Goal 12: Metabolism Of Fructose And Galactose

Question 1

What are examples of free forms of fructose?

Answer 1

Honey, corn syrup, fruits

Question 2

What is another name for table sugar? What enzyme digest it in the intestine?

Answer 2

• sucrose

- sucrase

Question 3

How is fructose absorbed from the intestine? By what? What is the difference compared to glucose?

Answer 3

• fructose: absorbed by facilitated transport, carried by GluT5
• glucose: secondary active transport (symport), carried by SGLT1

Question 4

Where is fructose metabolized?

Answer 4

-liver mostly, but kidneys and intestine as well

Question 5

What are the 3 enzymes utilized in fructose metabolism?

Answer 5

• fructokinase
• aldolase B or fructose-1 phosphatase aldolase
• Triokinase

Question 6

Fructokinase has a high affinity (low km) for what?

Answer 6

Fructose

Question 7

What enzyme also can phosphorylate but not as well because of it high km and low affinity to fructose?

Answer 7

Hexokinase

Question 8

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?

Answer 8

• PFK1 is bypassed in fructose metabolism
• increased triglycerides or hyperlipidemia
• increased pyruvate—> increased Acetyl CoA—> increased fatty acid synthesis—> increased TAG production

Question 9

Does fructose stimulate insulin secretion? Does glucose stimulate insulin secretion?

Answer 9

• fructose DOES NOT stimulate insulin

- glucose does stimulate insulin

Question 10

What is sorbitol? How is it formed? How is it broken down? Where does the sorbitol process happen?

Answer 10

• 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

Question 11

Where do you see free fructose? What secretes it? What utilizes it for mobility?

Answer 11

• see in seminal plasma
• secreted from seminal vesicles
• spermatozoa uses fructose as energy for mobility

Question 12

What enzyme deficiency causes fructosuria?

Answer 12

• fructokinase- can’t break down fructose without it
• fructose in urine
• benign

Question 13

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?

Answer 13

• 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

Question 14

How does decreased ATP affect gluconeogenesis?

Answer 14

Decreases gluconeogenesis

Question 15

Decreased gluconeogenesis and decrease glycogenolysis leads to?

Answer 15

Hypoglycemia

Question 16

What symptoms do you see in Hereditary Fructose Intolerance?

Answer 16

• 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!

Question 17

Why do you see hyperuricemia in Hereditary Fructose Intolerance?

Answer 17

• Low ATP and inorganic phosphate means there’s more AMP.

- AMP used to degrade purines to make uric acid

Question 18

How is galactose absorbed compared to glucose?

Answer 18

• they both are absorbed by secondary active transport using SGLT1
• symport

Question 19

Where is galactose further metabolized?

Answer 19

In cytosol of liver

Question 20

What are the 3 enzymes utilized in Galactose Metabolism?

Answer 20

• galactokinase
• galactose 1-P uridyl transferase
• UDP-galactose epimerase

Question 21

At what product can galactose metabolism enter gylcogenesis?

Answer 21

UDP-Glucose

Question 22

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?

Answer 22

• lactose
• keratan sulfate (example of a GAG)
• glycoproteins
• glycolipids
• UDP acts as a carrier

Question 23

What deficiency causes Galactosemia? What accumulates because of this? What symptoms do you see?

Answer 23

• 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

Question 24

What enzyme causes galactitol to form from galactose?

Answer 24

Aldose reductase

Question 25

What does galactitol accumulation in the lens lead to?

Answer 25

Cataract-causes opacity to lens

Question 26

What diet change is recommended with Galactosemia?

Answer 26

Withdrawal of lactose!

Question 27

What deficiency causes Galactosemia Type 2? How severe is this one? What symptom do you usually see solely?

Answer 27

• deficiency in galactokinase
• mild
• cataract since galactitol is still produced—see this more in type 2 than type 1

Question 28

What is deficient in Galactosemia type 3?

Answer 28

-UDP-galactose epimerase

Question 29

Where is lactose synthesized in the body? What part of the cell there? What enzyme synthesizes it and what are the steps of synthesis?

Answer 29

• lactating mammary glands
• golgi
• lactose synthase (aka galactosyl transferase)
• UDP-galactose + glucose—> lactose

Question 30

What protein makes lactose synthase? What hormone do you need to make this important protien?

Answer 30

• Alpha Lactalbumin

- prolactin needed to make it