Pentose Phosphate Pathway Flashcards
What is the purpose of the PPP?
The PPP can provide in cytosol NADPH and pentose phosphates.
Mostly ribose 5-P is required for nucleotide metabolism
Where in the cell does it take place? Where in the body does it take place? Which cells have a very low activity of glucose 6-P dehydrogenase?
The PPP takes place only in the cytosol.
This pathway is very active in the liver, it provides NADPH for fatty acid de-novo synthesis, cholesterol synthesis and cytochrome P450 and other enzymes, like aldose reductase as example.
In the adrenal cortex, testes and ovaries, NADPH is needed for steroid hormone synthesis.
In RBC it provides NADPH for glutathione reductase which generates 2 reduced glutathiones (GSHs) from oxidized glutathione (GSSG).
In macrophages NADPH is needed for the oxidative burst that generates on purpose superoxide catalyzed by NADPH oxidase.
On the other hand, muscle cells have a very low activity of glucose 6-P dehydrogenase.
What is special about this pathway?
The PPP has an oxidative irreversible part starting with glucose 6-P which generates two NADPH and finishes with an oxidative decarboxylation.
This is followed by a reversible part using pentose phosphates which can be used to form ribose 5-P. In cells that want both, NADPH and ribose 5-P, the PPP ends here.
If ribose 5-P is not wanted, then the PPP uses totally 3 glucose 6-P and forms eventually two fructose 6-P and one glyceraldehyde 3-P. The formed products can join glycolysis.
If NADPH is not wanted and the high cellular NADPH levels inhibit the oxidative part , then ribose 5-P is formed branching out from glycolysis using the reversible part.
Is there a regulated step and how is this step regulated?
The PPP is regulated at the level of glucose 6-P dehydrogenase. High NADPH levels inhibit this enzyme and high NADP+ levels activate the oxidative part.
What is normally high in cytosol, NADPH or NADP+? Compare it to the ratio of NAD+/NADH.
As a general rule, the ratio of NADPH/NADP+ is normally kept high, and when NADPH is used, then the PPP regenerates NADPH from NADP+.
In contrary, the NAD+/NADH ratio is normally kept high, and when NAD+ is used, then several reactions regenerate NAD+ from NADH.
[note: NADPH is used for reductive synthesis or specific enzymes, whereas NAD+ is used for oxidative degradation or other reactions.]
The PPP contains a decarboxylation step. At which part of the pathway is it performed?
At the second step of NADPH formation in the oxidative part catalyzed by 6-phosphogluconate dehydrogenase, oxidative decarboxylation takes place and ribulose 5-P is formed.
[note: the pathway is named HMP following the fact that the oxidative part starts with a hexose monophosphate (glucose 6-P).
The pathway is also named PPP following the fact that after decarboxylation at the end of the irreversible part, pentose phosphates are formed and also that the synthesis of ribose 5-P can be performed without the oxidative part.]
What is the action of transketolase and what is the action of transaldolase in the PPP? What is special about the substrates?
Both enzymes always need two substrates.
Transketolase transfers 2-carbon units from xylulose 5-P to the other substrate, this enzyme requires thiamine-PP from vitamin B1 for this reaction.
Transketolase is used two times in the reversible part of the PPP.
Transaldolase transfers 3-carbon units from one of its substrates to the other substrate.
Describe the concept and the reactions of the reversible part of the PPP that follows the oxidative part.
The concept is that if no ribose 5-P is wanted then molecules shall be formed that can join glycolysis. In that case, totally three glucose 6-P are used, 6 NADPH and three carbon dioxides are generated and two fructose 6-P and one glyceraldedyde 3-P join glycolysis.
The first pentose phosphate formed after the oxidative part is ribulose 5-P which can be changed to ribose 5-P.
[more in detail:
If ribose 5-P is not wanted for nucleotide metabolism, then another glucose 6-P is needed to form another ribulose 5-P. This time the ribulose 5-P is changed to xylulose 5-P instead of to ribulose 5-P.
Now Ribose 5-P and xylulose 5-P are present and can be used as substrates for transketolase which forms a 7 carbon-P and 3 carbon-P.
The 7 carbon-P is sedoheptulose 7-P which is normally not wanted in the cell and shall not accumulate. This is why both products of transketolase are now used as substrates for transaldolase which forms fructose 6-P and a 4 carbon-P.
Fructose 6-P joins glycolysis but the 4 carbon-P which is erythrose 4-P, needs to be taken care of and is substrate for transketolase which needs another xylulose 5-P. The third glucose 6-P is used to form this xylulose 5-P.
Another Fructose 6-P and one glyceraldehyde 3-P are formed which join glycolysis.]
How do cells generate ribose 5-P at high NADPH levels?
High NADPH levels inhibit the oxidative part at the enzyme glucose 6-P dehydrogenase.
In this case, intermediates of glycolysis are used in the reversible part to form ribose 5-P. Glucose 6-P and glyceraldehyde 3-P are used.
Which enzyme of the PPP needs thiamine-PP as coenzyme? How can deficiency of vitamin B1 be tested?
Transketolase needs TPP as coenzyme.
Transketolase activity in RBC can be tested for possible vitamin B1 deficiency.
[TPP is also coenzyme in the PDH and TCA cycle and also needed for eventual degradation of branched-chain amino acids]
What is generated in a cell that needs NADPH and also ribose 5-P?
How many molecules of glucose 6-P are used in the complete PPP in RBC? How many molecules of NADPH are generated?
Cells that only want NADPH and ribose 5-P stop the PPP after the formation of ribose 5-P, they only used one glucose 6-P and form 2 NADPH.
Three glucose 6-phosphates are needed in the complete PPP in RBC.
Ribose 5-P is not wanted in these cells and 6 NADPHs are generated.
Which molecules can join glycolysis when the PPP is performed in RBC?
Eventually two fructose 6-P and one glyceraldehyde 3-P are formed which join the glycolysis pathway. (The substrate level phosphorylation steps can still be performed)
Why is NADPH needed for eventual detoxification of hydrogen peroxide?
NADPH is coenzyme (cosubstrate) for glutathione reductase which uses GSSG and forms 2 GSH.
Reduced glutathione is needed for glutathione peroxidase (selenium is cofactor) when it detoxifies hydrogen peroxide or also acts on organic peroxides.
What is the importance of NADPH related to phagocytosis?
NADPH and oxygen are needed for NADPH oxidase which forms on purpose superoxide. This is described as respiratory burst and superoxide is used to eventually destroy microorganisms.
Describe the reaction catalyzed by nitric oxide synthase.
Nitric oxide synthase uses NADPH, oxygen and arginine for synthesis of nitric oxide.
