bisphosphateform dihydroxyacetone phosphate, whereas carbons 4-6 form glyceraldehyde 3-phosphate. The term "bis-," as in bisphosphate, indicates that the phosphate groups are separated, whereas diphosphate, as in adenosine diphosphate, indicates that they are joined.

Tracing the carbons of glucose to pyruvate gets complicated when fructose 1,6-bisphosphate (FBP) is cleaved into two 3-carbon fragments, glyceraldehyde 3-phosphate (G3P) and dihydroxyacetone phosphate (DHAP). The numbers of the original carbons of glucose are indicated by the superscripts next to the carbon. 

The natural substrate for the dehydrogenase, glyceraldehyde-3-phosphate (G-3-P), had been synthesized earlier by Hermann Fischer, Emil Fischer s son, and Baer in 1932. In 1934 Meyerhof and Lohmann synthesized hexose diphosphate, establishing it to be fructose 1,6 bisphosphate (F-l, 6 bis P). With F-1,6 bisP as substrate and hydrazine to trap the aldehydic and ketonic products of the reaction, G-3-P was identified in the mixture of G-3-P and dihydroxyacetone phosphate which resulted. Triose phosphate isomerase was then isolated and the importance of phosphorylated 3C derivatives established. 

This is interconverted to form glyceraldehyde 3-phosphate and both combine, via the enzyme aldolase, to produce fructose bisphosphate, en route to form glucose or glycogen. 

Ribose 5-phosphate ) Xylulose 5-phosphate (1 Sedoheptulose 7-phosphate Glyceraldehyde 3-phosphate (1 Erythrose 4-phosphate Fructose 6-phosphate Fructose 1,6-bisphosphate Glycerone-3-phosphate... 

D. The six-carbon fructose 1,6-bisphosphate is then cleaved into two three-carbon molecules, dihydroxyacetone phosphate and glyceraldehyde 3-phosphate, by... 

Based on the stereospecific transketolase-catalyzed ketol transfer from hydroxy-pyruvate (20) to D-glyceraldehyde 3-phosphate (18), we have thus developed a practical and efficient one-pot procedure for the preparation of the valuable keto-sugar 19 on a gram scale in 82% overall yield [29]. Retro-aldolization of D-fructose 1,6-bisphosphate (2) in the presence of FruA with enzymatic equilibration of the C3 fragments is used as a convenient in-situ source of the triose phosphate 18 (Scheme 2.2.5.8). Spontaneous release of CO2 from the ketol donor 20 renders the overall synthetic reaction irreversible [29]. 

Cleavage of Fructose 1,6-Bisphosphate The enzyme fructose 1,6-bisphosphate aldolase, often called simply aldolase, catalyzes a reversible aldol condensation (p. 485). Fructose 1,6-bisphosphate is cleaved to yield two different triose phosphates, glyceraldehyde 3-phosphate, an aldose, and dihydroxyacetone phosphate, a lcetose ... 

Glyceraldehyde 3-phosphate + dihydroxyacetone phosphate fructose 1,6-bisphosphate Fructose 1,6-bisphosphate —> fructose 6-phosphate + P ... 

Equivalence of THose Phosphates 14C-Labeled glyceraldehyde 3-phosphate was added to a yeast extract. After a short time, fructose 1,6-bisphosphate labeled with 14C at C-3 and C-4 was isolated. What was the location of the 14C label in the starting glyceraldehyde 3-phosphate Where did the second 14C label in fructose 1,6-bisphosphate come from Explain. 

FIGURE 20-10 Third stage of C02 assimilation. This schematic diagram shows the interconversions of triose phosphates and pentose phosphates. Black dots represent the number of carbons in each compound. The starting materials are glyceraldehyde 3-phosphate and dihydroxyacetone phosphate. Reactions catalyzed by transaldolase ( and ) and transketolase ((3) and ) produce pentose phosphates that are converted to ribulose 1,5-bisphosphate—ribose... 

One molecule of glyceraldehyde 3-phosphate is the net product of the carbon assimilation pathway. The other live triose phosphate molecules (15 carbons) are rearranged in steps to (S) of Figure 20-10 to form three molecules of ribulose 1,5-bisphosphate (15 carbons). The last step in this conversion requires one ATP per ribulose 1,5-bisphosphate, or a total of three ATP. Thus, in summary, for every molecule of triose phosphate produced by photosynthetic C02 assimilation, six NADPH and nine ATP are required. 

Photosynthesis in vascular plants takes place in chloroplasts. In the C02-assimilating reactions (the Calvin cycle), ATP and NADPH are used to reduce C02 to triose phosphates. These reactions occur in three stages the fixation reaction itself, catalyzed by rubisco reduction of the resulting 3-phosphoglycerate to glyceraldehyde 3-phosphate and regeneration of ribulose 1,5-bisphosphate from triose phosphates. 

---