Glyceraldehyde from fructose

The preparation of DHAP for synthetic applications has been accomplished both enzymatically and chemically (Scheme 5.7).27 DHAP can be generated enzymatically in situ from fructose 1,6-diphosphate, using FDP aldolase acting in its catabolic mode, and triosephosphate isomerase (TPI). Glyceraldehyde 3-phosphate (G3P) and DHAP are produced in this reaction, with the G3P rapidly undergoing isomerization to DHAP. 

When NADPH levels are high, the reversible nonoxidative portion of the pathway can be used to generate ribose 5-phosphate for nucleotide biosynthesis from fructose 6-phosphate and glyceraldehyde 3-phosphate. 

The dark reaction (Calvin cycle) uses the NADPH and ATP to make glyceraldehyde 3-phosphate (triose phosphate), which is metabolized initially to starch, sucrose, and cellulose. Starch and sucrose are the major plant storage products. Starch is synthesized from ADP-glucose in the chloroplast, sucrose from fructose 6-phosphate and UDP-glucose in the leaf cytosol. 

When fructose-1-phosphate enters the glycolytic pathway, it is first split into dihy-droxyacetone phosphate (DHAP) and glyceraldehyde by fructose-1-phosphate aldolase. DHAP is then converted to glyceraldehyde-3-phosphate by triose phosphate isomerase. Glyceraldehyde-3-phosphate is generated from glyceraldehyde and ATP by glyceraldehyde kinase. 

Acta 4, 924 (1921) cf. Baer, Fischer, Science 88, 108 (1938). Prepn of L-glyceraldehyde from L-sorbose and of D-glycer-atdehyde from D -fructose Perlin, Methods in Carbohydrate Chemistry 1, 61 (1962). 

Ribose 5-phosphate can be synthesized from fructose 6-phosphate and glyceraldehyde... 

Fig. 3.8. Formation of glyceraldehyde-3-phosphate and dihydroxyacetone-1-phosphate from fructose-1,6-diphosphate during fermentation...

Transketolase has been used for the key steps in chemoenzymatic syntheses of (+)-exo-brevicomin 111 from racemic 2-hydroxybutyraldehyde [236], and of the azasugars l,4-dideoxy-l,4-imino-D-arabinitol [196] or N-hydroxypyrrolidine 124 [265] from 3-azido (95) and 3-O-benzyl (122) derivatives, respectively, of glyceraldehyde (Figure 5.55). Such syntheses were all conducted with intrinsic racemate resolution of 2-hydroxyaldehydes and profited from utilization of 119. Further preparative applications include the synthesis of valuable ketose sugars, particularly fructose analogs [258]. Transketolase has also been used for in-situ generation of erythrose 4-phosphate from fructose 6-phosphate in a multi-enzymatic synthesis of DAHP (26 Figure 5.17) [131]. 

An indirect enzymatic pathway for the formation of uctose-6-phosphate from fructose-l-phosphate has recently been described by Leuthardt et al. Fructose-l-phosphate is cleaved to D-glyceraldehyde and dihydroxyacetone phosphate. The latter isomerizes to form glyceraldehyde-3-phosphate. The triose phosphates condense to fructose-1,6-diphosphate which is dephosphorylated at the 1 position. In the presence of ATP, the glyceraldehyde may be converted by means of a triosekinase to glyceraIdehyde-3-phosphate. 

The enzyme aldolase has found several uses, such as the preparation of glyceraldehyde 3-phosphate from fructose 1,6-diphosphate and of fructose I-phosphate from the 1,6-diphosphate plus glyceraldehyde. 

D-Fructose i-phosphate was first prepared by phosphatase action on fructose i,6-diphosphate °. It is formed enzymically by condensation of dihydroxyacetonephosphate and D-glyceraldehyde and from fructose and ATP " . Synthesis has been carried out by treating 2,3,4,5-diiso-propylidene-D-fructopyranose with phosphoryl chloride , or PjOj , or diphenyl phosphorochloridate . 

The transaldolase functions primarily to make a useful glycolytic substrate from the sedoheptulose-7-phosphate produced by the first transketolase reaction. This reaction (Figure 23.35) is quite similar to the aldolase reaction of glycolysis, involving formation of a Schiff base intermediate between the sedohep-tulose-7-phosphate and an active-site lysine residue (Figure 23.36). Elimination of the erythrose-4-phosphate product leaves an enamine of dihydroxyacetone, which remains stable at the active site (without imine hydrolysis) until the other substrate comes into position. Attack of the enamine carbanion at the carbonyl carbon of glyceraldehyde-3-phosphate is followed by hydrolysis of the Schiff base (imine) to yield the product fructose-6-phosphate. 

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]. 

Thus, in the course of reactions catalyzed by the intrinsic enzymes of the pentose phosphate cycle, two fructose 6-phosphate molecules, one glyceraldehyde 3-phosphate molecule, and three carbon dioxide molecules are produced from three glucose 6-phosphate molecules. In addition, six NADP -H2 molecules are formed. The overall scheme for the pentose phosphate cycle is ... 

Figure 4.17 The trioses D-glyceraldehyde (aldose) and dihydroxyacetone (ketose), the pentose D-ribose, the hexoses D-galactose and D-glucose (aldoses) and the ketohexose D-fructose in their open chain forms. The configuration of the asymmetrical hydroxyl group on the carbon, the furthest away from the aldehyde or ketone group, determines the assignment of D- or L-configuration.

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