Fructose, Galactose, and Pentose Posphate Lecture Sep 18

Question 1

What are the dietary sources of fructose?

Answer 1

Main source is from fruit, although it’s indirect–fruit contains sucrose which is a dissacharide of glucose and fructose.

We can get direct fructose form honey and high fructose corn syrup

Question 2

What pathway will synthesize fructose int he body?

Where does it happen?

What does it use?

Answer 2

The polyol pathway will take glucose and make fructose.

This occurs in seminal vesicles for sperm production and within the eye.

Question 3

What enzyme will cleave sucrose into glucose and fructose?

Answer 3

Sucrase

(also called isolmaltase)

Question 4

How can the polyol pathway lead to pathology in the eye?

Answer 4

Cataracts:

In cases of uncontrolled diabetes (hyperglycemia), the polyol pathway can lead to elevated sorbitol and fructose levels in the eye. This leads to water influx and high fluid pressure.

The excess sugar also results in non-enymatic glycosylation of lens proteins. The high pressure and glycolsylated lens proteins make an opaque spot on the lens.

Question 5

Where is fructose primarily metabolized?

How does it get into the cell?

Answer 5

Fructose is primarily metabolized in the liver

It enters the hepatocyte through GLUT V receptors (which are symporters with Na+)

Question 6

What is the first step in fructose metabolism? WHat enzyme does it?

Answer 6

Fructose is phosphorylated by fructokinase to yield fructose 1-phosphate.

This traps the fructose in the cell.

Question 7

Why is fructose metabolism mainly in the liver?

Answer 7

THe liver is the only tissue that expresses aldolase B.

Aldolase in the res tof the body works on fructose 1,69bisphosphate

Only aldolase B can act on fructose 1 phosphate, which is the substrate we have in fructose metabolism.

Question 8

What does aldolase B cleave frustose 1 phosphate into?

Answer 8

dihydroxyacetone phosphate

and

glyceraldehyde (which can then be converted to glyceraldehyde 3 phosphate)

Question 9

What enzyme will convert the glyceraldehyde produces by aldolase into glyceraldehyde 3 phosphate (which is what we want)?

Answer 9

triose kinase

Question 10

What can the glyceraldehyde 3 phosphate be used for?

What is it usually used for?

Answer 10

It can enter glycolysis to create pyruvate that can enter the TCA cycle to be oxidized (or generate intermediates for fatty acid sythesis)

Or it can enter the gluconeogenesis/glycogenesis pathway.

Since fructose levels are high in the fed state only (hardly any fructose is synthesized in the body), it usually enters the glyconeogenesis/glycogenesis pathway.

It’s converted to glucose 6 phosphate which can be used to make glucose or further worked on to make glycogen.

Question 11

If the glyceraldehyde 3 phosphate from fructose metabolism enters glycolysis, what important regulatory step is bypassed?

Answer 11

Phosphofructokinase 1

(inhiibited by ATP and citrate: excess energy to work)

(activated by AMP and fructose 2,6 bisphosphate: excess work to fuel)

Question 12

Even if the glyceraldehyde 3 phosphate from fructose metabolism enters glycolysis instead of gluconeogenesis/glycogenesis, there is still a way for it to be stored instead of burned in the TCA. How is it stored?

Answer 12

THe citrate is diverted out for fatty acid synthesis

Question 13

EVen though fructose is mainly metabolized in the liver, some of it can be metabolized in other tissues. How is this possible when none of the other tissues have aldolase B?

Answer 13

Hexokinase (which usually acts on glucose) can also act on fructose at a slower rate. This causes the phosphorylation of fructose to fructose 6 phosphate.

THis can either enter glycolysis (in which it DOES NOT bypass the PFK regulatory step) or glycogenogenesis

Question 14

What causes essential fructosuria?

What are the symptoms?

prognosis?

Answer 14

A mutation in fructokinase results in the inability to ocnvert frutose to fructose 1 phosphate.

This causes a buildup of fructose which is excreted in the urine - fructosuria.

THis is benign - people won’t even knwow they have it unless they have a positive reducing sugar test for diabetes. The glucose specific test will then be negative.

Question 15

What results from an aldolase B deficiency?

What are the symptoms?

Prognosis?

Treatment?

Answer 15

Hereditary fructose intolerance

This means you can’t convert fructose 1 phosphate into DHAP or G3P.

THe frustose 1 phosphate therefore has no metabolic fat and it accumulate in the liver.

THe bigger issue is that all the phosphate in the liver then gets traped as fructose 1 phosphate! But we need phosphate groups for almost every process! So the condition is quite serious.

It can be asymptomatic until fructose is ingested and if untreated it will cuase recurrent hypolgycemia episodes, FTT and heaptic failure.

Treat by avoiding fructose.

Question 16

What is the primary dietary souce of galactose?

Answer 16

It’s from the dissacharide lactose (milk sugar) which is galactose and glucose.

Question 17

What enzyme is used to cleave lactose into galactose and glucose?

Answer 17

lactase

Question 18

where does galactose metabolism mainly occur?

Answer 18

the liver

Question 19

Lactase deficiency or lactose intolerance comes in three forms. What are they?

Answer 19

Primary lactase deficiency - an autosomal recessive condition in which lactase activity declines over many years, usually beginning in acolescence. THis is the most common.

Secondary lactase deficiency i due to damage of brush border of intestinal anterocytes due to intestinal disease.

Congeintal lactase deficiency, a very rare genetic conditions, is the complete abscence of lactase

Question 20

What is the first step in galactose metabolism?

Answer 20

Galactokinase phosphorylates galactose to galactose 1 phosphate to trap it in the cell.

Question 21

Which enzyme works on galactose 1 phosphate?

WHat is the produce?

Answer 21

Galactose 1 phosphate uridyl transferase

It takes the UDP off UDP-Glucose and adds it to galactose, making UDP galactose

Question 22

What enzyme will interconvert UDP-galactose and UDP-glucose?

Answer 22

epimerase

Question 23

What is the end produce of galactose metabolism?

Answer 23

Glucose 1-phosphate for glycogenesis or gycolysis

Question 24

What causes nonclassical galactosemia?

symptoms?

treatment?

Answer 24

A galactokinase deficiency

You can’t convert galactose to galactose 1-phosphate

The galactose accumulates and is then converted to galactitol through the polyol pathway

This causes cataracts

The treatment is to eliminate lactose form the diet

good prognosis

Question 25

WHat causes classical galactosemia?

Answer 25

This is a galactose 1-phosphate uridyl transferase deficiency

This means that the conversion of galactose 1-phosphate to UDP galactose cannot occur. This is similar to the hereditary fructose intolerance in that all the phosphate in the liver gets stuck as galactose 1 phosphate and you have major issues.

It’s autosomal recessive and VERY serious

Early onset (1-2 weeks) of jaundice, hepatomegaly, hypoglycemia, lethargy, and FTT

Treatment: try to eliminate galactose from the diet; prognosis is poor.

Question 26

What results from an epimerase deficiency?

Answer 26

It comes in two forms:

The benign form affects only leukocytes and erythrocytes and is not that awful

The serious form affects all tissues and has syptoms similar to transferase deficiency/classical galactosemia.

You get a buildup of UDP galactose which pushes back to lead to an increase in galactose 1 phosphate, again trapping the liver’s phosphate.

The treatment for this is a galactose restricted diet.

Question 27

Why is classical galactossemia treated with an elimination of galactose form the diet, while epimirase deficiency is treated with a reduction in diet galactose?

Answer 27

In transferase deficiency, any dietary galactose that comes in will result in phosphate trapping, but the body will still be able to synthesize some galactose by the interconversion of UDP glucose with epiminerase to form UDP galactose.

In epimerase deficiency, the body can’t do the comversion from UDP glucose to UDP galactose, so the only way you can get UDP galactose is thorugh DIETARY galactose. So you still need to eat some of it - just not soo much that it builds up and pushes back into galactose 1 phosphate.

Question 28

What percentage of the body’s glucose is metabolized throuh the pentose phosphate pathway?

Answer 28

10%

Question 29

What are the two main purposes of the pentose phosphate pathway?

Answer 29

generates NADPH to provide reducing power for a number of reactions in the body

Generation of 5 carbon sugars for the synthesis of nucleotides

Question 30

What is the reducing power of NADPH used for in the body?

Answer 30

Biosynthesis/detoxification:

fatty acid synthesis

cholesterol synthesis

nucleotide synthesis

oxidation of toxins by Cyt P450 enzymes

regeneration of reduced glutathione

Question 31

In what tissues is the pentose phosphate pathways very active?

Answer 31

liver

adipose

lactating mammary gland

adrenal cortex

RBCs

Question 32

What are the two phases of the PPP?

Which phase is reversible?

Answer 32

The oxidative phase (converts glucose to ribulose 5 phosphate and generates NADPH - NOT reversible)

THe regenerative phase: (converts ribulos 5 phosphate to the glycolysis intermediates fructose 6 phosphate and glyceraldehyde 3 phosphate. This IS reveresible - the glycolysis intermediates can then be used to regenerate 5 carbon sugars for nucleotide synthesis.

Question 33

What are the two key enzymes of the regenerative phase in the PPP?

Answer 33

transketolase

transaldolase

Question 34

What is the key metabolic regulator of the PPP?

Answer 34

xylulose 5 phosphate

Question 35

What are the three steps of the oxidative phase of the PPP?

Answer 35

1. glucose 6 phosphate is acted on by G6P dehydrogenase to give 6-phosphoglucano delta-lactone (reducing NADP to NADPH)
2. 6 phosphoglucano delta-lactone is acted on by gluconolactonase to form the linear version = 6 phosphogluconate
3. 6 phosphogluconate is aced on by 6-phosphogluconate dehydrogenase to oxidize and decarboxylate it to form ribulose 5 phosphate (a 5 carbon sugar with a phosphate group). This reduces another NADP to NADPH

So you start with a 6 carbon sugar, you get a a 5 carbon sugar and two reduced NADPH.

Question 36

WHat comprises glutathione?

What is glutathione important for?

What form does glutathione need to be in in order to carry out this function?

What is required to keep glutathione in that form?

Answer 36

Glutathione is a tripeptide of glutamic acid, cytsein and glycine (with glutamic acid bound in an atypical peptide bond through glutamid acid’s side chain and cystein’s amine)

It’s role is to neutralize free radicals sot hey do not damage the cell. It especially takes care of hydrogen peroxide

glutathione can either be dimerized through a disulfide bond GS-SG, or in the reduced, unbound form or GSH.

In order to neutralize free radicals, it needs to be able to accept electrons so the unbound, reduced GSH is required.

NADPH’s reducing power is used to maintain glutathione in a reduced state so that it can remove hydrogen peroside from the cell

Question 37

How is bound GS-SG reduced to 2 GSH?

Answer 37

Through the enzyme glutathione reductase, which uses NADPH as a cofactor

Question 38

Through what reaction does GSH take care of H2O2?

WHy is this so important for the body?

Answer 38

Through glutahione peroxidase.

This is very important because hydrogen peroxide can dissociate to form the hydroxyl radical, which is the worse one to have in the cell in terms of its ability to cause damage to DNA and proteins

Question 39

What results from glucose 6 phosphate dehydrogenae deficiency?

When does it manifest?

What are the symptoms?

Answer 39

With G6P dehydrogenase deficiency (favism), you do not have a functional PPP, thus you don’t generate NADPH for biosynthesis, detoxification, or oxidative protection.

THe enzyme is not completely inactive, so patients will still be able to produce NADPH to maintain GSH in the reduced form to some extent, but if they are suddenly put in conditions of high oxodative stress, they will not be able to keep enough GSH int he reduced form and the oxidative stress on their RBCs in particular results in hemolysis and hemolytic anemia.

Men with this condition are typically asymptomatic until they are put on anything that will cause high oxidative stress, such as xenobiotics, sulfa drugs, fava beens, nalidixic acid, TNT, primaquine, methylene blue

Question 40

What are the functions of transketolase and transaldolase?

Answer 40

transketolase transfers two carbon groups.

transaldolase transfers 3 carbon groups.

THe goal of these reactions is to allow the 5 carbon sugar to be turned into glcolysis intermediates in situations when nucleotide synthesis not requires (needs to turn the 5 C into either a 6C or 3C molecule)

WHen nucleotide synthesis is required, the reverse reactions convert the glycolysis intermediates into fie carbon sugars.

Question 41

What two things can happen to rubulose 5 phosphate after the oxidative phase of the PPP?

Answer 41

It can either be isomerized into ribose 5-phosphate for nucleotide synhtesis

or it can be epimerized to xylulose 5 phosphate, which is a transcriptional regulator

Question 42

What are the two fates of xyluluose 5 phsophate?

Answer 42

It can either be acted on by transaldolase and transketolase for glycolytic intermediates. OR it can act as a transcription regulator.

Question 43

How does X5P act as a transcripition regulator?

Answer 43

It acts indirectly on the charbohydrate response element binding protein (ChREBP), which is a transcription factor

ChREBP is inhibited by phosphorylation through PKA and AMPK and activated by by Phosphatase 2A, which removes the phosphate groups.

X5P allosterically activates PP2A.

Once ChREBP is dephosphorylated, it will translocate ot the nucleus and influence gene transcription.

Specifically, it turns on transcription of genes that are involved in the storage of sugar as fat.

Question 44

WHat genes are upregulated by ChREBP?

Answer 44

genes involved in the storage of sugar as fats in the body

pyruvate kinase

malic enzyme

citrate lyase

acetyl CoA carboxylase

fatty acid synthase