Goal 12: Metabolism Of Fructose And Galactose Flashcards
What are examples of free forms of fructose?
Honey, corn syrup, fruits
What is another name for table sugar? What enzyme digest it in the intestine?
- sucrose
- sucrase
How is fructose absorbed from the intestine? By what? What is the difference compared to glucose?
- fructose: absorbed by facilitated transport, carried by GluT5
- glucose: secondary active transport (symport), carried by SGLT1
Where is fructose metabolized?
-liver mostly, but kidneys and intestine as well
What are the 3 enzymes utilized in fructose metabolism?
- fructokinase
- aldolase B or fructose-1 phosphatase aldolase
- Triokinase
Fructokinase has a high affinity (low km) for what?
Fructose
What enzyme also can phosphorylate but not as well because of it high km and low affinity to fructose?
Hexokinase
What step is bypassed in the liver, allowing the rate of fructose metabolism to move way faster than glucose metabolism? What can this lead to if this happens often? How does this happen?
- PFK1 is bypassed in fructose metabolism
- increased triglycerides or hyperlipidemia
- increased pyruvate—> increased Acetyl CoA—> increased fatty acid synthesis—> increased TAG production
Does fructose stimulate insulin secretion? Does glucose stimulate insulin secretion?
- fructose DOES NOT stimulate insulin
- glucose does stimulate insulin
What is sorbitol? How is it formed? How is it broken down? Where does the sorbitol process happen?
- sugar alcohol
- glucose turned to sorbitol by aldose reductase to form sorbitol
- sorbitol is turned to fructose by sorbitol dehydrogenase
- happens in seminal vesicles, liver, sperm, ovaries
Where do you see free fructose? What secretes it? What utilizes it for mobility?
- see in seminal plasma
- secreted from seminal vesicles
- spermatozoa uses fructose as energy for mobility
What enzyme deficiency causes fructosuria?
- fructokinase- can’t break down fructose without it
- fructose in urine
- benign
What enzyme deficiency causes Hereditary Fructose Intolerance? What’s accumulating because if this? What are later complications of this? How does this affect inorganic phosphate?
- deficient in aldolase B
- fructose 1-P accumulates
- fructose 1-P inhibits glycogen phosphorylase and leads to the decrease rate of glycogenolysis
- inorganic phosphate is sequestered (trapped) so there’s a decrease level of it and ATP—-leads to a decreased rate of glycogenolysis too
How does decreased ATP affect gluconeogenesis?
Decreases gluconeogenesis
Decreased gluconeogenesis and decrease glycogenolysis leads to?
Hypoglycemia
What symptoms do you see in Hereditary Fructose Intolerance?
- Fructose in urine, hyperuricemia, vomiting, hepatomegaly, hepatic damage, jaundice
- may lead to death from liver failure if diet is not change to limit fructose/sucrose!
Why do you see hyperuricemia in Hereditary Fructose Intolerance?
- Low ATP and inorganic phosphate means there’s more AMP.
- AMP used to degrade purines to make uric acid
How is galactose absorbed compared to glucose?
- they both are absorbed by secondary active transport using SGLT1
- symport
Where is galactose further metabolized?
In cytosol of liver
What are the 3 enzymes utilized in Galactose Metabolism?
- galactokinase
- galactose 1-P uridyl transferase
- UDP-galactose epimerase
At what product can galactose metabolism enter gylcogenesis?
UDP-Glucose
Glaactose is essential in the body to synthesize what 4 listed compounds? What acts as the carrier of galactose during the synthesis of these compounds?
- lactose
- keratan sulfate (example of a GAG)
- glycoproteins
- glycolipids
- UDP acts as a carrier
What deficiency causes Galactosemia? What accumulates because of this? What symptoms do you see?
- deficiency of Galactose-1-P Uridyl Transferase
- accumulation of galactose-1-phosphate (Gal-1-P) and galactitol
- hepatomegly, jaundice, vomiting, diarrhea, hypoglycemia, failure to thrive, mental retardation, urinate galactose
What enzyme causes galactitol to form from galactose?
Aldose reductase
What does galactitol accumulation in the lens lead to?
Cataract-causes opacity to lens
What diet change is recommended with Galactosemia?
Withdrawal of lactose!
What deficiency causes Galactosemia Type 2? How severe is this one? What symptom do you usually see solely?
- deficiency in galactokinase
- mild
- cataract since galactitol is still produced—see this more in type 2 than type 1
What is deficient in Galactosemia type 3?
-UDP-galactose epimerase
Where is lactose synthesized in the body? What part of the cell there? What enzyme synthesizes it and what are the steps of synthesis?
- lactating mammary glands
- golgi
- lactose synthase (aka galactosyl transferase)
- UDP-galactose + glucose—> lactose
What protein makes lactose synthase? What hormone do you need to make this important protien?
- Alpha Lactalbumin
- prolactin needed to make it
