Food Chemistry and Fish in Nutrition

6.1.4. Metabolisms involving glucose

1.      Metabolisms involving glucose

The absorbed free glucose is phosphorylated to glucose 6 phosphate by hexokinase using ATP. The fructose, galactose and mannose absorbed are also converted into glucose 6 phosphate. This compound is metabolized by five major metabolic path ways.

i.        Conversion into blood glucose: Glucose 6-phosphate is dephosphorylated by glucose 6-phosphate to yield free-D-glucose, which passes into the systemic blood to be transported to other tissues.

Glucose 6 phosphatase

Glucose 6-phosphate   → Glucose  + Pi

ii.      Conversion into glycogen: Glucose 6-phosphate not immediately needed to form blood glucose is converted into liver glycogen by the sequential action of phosphoglucomutase and glycogen synthase.

iii.    Conversion into fatty acids and cholesterol: Excess glucose 6-phosphate not used to make blood glucose or liver glycogen is degraded via glycolysis and pyruvate dehydrogenase into acetyl-CoA, which is converted into malonyl CoA, and thence into fatty acids. These are used to form triglycerides and phospholipids, which are in part exported to other tissues by plasma lipoproteins. Some acetyl-CoA is also used by the liver to make cholesterol.

iv.    Oxidative degradation to CO₂: Acetyl-CoA, yielded from glucose 6-phosphate via glycolysis and decarboxylation of pyruvate, may be oxidized via the citric acid cycle. The ensuing electron transport and oxidative phosphorylation yield energy in the form of ATP. Normally, however, fatty acids are the major oxidative fuel for the citric acid cycle in the liver.

v.      Degradation via the Pentose Phosphate Pathway: Glucose 6-phosphate is the substrate for the pentose phosphate pathway, yielding (1) reducing power in the form of NADPHI, needed in the reducing steps in the biosynthesis of fatty acids and cholesterol and (2) D-ribose 5-phosphate, a precursor in nucleotide biosynthesis. Through the action of various regulatory enzymes and through hormonal regulation, the liver directs the flow of glucose residues into these different pathways according to the prevailing supply and demand economy of the organism.