Fructose and Galactose Metabolism Flashcards
Where do we get fructose from?
Mostly from dietary intake of disaccharide sucrose in table sugar or fruit.
Also from the monosaccharide in corn syrup which is used as a sweetener.
How do we get fructose to be useful?
Metabolize in the liver!
Phosphorylate is with ATP and fructokinase to make fructose 1-phosphate (hmmm, really original here)
Aldolase B turns fructose-1-phosphate to DHAP and glyceraldehyde, which are intermediates for glycolysis
When we are not in the liver, we just use hexokinase to turn fructose to fructose 6-phosphate and let it enter glycolysis. Hexokinase has 1/20th the affinity for it though when compared to glucose.
How do we turn glucose to fructose?
Glucose is reduced to sorbitol by aldolase reductase.
Sorbitol is then reoxidized at carbon 2 by sorbitol dehydrogenase to make fructose.
We do this in seminal vesicles because sperm need fructose.
What two fructose conditions do we worry about?
Essential fructosuria - Fructose builds up in the urine because we are deficient in fructokinase. We cannot metabolize fructose as swiftly. This is rather benign.
Hereditary fructose intolerance - Aldolase B, an isozyme in the liver needed for aldolase, is defective (still functions for glycolysis, just can’t break down fructose). Thus, fructose 1-phosphate builds up and inhibits glucose production, causing severe hypoglycemia when fructose is ingested.
We get fructose from sucrose (Glucose + fructose = sucrose).
How do we get galactose?
Source of galactose = lactose (Galactose + glucose = lactose) from milk.
How do we metabolize galactose?
Similar to fructose. Galactose is phosphorylated via ATP and galactokinase to Galactose 1-phosphate.
Galactose-1-phosphate reacts with UDP-glucose and forms glucose-1-phosphate and UDP-galactose via galactose-1-phosphate uridylyl transferase.
UDP-galactose is epimerized to UDP-glucose via UDP-glucose epimerase
2 conditions we worry about with galactose metabolism
The appearence of galactose in the blood after lactose ingestion can be due to one of the two enzymes being defective. In both conditions, the buildup of galactose leads to it being turned in to galacticol by aldolase reductase, which causes cataracts.
However, with Uridylyl transferase deficiency (not in galactokinase deficiency), we get an elevation of galactose-1-phosphate, which inhibits phosphoglucomutase, interfering with glycogen synthesis and degradation, leading to hypoglycemia after ingesting galactose.
