Fructose-6-phosphate

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Fructose-6-phosphate is one of the metabolites of the carbohydrates metabolism pathway, which has glucose-6-phosphate as a precursor and fructose-1,6-biphosphate as a postcursor . More specifically, it is found in the glycolysis stage of the carbohydrates metabolism. 

Contents

Production of fructose-6-phosphate from glucose-6-phosphate

Fructose-6-phosphate results from a aldopyranose to ketofuranose isomerisation reaction which involves a sequence of events. These events are catalysed by phosphoglucose isomerase. The first one is a conversion from the ring form of glucose-6-phosphate to the linear form. The second one is the conversion from this linear form to a linear ketohexose form. Then, the linear ketohexose is converted to its ring form by carbon 2 binding covalently to the oxygen atom on carbon 5[1].

Conversion of fructose-6-phosphate to fructose-1,6-biphosphate

This conversion involves an enzyme called phosphofructokinase. It phosphorylates carbon 1 of fructose-6-phosphate by adding a phosphate group from ATP, forming fructose-1,6-biphosphate[2]. The process involves the ATP forming a complex with Mg2+[3]in which the ion neutralises the charges of the alpha and beta phosphoryl groups of ATP .This way, only the gamma phosphoryl group of ATP is used for phosphorylation.

Importance of conversion between glucose-6-phosphate and fructose-1,6-biphosphate

The advantages of converting glucose-6-phosphate to fructose-6-phosphate, and then phosphorylating it to fructose-1,6-biphosphate is that, in the end, a symmetrical molecule is produced which, by cleavage (catalysed by the enzyme aldolase), will produce 2 phosphorylated interconvertible three-carbon compounds. Therefore, only one metabolic pathway is required for the further oxidation of these 2 fragments. The isomerisation step is responsible for creating the form of the molecule able to yield 2 three-carbon compounds, while the subsequent phosphorylation is important so that fructose-6-phosphate will not isomerise back to glucose-6-phosphate[4].

If the aldol cleavage had taken place in the aldose glucose, a 2-carbon compound and a 4-carbon compound would result, which would require two different metabolic pathways to further process and oxidise the two fragments[5].

References

  1. Biochemistry, Berg et al., 8th edition, W. H. Freeman and Company, page 454
  2. Biochemistry, Berg et al., 8th edition, W. H. Freeman and Company, page 454
  3. Biochemistry, Berg et al., 8th edition, W. H. Freeman and Company, page 453
  4. Biochemistry, Berg et al., 8th edition, W. H. Freeman and Company, page 454
  5. Biochemistry, Berg et al., 8th edition, W. H. Freeman and Company, page 454
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