Fructose-6-phosphate, formation

The experiments of Barrenscheen and Pany (33), Bonner and Wildman (52), and Holzer and Holzer (170) make it almost a matter of certainty that phosphohexoisomerase is present in leaves and other green plant organs, since all these investigators found fructose phosphate formation when glucose was utilized. However, the only specific study of this enzyme in plants was carried out by Somers and Cosby (322), who demonstrated its presence in pea seeds. 

Aldolase 4.1.2.3 Fructose-1,6-diphosphate Dihydroxyacetone phosphate Formation of a hydrazone... 

Figure 20.9. Hexose Phosphate Formation. 3-Phosphoglycerate is converted into fructose 6-phosphate in a pathway parallel to that of glyconeogenesis.

Mannose-6-phosphate formation (catalyzed by hexokinase) Conversion of mannose-6-phosphate to fructose-6-phosphate... 

Following Its formation D fructose 6 phosphate is converted to its corresponding 1 6 phosphate diester which is then cleaved to two 3 carbon fragments under the mflu ence of the enzyme aldolase... 

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 most convenient method is the formation of two equivalents of (25) by retro-aldol cleavage from commercially available (26) by the combined action of FruA and triose phosphate isomerase (Figure 10.18 inset) [84]. This scheme has been extended into a highly integrated, artificial metabolism for the efficacious in situ preparation of (25) from inexpensive feedstock such as glucose and fructose (two equivalents of... 

Figure 10.18 Enzymatic in situ generation of dihydroxyacetone phosphate from fructose 1,6-bisphosphate (b), with extension to an in vitro artificial metabolism for its preparation from inexpensive sugars alongthe glycolysis cascade (a), and utilization for subsequent stereoselective carbon-carbon bond formation using an aldolase with distinct stereoselectivity (c).

The pentose phosphate pathway is an alternative route for the metabolism of glucose. It does not generate ATP but has two major functions (1) The formation of NADPH for synthesis of fatty acids and steroids and (2) the synthesis of ribose for nucleotide and nucleic acid formation. Glucose, fructose, and galactose are the main hexoses absorbed from the gastrointestinal tract, derived principally from dietary starch, sucrose, and lactose, respectively. Fructose and galactose are converted to glucose, mainly in the liver. 

The second indirect route involves the formation of fructose 6-phosphate from fructose 1,6-bisphosphate without the intervention of phosphofructokinase reaction. This route is catalyzed by fructose bisphosphatase ... 

Harrison, Tarr and Hibbert96 investigated the production of levan from sucrose by the action of Bacillus subtilis Cohn and B. mesentericus Trevisan. Nutrient solutions containing 10% carbohydrate, 0.1% peptone, 0.2% disodium hydrogen phosphate and 0.5% potassium chloride were incubated at 37° for six days. Levan formation occurred only with sucrose and raffinose, and not with melezitose, lactose, maltose, D-xylose, D-glucose or D-fructose. It was therefore suggested that only those carbohydrates with a terminal D-fructofuranose residue were satisfactory substrates for levan formation. 

The essentially nonreversible formation of D-fructose 1-phosphate in the muscle-aldolase system is probably attributable to thermodynamic stabilization. D-Fructose 1-phosphate can form a stable pyranose structure, whereas D-fructose 1,6-diphosphate can exist only in the less stable furanose or acyclic forms.72(,) Only when the cleavage products are removed is the monophosphate effectively split under the influence of aldolase. 

I, 7-diphosphate.170 1 (f> This tetrose phosphate is involved with phosphoenol pyruvate in the formation of shikimic acid via 3-deoxy-2-keto-D-ara6ino-heptonic acid 7-phosphate and, hence, of aromatic compounds.170(d) A synthesis of the tetrose phosphate has been described.170 1 Aldolase shows a high affinity for the heptulose diphosphate and, compared with that for D-fructose 1,6-diphosphate, the rate of reaction is about 60 %. The enzyme transaldolase, purified 400-fold from yeast, catalyzes the following reversible reaction by transfer of the dihydroxyacetonyl group.l70(o>... 

D-erythro-Pentulose 5-phosphate (XLIV) has been formed by the action of transketolase on hydroxypyruvate (XLII) and D-glycerose 3-phosphate, the hydroxypyruvate being decarboxylated196 to active glycolaldehyde which then reacts with the triose phosphate by an acyloin reaction.28 The active glycolaldehyde is also formed from L-glycero-tetrulose, d-altro-heptulose 7-phosphate, D-fructose 6-phosphate, and D-i/ireo-pentulose 5-phosphate and it reacts with various aldehydes (acceptors) to give ketoses.198, 200 Thus, substitution of L-gfh/cero-tetrulose for hydroxypyruvate in the above experiment also resulted in formation of D-en/i/iro-pentulose... 

It s unfortunate that we have to deal with PFK-1 and PFK-2. PFK-1 is the enzyme that catalyzes the formation of fructose 1,6-bisphospate from fructose 6-phosphate. PFK-2 makes the 2,6-bisphosphate. 

Even more interesting is the observed regioselectivity of 37 its reaction with 2, 3 -cCMP and 2, 3 -cUMP resulted in formation of more than 90% of 2 -phosphate (3 -OH) isomer. The postulated mechanisms for 37 consists of a double Lewis-acid activation, while the metal-bound hydroxide and water act as nucleophilic catalyst and general acid, respectively (see 39). The substrate-ligand interaction probably favors only one of the depicted substrate orientations, which may be responsible for the observed regioselectivity. Complex 38 may operate in a similar way but with single Lewis-acid activation, which would explain the lower bimetallic cooperativity and the lack of regioselectivity. Both proposed mechanisms show similarities to that of the native phospho-monoesterases (37 protein phosphatase 1 and fructose 1,6-diphosphatase, 38 purple acid phosphatase). 

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