Metabolism of Hexoses Other than Glucose Flashcards
What kind of carbohydrates are mannose, glucose, galactose, and fructose?
Hexoses
Where does galactose enter glycolysis at?
It is converted to glucose 6-phosphate and begins glycolysis there.
Where do mannose and fructose (in muscle) enter glycolysis at?
They are converted to fructose 6-phosphate and enters glycolysis there.
Where does fructose enter glycolysis at in the liver?
It is converted to glyceraldehyde 3-phosphate and enters glycolysis there.
What are the primary sources of fructose?
1) Fruit
2) Sucrose (disaccharide of glucose and fructose)
What enzyme converts fructose into fructose-6-phosphate in muscle?
Hexakinase
What enzyme catalyzes the conversion of fructose to fructose-1-phosphate in liver?
Fructokinase
True or false: Glucokinase in the liver will react with fructose?
False
Which type of aldolase is specific for fructose-1,6-bisphosphate?
Type A in muscle
Which type of aldolase in the liver will use both FBP and fructose 1-phosphate as substrates?
Type B
What are the two options for fructose in the liver after it has been converted to glyceraldehyde?
1) Glyceraldehyde kinase converts it to GAP (which allows it to enter glycolysis)
2) It is converted to glycerol by ADH, then to glycerol-3-phosphate by glycerol kinase, then to DHAP by glycerol phosphate dehydrogenase, then to GAP by triose phosphate isomerase (where it enters glycolysis).
True or false: Both pathways in the liver from glyceraldehyde to GAP have the same net energy cost.
True
True or false: Both pathways in the liver from glyceraldehyde to GAP produce ATP.
False
Which of the two pathways in the liver from glyceraldehyde to GAP oxidize/reduce NADH?
The longer pathway
Which of the two pathways in the liver from glyceraldehyde to GAP produce glycerol-3-phosphate and glycerol?
The longer pathway?
What can glycerol-3-phosphate become the backbone for?
Glycerophospholipids and triacylglycerols
What happens to fructose if there are high concentrations of ATP?
It will be converted to fat.
True or false: Fructose is converted to fructose-1-phosphate by phosphofructokinase in the liver.
False; fructose bypasses PFK in the liver. Fructose kinase takes it to F1P.
What is fructose intolerance caused by?
Deficiency of Type B aldolase
Why does a deficiency of Type B aldolase cause when fructose is consumed?
1) F1P accumulates and depletes liver stores of Pi.
2) Reduced ATP causes liver damage.
3) Increased F1P inhibits glycogen phosphorylase (breaks down glycogen) and fructose-1,6-bisphosphate (for gluconeogenesis).
4) This causes severe hypoglycemia.
How is fructose ingestion inhibited in someone with a deficiency of Type B aldolase?
They develop a strong distaste for anything sweet.
Where does galactose come from?
Hydrolysis of lactose (galactose and glucose)
What is the difference between galactose and glucose?
They differ only in configuration at carbon 4.
True or false: Hexokinase converts galactose to glucose-6-phosphate.
False; it works on glucose, fructose, and mannose, but not galactose.
What are the 4 steps of the epimerization that convert galactose to glucose-6-phosphate?
1) Galactokinase converts galactose to galactose 1-phosphate
2) Galactose-1-phosphate uridyl transferase converts galactose-1-phosphate to UDP-galactose.
3) UDP-galactose-4-epimerase converts UDP-galactose to glucose-1-phosphate.
4) Phosphoglucomutase converts G1P to G6P, which enters glycolysis.
What is the disease that occurs from the inability to convert galactose to glucose?
Galactosemia
What are the symptoms of galactosemia?
1) Failure to thrive
2) Mental retardation
3) Death
4) Cataracts
What usually causes galactosemia?
Deficiency of Gal-1-P uridyl transferase
What does galactosemia cause to happen in the lens of the eye?
High [galactitol] (from high blood levels of galactose), leading to cataracts.
What is the treatment for galactosemia?
Galactose-free diet (which can reverse all symptoms except mental retardation).
Where does one’s required galactose come from if one is on a galactose-free diet?
Glucose
Where does mannose come from?
Digestion of polysaccharides and glycoproteins
Where does mannose enter glycolysis?
As fructose-6-phosphate
What enzyme converts mannose into mannose-6-phosphate?
Hexokinase
What enzyme converts mannose-6-phosphate to fructose-6-phosphate?
Phosphomannose isomerase.
