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Flashcards in Hexose Metabolism Deck (47):

How does fructose enter the cell?

Glut 5 transporter


1. What deficiently leads to fructokinase deficiency?
2. What is the result?

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


What are some of the results of Hereditary fructose intolerance?

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


What is the net reaction for galactose metabolism?

Galactose + ATP ----> Glucose 1-P + ADP


What enzyme phosphorylates Galactose?



What is the second most common sugar in our diet?



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

GLUTV, small intestine


Describe, briefly, how fructose enters glycolysis.

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


What is the major fuel source for sperm?

Fructose (sertoli cells)


What is the presentation of fructosuria?

Pretty benign, fructose appears in urine


What is the presentation for hereditary fructose intolerance?

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


How does galactose enter glycolysis?

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


What causes classical galactosemia? Nonclassical?

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


What is the presentation of classical galactosemia?

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


What is the presentation of nonclassical galactosemia?

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


What is the purpose of the pentose phosphate pathway?

Produce reducing equivalents, ribose phosphates for nucleotide biosynthesis,


How many NADPH are made per glucose6phosphate oxidized?



What stage produces NADPH? Ribose 5 phosphate?

Oxidative, non oxidative stages respectively


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

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


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



What is the rate limiting step for fructose metabolism?

Aldolase B cleavage to G3P and DHAP


What pathway synthesizes fructose?

Glucose-polyol pathway


Describe the pathway of fructose synthesis

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


Describe the oxidative step for the PPP.

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)


Describe non-oxidative step.

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)


What happens in glucose 6 phosphate dehydrogenase deficiency?

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


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

metabolized in the liver and converted to pyruvate under fasting conditions


Describe the 2 steps of fructose metabolism

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


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

glyceraldhyde 3-phosphate


How is fructose produced from glucose?

1. glucose reduced to sorbitol by aldose reductase
2. Sorbitol reoxidized to form fructose


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

seminal vesicles and is the major energy source for sperm


Describe essential fructosuria and explain the result of the disorder

inherited with accumulation of fructose
fructose is excreted in urine where some may be phosphorylated by HK for glycolysis


Describe hereditary fructose intolerance and explain the results of the disorder

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


Describe galactose metabolism and energy req.

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


Describe classical galactosemia

-deficiency of galactose-1-phosphate uridyl transferase
-accumulation of gal-1-P in tissues and will result in hypoglycemia


Describe nonclassical galactosemia

-Deficiency of galactokinase which converts gal => gal-1-P
-results in galactose accumulation in blood and tissue with cataract formation


Describe the irreversible oxidative reactions of the pentose phosphate pathway

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


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

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


Where does the pentose phosphate pathway occur?

most tissues especially ones that use lots of NADPH


Describe the first 2 stages of the pentose phosphate pathway

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


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



The NADPH generation will provide what other reactions to occur?

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


T/F The cell dictates the direction of the pathway



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

oxidative rxns produce;

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


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

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


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

oxidative and nonoxidative rxns are used

Nonoxidative=>convert to intermediates to pyruvate via glycolysis


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

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