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Metabolite Card

Note

Fructose, or levulose, is a levorotatory monosaccharide and an isomer of glucose (C6H12O6). Pure fructose has a sweet taste similar to cane sugar, but with a "fruity" aroma. Pure, dry fructose is a sweet, white, odorless, crystalline solid, and is the most water-soluble of all the sugars. Although fructose is a hexose (6-carbon sugar), it generally exists as a 5-member hemiketal ring (a furanose). This structure is responsible for the long metabolic pathway and high reactivity compared to glucose. Fructose is a reducing sugar, as are all monosaccharides. Fructose is found in many foods including honey, tree fruits, berries, melons, and some root vegetables, such as beets, sweet potatoes, parsnips, and onions. Commercially, fructose is derived from sugar cane, sugar beets, and maize. Fructose is also derived from the digestion of sucrose, a disaccharide consisting of glucose and fructose that is broken down by enzymes during digestion. Fructose is the sweetest naturally occurring sugar, estimated to be twice as sweet as sucrose. It is used as a preservative and an intravenous infusion in parenteral feeding. Excessive consumption of fructose (especially from sugar-sweetened beverages) may contribute to insulin resistance, obesity, elevated LDL cholesterol and triglycerides, leading to metabolic syndrome (PMID: 26429086). Fructose exists in foods either as a monosaccharide (free fructose) or as a unit of a disaccharide (sucrose). Free fructose is absorbed directly by the intestine. When fructose is consumed in the form of sucrose, it is digested (broken down) and then absorbed as free fructose. As sucrose comes into contact with the membrane of the small intestine, the enzyme sucrase catalyzes the cleavage of sucrose to yield one glucose unit and one fructose unit, which are then each absorbed. After absorption, it enters the hepatic portal vein and is directed toward the liver. fructose absorption occurs on the mucosal membrane via facilitated transport involving GLUT5 transport proteins. Since the concentration of fructose is higher in the lumen, fructose is able to flow down a concentration gradient into the enterocytes, assisted by transport proteins. Fructose may be transported out of the enterocyte across the basolateral membrane by either GLUT2 or GLUT5, although the GLUT2 transporter has a greater capacity for transporting fructose, and, therefore, the majority of fructose is transported out of the enterocyte through GLUT2. The catabolism of fructose is sometimes referred to as fructolysis. In fructolysis, the enzyme fructokinase produces fructose 1-phosphate, which is split by aldolase B to produce the trioses dihydroxyacetone phosphate (DHAP) and glyceraldehyde. Unlike glycolysis, in fructolysis the triose glyceraldehyde lacks a phosphate group. A third enzyme, triokinase, is therefore required to phosphorylate glyceraldehyde, producing glyceraldehyde 3-phosphate. The resulting trioses can enter the gluconeogenic pathway for glucose or glycogen synthesis, or be further catabolized through the lower glycolytic pathway to pyruvate. Fructose metabolism leads to significant increases of plasma uric acid levels (PMID: 28420204). In fructolysis, fructose 1-phosphate accumulates, and intracellular phosphate decreases. This decrease stimulates AMP deaminase (AMPD), which catalyzes the degradation of AMP to inosine monophosphate, increasing the rate of purine degradation (PMID: 28420204). The purine degradation produces uric acid and generates mitochondrial oxidants. Mitochondrial oxidative stress then induces aconitase inhibition in the Krebs cycle, with accumulation of citrate and stimulation of ATP citrate lyase and fatty acid synthase (PMID: 28420204). The result is de novo lipogenesis and hepatic fat accumulation. Physiologically, the increase in intracellular uric acid is followed by an acute rise in circulating levels of uric acid, which is likely due to its release from the liver. Fructose also stimulates uric acid synthesis from amino acid precursors such as glycine. Moreover, long-term fructose administration suppresses renal excretion of uric acid, resulting in elevated serum uric acid levels.