Metabolism of other Mono-and Disaccharides Flashcards Preview

Biochem BLOCK 2 > Metabolism of other Mono-and Disaccharides > Flashcards

Flashcards in Metabolism of other Mono-and Disaccharides Deck (34):
1

fructose and galactose

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

2

What is an important component of structural carbohydrates?

galactose

3

fructose

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

4

fructose entry into cells

NOT insulin dependent

5

fructose mediated by what transporter?

GLUT 5

6

fructose and insulin secretion

does not promote insulin secretion

7

fructose metabolization

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

8

step 1 of fructose metabolism

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

9

fructokinase

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

10

hexokinase

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

11

step 2 of fructose metabolism

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

12

DHAP

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

13

glyceraldehyde

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

14

Conversion of glucose to fructose via sorbitol

-enzyme: aldose reductase and sorbitol dehydrogeanse

15

aldose reductase

-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

16

sorbitol dehydrogenase

-conversion of glucose to fructose via sorbitol
-ONLY found in the liver, ovaries, and seminal vesicles

17

Tissues that express only aldose reductase

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

18

excess glucose

converted to sorbitol

19

sorbitol accumulation

results in osmotic uptake of water, which can account for some of the symptoms seen in DM patients
-cataracts
-retinopathy
-nephropathy
-peripheral neuropathy

20

conversion of mannose to fructose 6-P

-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

21

substrate for hexokinase

mannose. phosphorylated to mannose 6-P

22

substrate for phosphomannoseisomerase

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

23

essential fructosuria

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

24

Hereditary fructose intolerance (fructose poisoning
)

-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

25

Why does fructose get trapped in cell in fructose poisoning?

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

26

production of fructose 1-P in someone with fructose poisoning

-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

27

galactose metabolism

-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

28

step 1 of galactose metabolism

-phosphorylation of galactose with enzyme galactokinase

29

step 2 of galactose metabolism

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

30

galactokinase deficiency

-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

31

aldose reductase in galactose metabolism

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

32

classic galactosemia

-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

33

synthesis of lactose

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

34

lactose synthase

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