Fructose 2,6-bisphosphate Fru

Fructose 2,6-bisphosphate (Fru-2,6-bP) plays an important part in carbohydrate metabolism. This metabolite is formed in small quantities from fructose 6-phosphate and has purely regulatory functions. It stimulates glycolysis by allosteric activation of phosphofructokinase and inhibits gluconeogenesis by inhibition of fructose 1,6-bisphosphatase. 

Figure 11-2 Roles of phosphofructose kinase and fructose 1,6-bisphosphatase in the control of the breakdown and storage (—+) of glycogen in muscle. The uptake of glucose from blood and its release from tissues is also illustrated. The allosteric effector fructose 2,6-bisphosphate (Fru-2,6-P2) regulates both phosphofructokinase and fructose 2,6-bisphosphatase. These enzymes are also regulated by AMP if it accumulates. The activity of phosphofructokinase-2 (which synthesizes Fru-2,6-P2) is controlled by a cyclic AMP-dependent kinase and by dephosphorylation by a phosphatase.

The activity of the F. hepatica PFK is also modified by phosphorylation and the phosphorylated enzyme is still stimulated by AMP and fructose-2,6-bisphosphate (Fru-2,6-P2) (21). Phosphorylation may also play a physiological role in the regulation of the F. hepatica PFK, since serotonin exhibits a marked stimulatory effect on PFK activity in intact F. hepatica (22). The D. immitis PFK is also regulated by phosphorylation, Fru-2,6-P2 and AMP (23). 

As in the case of the dikinase regulatory cascade (see Fig. 1), little is known with certainty with respect to how light modulates the phosphorylation-status (activation-state) of cytoplasmic PEPC other than it, too, is dependent, either directly or indirectly, on photosynthetic electron transport and/or photophosphorylation (19, 20). Neither dithiothreitol-reduced spinach leaf thioredoxin h [Td (30)], PPi, fructose 2,6-bisphosphate (Fru 2,6-P2), various combinations of EGTA/calcium/calmodulin (CaM) [see Table 1 (Refs. 29, 31)] nor the CaM-antagonist calmidazolium (R. Chollet and J. Vidal, unpublished) has any significant effect on the regulatory phosphorylation and/or activation of purified dark-form PEPC in our reconstituted in vitro system. A one-hour preincubation of the dark-form PK with Mg-ATP is also without effect on its subsequent activation of PEPC. Thus, we currently have no supportive evidence... 

Although the bundle sheath chloroplasts contain all the enzymes of the RPP cycle, there is now evidence that some of the 3-PGA formed by the activity of rubisco is exported to the mesophyll cells [9]. Bundle sheath chloroplasts of maize are deficient in photosystem II activity and apparently cannot produce sufficient NADPH to reduce all of the 3-PGA formed to triose phosphate. Responsibility for this step is thus shared with the mesophyll chloroplasts which recycle triose phosphate to the bundle sheath as DHAP. This transport of 3-PGA from bundle sheath to mesophyll permits the synthesis of sucrose in the mesophyll cell cytoplasm. The evidence suggests that the mesophyll cells are the major site of sucrose synthesis [10-13]. Sucrose phosphate synthetase, one of the regulatory enzymes of sucrose synthesis and fructose 6-phosphate, 2-kinase (Fru-6-P,2K), the enzyme synthesizing fructose 2,6-bisphosphate — a potent regulator of cytoplasmic sucrose synthesis (see Section 5.4.1) — are both almost completely confined to the mesophyll cells. 

FIGURE 1.2 (continued from page 10) PDH complex activity, leading to the production of more acetyl-CoA, resulting in enhanced accumulation of seed oil (Zou et al. 1999, Marillia et al. 2003). The precise mechanism by which mitochondrial acetyl-CoA promotes enhanced seed oil accumulation remains to be elucidated. Additional metabolites Fru 1,6BP, fructose-1, 6-bisphosphate (Frul,6BP) OAA, oxaloacetate PEP, phosphoenolpyruvate. Additional information for the depicted scheme is available in the work of Ruuska et al. (2004). 

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