fructose/galactose/pentose phosphate pathways

Question 1

Describe the dietary sources of fructose

Answer 1

In the diet, the main sources of fructose are honey and high fructose corn syrup. Fruit contain sucrose, a disaccharyde of glucose and fructose

Question 2

Describe the two major functions of the pentose phosphate pathway

Answer 2

Generation of NADPH + H+ 
The reducing power of NADPH is used for:
 fatty acid synthesis
 cholesterol synthesis
 nucleotide synthesis
 oxidation of toxins by cytochrome P450 enzymes
 generation of reduced glutathione

Generation of five carbon sugars
Five carbon sugars are used primarily for synthesizing nucleotides.

Question 3

compare the two phases of the pentose phosphate pathway

Answer 3

The oxidative phase:

converts glucose to ribulose 5-phosphate
generates NADPH + H+
reactions are not reversible

The regenerative phase:
converts ribulose 5-phosphate to the glycolysis intermediates fructose 6-phosphate and glyceraldehyde 3-phosphate

is reversible– glycolysis intermediates can be used to generate five carbon sugars for nucleotide synthesis

the key enzymes are transketolase and transaldolase; their reactions are reversible

xylulose 5-phosphate is a key metabolic regulator

Question 4

essential fructosuria

Answer 4

benign deficiency in fructokinase

Question 5

hereditary fructose intolerance

Answer 5

deficiency in aldolase B, fructose 1-phosphate builds up in liver, not benign

Question 6

galactose 1-phosphate uridyl transferase deficiency

Answer 6

“classical galactosemia”; autosomal recessive; very serious
early onset (1-2 weeks old) jaundice, hepatomegaly, hypoglycemia, lethargy, FTT
treatment:  eliminate galactose from diet; prognosis is poor

Question 7

galactokinase deficiency

Answer 7

galactose accumulates, is converted to galactitol through polyol pathway -> catacts
treatment is to eliminate lactose from diet; good prognosis

Question 8

uridine diphosphate galactose 4-epimirase deficiency

Answer 8

benign form: only affects leukocytes and erythrocytes
serious form: all tissues; symptoms similar to transferase deficiency
treatment: A galactose restricted diet

Question 9

glucose 6-phosphate dehydrogenase deficiency

Answer 9

If people with glucose 6-phosphate dehydrogenase deficency are exposed to xenobiotics which are detoxified by NAPDH consuming enzymes, their capacity to regenerate NAPDH through the pentose phosphate pathway is limited, and NAPDH is rapidly depleted.

This leads to severe oxidative stress; red blood cells are particularly affected. 50% or more of a person’s red blood cells may be destroyed– this condition is called hemolytic anemia

Question 10

rate limiting step of glycolysis

Answer 10

fructose 6-phosphate -> fructose 1,6-bis-phosphate. Fructose metabolism bipasses this