Metabolism Of Monosaccharides

Question 1

Sources of fructose?

What part of body requires fructose?

Answer 1

Honey, fruit/fruit juices

Spermatozoa (mostly in head)

Question 2

What converts glucose to fructose?

Fructose can give rise to glucose because?

Answer 2

Sorbitol polyol pathway (De novo synthesis)

It can enter glycolysis

Question 3

What is fructose+glucose?

Answer 3

Sucrose

Question 4

What needs to happen first for fructose metabolism?

What enzymes do this?

Answer 4

Fructose needs to be phosphorylated

Fructokinase (primary) - in liver, kidney, and small intestine
Hexokianse - present in other cells and skeletal muscle

Question 5

Why is fructokinase primary and not hexokinase for fructose?

Answer 5

Because hexokinase has a high Km for fructose (which means low affinity)

Question 6

Difference in initial steps of metabolism for glucose vs fructose

Metabolism rate is faster for glucose or fructose? Why

Answer 6

Fructose is not mediated by insulin (absorption by GLUT5) like glucose is

Fructose (gatorade); bypasses PFK1 (don’t need to input as much energy)

Question 7

Fructose gets converted into what by hexokinase? Fructokinase?

Next step and why it is significant

Answer 7

Hexokinase-fructose 6 phosphate
Fructosekinase- fructose 1 phosphate

Next, aldolase B converts fructose 1 phosphate to glyceraldehyde + DHAP. Significant bc aldolase B does not work on fructose 6 phosphate

Question 8

Elevated levels of dietary fructose will increase rate of?

Answer 8

Lipogenesis in the liver

Question 9

Glucose is able to produce fructose by converting to?

When would this happen and explain

Answer 9

Sorbitol (accumulates rapidly but degrades slowly)

This is minor- but will happen with excess glucose. Glucose gets acted on by aldose reductase to give sorbitol

Question 10

Clinical relevance of excess sorbitol

Answer 10

All complications of diabetes are due to excess sorbitol/glucose in the organs affected (absence of sorbitol dehydrogenase)

Question 11

1. Aldose reductase is located where in the body?

2. Sorbitol dehydrogenase is located where?

Answer 11

1. Lens, retina, peripheral nerves, kidney, placenta, RBCs, ovaries, seminal vesicles (all non insulin dependent tissue)
2. Liver, ovaries, sperm, seminal vesicle cells

Question 12

Do diabetic patients tolerate fructose or glucose better? Why?

Answer 12

Fructose because enzymes of fructose metabolism are NOT insulin dependent

Question 13

Fructosuria is caused by deficiency of?

What happens

Answer 13

Fructokinase deficiency- excess fructose in blood; kidneys remove it by putting it in urine (harmless)

Question 14

Fructose intolerance would be caused by deficiency of

What happens?

Answer 14

Aldolase B (potentially lethal)(in children will lead to liver failure)

Fructose 1 phosphate cannot be broken down and will stay in the cell and take up all the cell’s phosphate. This will lead to ATP not being produced so you will have high levels of ADP and AMP. ADP and AMP will be broken down to uric acid which could lead to GOUT

Question 15

Source of galactose

Answer 15

Dietary lactose

Question 16

Path of galactose metabolism: with 3 main enzymes

Answer 16

1. Galactose converts to galactose 1 P by GALK (galactokinase)
2. Galactose 1 P converts to Glucose 1 P by GALT - during this step GALE recycles UDP galactose back to UDP glucose
3. Glucose 1 P enters glycolysis

Question 17

Important of UDP galactose?

Answer 17

Required for biosynthesis of lactose, glycoproteins, and glycosaminoglycans

Question 18

3 types of galactosemia (a lot of galactose in blood) and their prevalence

Answer 18

1. GALT deficiency - most common/severe
2. GALK deficiency - milder
3. GALE deficiency - rarest and least lethal

Question 19

GALT deficiency is similar to ?

Answer 19

Deficient aldolase B from fructose metabolism (similar mechanism of what is happening)

Question 20

1. Effects of aldoase B deficiency on body

2. Effects of GALT deficiency on body?

Answer 20

1. Hypoglycemia, vomiting (after fructose), poor feeding, jaundice, hepatomegaly, hemorrhage, and eventual hepatic failure
2. Failure to thrive, lethargy, vomiting and diarrhea (after milk), eventually result in cataract, blindness, mental retardation, and fatal liver damage

Question 21

Patients with GALT deficiency will have persistently elevated ____ levels

Answer 21

Erythrocyte galactose-1-phosphate levels