Fructose-2,6-biphosphate

Recently, a chemoenzimatic catalized Henry reaction has been reported by El Blidi et al.53 Nitroaldol cyclization between the masked 3-hydroxy-4-nitrobutyraldehyde 72 and dihydroxyacetone phosphate (DHAP) 73, catalyzed by fructose-1,6-biphosphate aldolase (RAMA), afforded the nitro-cyclohexane 74 (Scheme 24). 

Figure 30. A medium complexity model of yeast glycolysis [342], The model consists of nine metabolites and nine reactions. The main regulatory step is the phosphofructokinase (PFK), combined with the hexokinase (HK) reaction into a single reaction vi. As in the minimal model, we only consider the inhibition by its substrate ATP, although PFK is known to have several effectors. External glucose (Glc ) and ethanol (EtOH) are assumed to be constant. Additional abbreviations Glucose (Glc), fructose 1,6 biphosphate (FBP), pool of triosephosphates (TP), 1,3 biphosphogly cerate (BPG), and the pool of pyruvate and acetaldehyde (Pyr).

A typical chemical system is the oxidative decarboxylation of malonic acid catalyzed by cerium ions and bromine, the so-called Zhabotinsky reaction this reaction in a given domain leads to the evolution of sustained oscillations and chemical waves. Furthermore, these states have been observed in a number of enzyme systems. The simplest case is the reaction catalyzed by the enzyme peroxidase. The reaction kinetics display either steady states, bistability, or oscillations. A more complex system is the ubiquitous process of glycolysis catalyzed by a sequence of coordinated enzyme reactions. In a given domain the process readily exhibits continuous oscillations of chemical concentrations and fluxes, which can be recorded by spectroscopic and electrometric techniques. The source of the periodicity is the enzyme phosphofructokinase, which catalyzes the phosphorylation of fructose-6-phosphate by ATP, resulting in the formation of fructose-1,6 biphosphate and ADP. The overall activity of the octameric enzyme is described by an allosteric model with fructose-6-phosphate, ATP, and AMP as controlling ligands. 

HDF has been identified as reaction product of a thermal treatment of sugars, especially the 6-desoxy sugar rhamnose [90], Analysis of the free sugars in the LMW-fraction revealed fructose-1,6-biphosphate (FBP) as the predominating carbohydrate in the LMW fraction (5.75 g/kg yeast), but no rhamnose was present [88]. To elucidate the contribution of FBP as precursors of HDF in the yeast fraction, the sugar phosphate was thermally degraded under the same conditions as used for the LMW fraction of yeast. The results revealed FBP, as effective precursor of HDF in aqueous model systems at lower reaction temperatures (100°C Table 19). It should be stressed that additions of the amino acids proline or alanine did not increase the concentrations of HDF from the carbohydrates listed in Table 19 (unpublished results). The data implied that FBP which was the predominant carbohydrate in yeast, is the... 

On the basis of these results a general pathway for the formation of HDF from sugars was assumed as exemplified for fructose-1,6-biphosphate in Figure 11 [88]. Elimination of the phosphate group at C-6 of the l-deoxyosone-6-phosphate, results in acetylformoine which was established as the key intermediate in HDF formation [88]. Reduction of acetylformoine either by a disproportionation reaction with a second molecule of acetylformoine or by further reductive agents present in foods, like vitamin C, then, after elimination of water, generates HDF. 

Figure II. Formation of 4-hydroxy-2,5-dimethyI-3(2H)-furanone (HDF) from fructose-1,6-biphosphate via acetylformoine as the intermediate.

Fructose 1,6-biphosphate aldolase from rabbit muscle in nature reversibly catalyzes the addition of dihydroxyacetone phosphate (DHAP) to D-glyceraldehyde 3-phosphate. The tolerance of this DHAP-dependent enzyme towards various aldehyde acceptors made it a versatile tool in the synthesis of monosaccharides and sugar analogs [188], but also of alkaloids [189] and other natural products. For example, the enzyme-mediated aldol reaction of DHAP and an aldehyde is a key step in the total synthesis of the microbial elicitor (—)-syringolide 2 (Fig. 35a) [190]. 

For the described approach, it is important to note that aldolases of different origin were tested and that, in contrast to L-rhamnulose-1-phosphate aldolase (RhuA), the D-fructose-1,6-biphosphate aldolase from rabbit muscle and L-fucu-lose-1-phosphate aldolase from E. coli were not active for DHAP/(R)-N- and (S)-iV-Cbz-prolinal condensation. Since RhuA accepts both, (S)-N- and (R)-N-Cbz prolinals, the chemo-enzymatic synthesis of both, hyacinthacines A and A2 isomers could be achieved. In conclusion, the origin and the particular enzyme itself... 

Thus the equilibrium concentrations of glucose, glucose 6-phosphate, fructose 6-phosphate, and fructose 1,6-biphosphate are 8.21 xl0 7, 1.56 xl0 4,... 

It is perhaps surprising that raising the concentration of ATP by a factor of 100 makes so little difference, but of course it does make a big difference for the first three reactants. The concentration of fructose 1,6-biphosphate cannot increase very much because it already dominates, and that limits the effects on GAP and GlycP. These calculations can be applied to larger systems and can include the specification of the concentrations of other coenzymes like NAD0X and NADred. 

In nature, many enzymes that hydrolyze phosphate monoesters are activated by two or more metal ions. They include alkaline phosphatase [79], purple acid phosphatase [80], inositol monophosphatase [81], and D-fructose 1,6-biphosphate 1-phosphatase [82]. The active sites of protein serine and threonine phosphatases also consist of dinuclear... 

Figure 6.18 Active site ofD-fructose 1,6-biphosphate 1-phosphatase.

The structure of D-fructose 1,6-biphosphate 1-phosphatase has been reported [82]. Proposedly, the two metal centers at the active site of the enzyme are bridged by the substrate phosphate monoester, and a metal hydroxide is involved in the hydrolysis (Figure 6.18). 

In vivo regulation of monomer-tetramer conversion of pyruvate kinase subtype M2 by glucose is mediated via fructose 1,6-biphosphate./. Biol. Chem. 266, 16842-16846. 

---