Galactose and fructose metabolism is a logical place to begin looking at carbohydrate metabolism, before shifting focus to the preferred monosaccharide glucose. The figure below reminds you that in the liver, galactose and fructose have been phosphorylated.

Figure 6.211 Uptake of monosaccharides into the hepatocyte

Galactose

In the liver, galactose-1-phosphate is converted to glucose-1-phosphate, before finally being converted to glucose-6-phosphate¹. As shown below, glucose 6-phosphate can then be used in either glycolysis or glycogenesis, depending on the person’s current energy state.

Figure 6.212 Conversion of galactose-1-phosphate to glucose-6-phosphate

Fructose

Unlike galactose, fructose cannot be used to form phosphorylated glucose. Instead, fructose-1-phosphate is cleaved in the liver to form glyceraldehyde 3-phosphate, a glycolysis (pathway that breaks down glucose) intermediate . This occurs through multiple steps, as depicted below.

Figure 6.213 Conversion of fructose-1-phosphate to glyceraldehyde 3-phosphate

Glucose-6-Phosphate

Within hepatocytes or myocytes (muscle cells), glucose-6-phosphate can be used either for glycogenesis (glycogen synthesis) or glycolysis (breakdown of glucose for energy production). If the person is in an anabolic state, they will use glucose-6-phosphate for storage. If they are in a catabolic state, they will use it for energy production.

Figure 6.214 The “fork in the road” for glucose-6-phosphate

References & Links

1. Gropper SS, Smith JL, Groff JL. (2008) Advanced nutrition and human metabolism. Belmont, CA: Wadsworth Publishing.