Metabolism of other Mono-and Disaccharides

Question 1

fructose and galactose

Answer 1

• occur in significant amounts in the diet (primarily in disaccharides)
• make important contributions to energy metabolism

Question 2

What is an important component of structural carbohydrates?

Answer 2

galactose

Question 3

fructose

Answer 3

• significant source of calories (10%) in typical western diet
• comes from sucrose, high fructose corn syrup, honey, fruits

Question 4

fructose entry into cells

Answer 4

NOT insulin dependent

Question 5

fructose mediated by what transporter?

Answer 5

GLUT 5

Question 6

fructose and insulin secretion

Answer 6

does not promote insulin secretion

Question 7

fructose metabolization

Answer 7

Bypasses the PFK-1 step, which means that it is metabolized more rapidly than glucose

Question 8

step 1 of fructose metabolism

Answer 8

• phosphorylation of fructose
• enzymes-fructokinase and hexokinase
• IRREVERSIBLE

Question 9

fructokinase

Answer 9

• phosphorylation of fructose
• in liver (mostly), kidney, and small intestinal mucosal cells
• uses ATP

Question 10

hexokinase

Answer 10

• phosphorylation of fructose
• high Km (low affinity) for fructose, therefore it does not usually contribute significantly
• acts in all other tissues

Question 11

step 2 of fructose metabolism

Answer 11

• cleavage of fructose 1-phosphate
• enzyme: aldolase B
• DHAP and glyceraldehyde formed

Question 12

DHAP

Answer 12

formed from the cleavage of fructose 1-P with enzyme Aldolase B
can directly enter glycolysis (or gluconeogensis)

Question 13

glyceraldehyde

Answer 13

product of the cleavage of fructose 1-P with enzyme aldolase B

• can be phosphorylated to glyceraldehyde 3-P (via triose kinase) and enter glycolysis
• can also be converted to glycerol to enter other pathways

Question 14

Conversion of glucose to fructose via sorbitol

Answer 14

-enzyme: aldose reductase and sorbitol dehydrogeanse

Question 15

aldose reductase

Answer 15

• conversion of glucose to fructose via sorbitol
• found in many tissues including lens, retina, schwann cells of peripheral nerves, liver, kidney, cells of ovaries and seminal vesicles

Question 16

sorbitol dehydrogenase

Answer 16

• conversion of glucose to fructose via sorbitol

- ONLY found in the liver, ovaries, and seminal vesicles

Question 17

Tissues that express only aldose reductase

Answer 17

will be damaged by long term elevation of blood glucose because it can make sorbitol but not further metabolize it.

Question 18

excess glucose

Answer 18

converted to sorbitol

Question 19

sorbitol accumulation

Answer 19

results in osmotic uptake of water, which can account for some of the symptoms seen in DM patients

• cataracts
• retinopathy
• nephropathy
• peripheral neuropathy

Question 20

conversion of mannose to fructose 6-P

Answer 20

• mannose is C-2 epimers of glucose
• very little in the diet, most is synthesized intracellularly by fructose or recycled
• important component of glycoproteins
• substrate for hexokinase
• mannose is substrate for phosphomannoseisomerase

Question 21

substrate for hexokinase

Answer 21

mannose. phosphorylated to mannose 6-P

Question 22

substrate for phosphomannoseisomerase

Answer 22

mannose 6-P (hexokinase phosphorylated mannose to form this)

-converts mannose to fructose 6-P

Question 23

essential fructosuria

Answer 23

• lack of fructokinase
• autosomal recessive (1:130,000)
• benign
• fructose accumulates in urine

Question 24

Hereditary fructose intolerance (fructose poisoning

)

Answer 24

• autosomal recessive (1:20,000)
• absence of adores B, intracellular trapping fructose
• severe hypoglycemia, vomiting, jaundice, hemorrhage, hepatomegaly, renal dysfunction, hyperuricemia, and lacticacidemia
• fructose, sucrose, and sorbitol can cause hepatic failure and death
• therapy: rapid detection and removal and fructose and sucrose from diet

Question 25

Why does fructose get trapped in cell in fructose poisoning?

Answer 25

absence of aldolase B, fructose is phosphorylated as in normal pathway, however builds up because can't be further metabolized

Question 26

production of fructose 1-P in someone with fructose poisoning

Answer 26

- ties up much of the available cellular phosphate, which means the liver and kidney cells are comprised - leads to hepatomegaly, jaundice, hypoglycemia, and renal dysfunction - large amount go AMP building up due to lack of ATP production results in AMP being degraded and resulting hyperuricemia

Question 27

galactose metabolism

Answer 27

- isomer of glucose (C-4 epimer) - dietary source-lactose from milk and milk products - some from lysosomal degradation of complex carbs - transport into cells is NOT insulin dependent - metabolism is 2 steps to UDP galactose

Question 28

step 1 of galactose metabolism

Answer 28

-phosphorylation of galactose with enzyme galactokinase

Question 29

step 2 of galactose metabolism

Answer 29

formation of UDP galactose - enzyme: galactose 1-phosphate uridyltransferase (GALT) - transfer reaction - UDP galactose can be converted to UDP-glucose and enter glucose metabolic pathways

Question 30

galactokinase deficiency

Answer 30

- rare autosomal recessive - elevation og galactose in blood and urine - causes galactitol accumulation if galactose is present in the diet - elevated galactitol can cause cataracts - dietary restriction

Question 31

aldose reductase in galactose metabolism

Answer 31

- physiologically unimportant in galactose metabolism unless galactose levels are too high - elevated galatitol can cause cataracts

Question 32

classic galactosemia

Answer 32

- GALT deficiency - autosomal recessive - galactosemia and galactosuria, vomitting, diarrhea, jaundice - accumulation of galactose 1-P and galactitol in nerve, liver, lens, and kidney tissue causes liver damage, severe mental retardation, and cataracts - prenatal Dx is possible by chorionic villus sampling - rapid dx and removal of galactose from diet

Question 33

synthesis of lactose

Answer 33

- milk sugar produced by lactating mammary glands - synthesized in golgi - enzyme: lactose synthase

Question 34

lactose synthase

Answer 34

complex of 2 proteins - A: B-D-galactosyltransferase found in a number of body tissues - B: a-lactalbumin: its synthesis is stimulated by the peptide hormone prolactin