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MBOD Block 4 > Hexose Metabolism > Flashcards

Hexose Metabolism Flashcards

1
Q

How does fructose enter the cell?

A

Glut 5 transporter

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2
Q
  1. What deficiently leads to fructokinase deficiency?

2. What is the result?

A
  1. Fructose is unable to be converted to fructose 1-phosphate
  2. Fructose is excreted in the urine
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3
Q

What are some of the results of Hereditary fructose intolerance?

A
  1. Hereditary deficency of hepatic fructose 1- Phosphate aldolase B
  2. Build up of F-1-P in the liver and kidney
  3. Inhibition of gluconeogenesis and glycogenolysis
  4. Depletion of ATP
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4
Q

What is the net reaction for galactose metabolism?

A

Galactose + ATP —-> Glucose 1-P + ADP

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5
Q

What enzyme phosphorylates Galactose?

A

Galactokinase

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6
Q

What is the second most common sugar in our diet?

A

Fructose

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7
Q

What transporter does fructose enter on? Where is this located (recall from earlier lecture)

A

GLUTV, small intestine

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8
Q

Describe, briefly, how fructose enters glycolysis.

A

FructoseF1P via frucokinase  aldolase B produces G3P and DHAP which enters via triose phosphate isomerase

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9
Q

What is the major fuel source for sperm?

A

Fructose (sertoli cells)

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10
Q

What is the presentation of fructosuria?

A

Pretty benign, fructose appears in urine

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11
Q

What is the presentation for hereditary fructose intolerance?

A

Toxic deficiency of aldolase B, f1P is toxic in the liver, vomiting, hypoglycemia, etc. uses up ATP stores and inhibits gluconeogenesis and glycogenolysis

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12
Q

How does galactose enter glycolysis?

A

Galactosegalactose 1 phosphate via galactokinase glucose 1 phosphate via galactose 1 phosphate uridilyl transferase (Leloir pathway) via UDP glucose/galactose exchange

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13
Q

What causes classical galactosemia? Nonclassical?

A

Deficiency of galactose-1-phosphate uridyl transferase; defeiciency of galactokinase

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14
Q

What is the presentation of classical galactosemia?

A

Accumulation of Galactose1P, vomiting, hypoglycemia, lots of other symptoms, galactose in blood and tissues

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15
Q

What is the presentation of nonclassical galactosemia?

A

Galactose in blood and tissues, galactitol accumulates in lens which can lead to cataracts, must avoid galactose in diet completely

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16
Q

What is the purpose of the pentose phosphate pathway?

A

Produce reducing equivalents, ribose phosphates for nucleotide biosynthesis,

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17
Q

How many NADPH are made per glucose6phosphate oxidized?

A

2

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18
Q

What stage produces NADPH? Ribose 5 phosphate?

A

Oxidative, non oxidative stages respectively

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19
Q

What is the predominant form of aldolase in liver? Muscle?

A

B, A, respectively: B is used for glycolytic reasons, A is used for gluconeogenic reasons

20
Q

T or F: aldolase B has a higher affinity for Fructose 1 phosphate than it does for G3P and DHAP.

A

True

21
Q

What is the rate limiting step for fructose metabolism?

A

Aldolase B cleavage to G3P and DHAP

22
Q

What pathway synthesizes fructose?

A

Glucose-polyol pathway

23
Q

Describe the pathway of fructose synthesis

A

Glucosesoribitol/polyol via aldolase reductasefructose via sorbitol DH

24
Q

Describe the oxidative step for the PPP.

A

Glucose 6 phosphateto 6 phosphoglucono-delta-lactone with generation of NADPH via glucose 6 DH (NADPH inhibits this step)6 phosphogluconolactone to 6 phosphogluconate via glucolactonase reduced to ribulose 5 phosphate via 6 phosphoglucnate DH (therefore generate 2 NADPH and 1 mol ribulose 5 phosphate)

25
Q

Describe non-oxidative step.

A

Transketolase takes 2 C fragment from xyulose 5 phosphate (which epiremized from ribose 5 phosphat) and puts it on another ribose 5 phosphate which forms sedoheptulose 7 phosphate and G3P, Sedoheptulose 7P and G3P then form erythrose 4 phosphate and F6P via a transaldolase, eryhthrose 4P then gains another2C fragment from another xyulose 5 phosphate to form F6P and G3P (basically forming 3 glycolytic intermediates)

26
Q

What happens in glucose 6 phosphate dehydrogenase deficiency?

A

Less NADPHOxidative damage, loss of reducing power to eliminate radicalsprecipitation of Hb in RBC’s cause Heinz bodies and hemolytic anemia

27
Q

Where is fructose mainly metabolized and what is it converted to?

A

metabolized in the liver and converted to pyruvate under fasting conditions

28
Q

Describe the 2 steps of fructose metabolism

A
  1. Fructose phosphorylated by fructose-1-phosphate

2. fructose 1-phosphate cleaved by aldolase B to form DHAP and glyceraldehyde => glyceraldehyde 3-phosphate

29
Q

Which intermediate from fructose metabolism will produce pyruvate leading to the TCA cycle

A

glyceraldhyde 3-phosphate

30
Q

How is fructose produced from glucose?

A
  1. glucose reduced to sorbitol by aldose reductase

2. Sorbitol reoxidized to form fructose

31
Q

Where would you find fructose derived from glucose? What is the function of fructose in this location?

A

seminal vesicles and is the major energy source for sperm

32
Q

Describe essential fructosuria and explain the result of the disorder

A

inherited with accumulation of fructose

fructose is excreted in urine where some may be phosphorylated by HK for glycolysis

33
Q

Describe hereditary fructose intolerance and explain the results of the disorder

A

hereditary deficiency of hepatic fru-1-P aldolase B leading to an accumulation in liver and kidney
-will inhibit gluconeogenesis and glycogenolysis

34
Q

Describe galactose metabolism and energy req.

A

Galactose + ATP => Galactose 1-P + UDP-glucose => Glucose-1-P + ADP => G-6-P (liver) for glucose in blood

35
Q

Describe classical galactosemia

A
  • deficiency of galactose-1-phosphate uridyl transferase

- accumulation of gal-1-P in tissues and will result in hypoglycemia

36
Q

Describe nonclassical galactosemia

A
  • Deficiency of galactokinase which converts gal => gal-1-P

- results in galactose accumulation in blood and tissue with cataract formation

37
Q

Describe the irreversible oxidative reactions of the pentose phosphate pathway

A
  • 1 carbon of G-6-P is released as CO2;
  • NADPH is generated;
  • ribulose 5-phosphate is produced
38
Q

Why are NADPH, ribose 5-phosphate, and pentose sugars formed?

A
  • NADPH: required for reducing equivalents
  • Ribose 5-phosphate: nucleotide biosynthesis
  • pentose sugars: rearrange for glycolytic/gluconeogenic intermediates
39
Q

Where does the pentose phosphate pathway occur?

A

most tissues especially ones that use lots of NADPH

40
Q

Describe the first 2 stages of the pentose phosphate pathway

A

1st=>oxidative and generates 2 mol of NADPH per -G-6-P oxidized

2nd=>non-oxidative and generates ribose 5-P that is converted unused to intermediates to glycolytic pathway

41
Q

T/F NADPH is a strong inhibitor to G-6-P dehydrogenase

A

True

42
Q

The NADPH generation will provide what other reactions to occur?

A

fatty acid synthesis, glutathione reduction (protection against oxidative damage)

43
Q

T/F The cell dictates the direction of the pathway

A

True

44
Q

Wrt NADPH only, how is the direction of the pathway dictated?

A

oxidative rxns produce;

nonoxidative rxns convert ribose-5-P to G-6-P to produce more NADPH

45
Q

Wrt to ribose 5-P, how is the direction of the pathway dictated?

A

only nonoxidative rxns as NADPH inhibits G-6-P dehydrogenase so fruc-6-P and glyceraldehyde 3-P and ribose 5-P are produced

46
Q

Wrt to NADPH and pyruvate, how is the direction of the pentose phosphate dictated?

A

oxidative and nonoxidative rxns are used

Nonoxidative=>convert to intermediates to pyruvate via glycolysis

47
Q

Describe the metabolic disorder associated with pentose phosphate shunt along with results

A

Glucose -6- Phosphate dehydrogenase deficiency
Results: more oxidative stress on the cell due to the disulfide bonds being formed bw cysteine residues of 2 GSH molecules

MBOD Block 4 (15 decks)

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