Carbohydrate Metabolism: Pentose Phosphate Pathway Flashcards
What is the Pentose Phosphate Pathway (PPP) also known as?
The Phosphogluconate Pathway
The Hexose Monophosphate Shunt/Pathway
Where does the PPP occur?
Cytosol
PPP is the oxidation of _____
Oxidation of Glucose-6-P
What is the electron acceptor in PPP?
NADP+
What phases are included in the PPP?
Oxidative
Non oxidative
How is flux controlled in the PPP?
Dependent on
- If NADPH is required
- If nucleotide pool needs to be replenished
- If ATP levels are low
What is Ribose-5-phosphate (R5P) and where is it utilized in PPP?
R5P is a biosynthetic precursor of nucleotides needed for
- DNA and RNA synthesis
- Synthesis of ATP, NAD+, FAD, Coenzyme A, etc.
Who are the main users of R5P?
Rapidly dividing cells
- Such as bone marrow, skin, intestinal mucosa, tumors
Where is NADPH utilized in PPP?
Tissues with active reductive biosynthesis
- Liver, adipose, lactating mammary glands (fatty acid synthesis)
- Liver, adrenal glands, gonads (cholesterol and steroid hormones synthesis)
Tissues that need to counter damaging effects of oxygen radicals
- Erythrocytes, Cells of the lens and cornea: directly exposed to O2
Describe the oxidative phase of the PPP.
Converts 6 molecules of glucose-6-P to 6 molecules of ribulose-5-P, resulting in the production of 2 NADPH and 1 CO2
The enzyme Glucose-6-phosphate dehydrogenase catalyzes an irreversible reaction that represents the commitment step in the pathway.
What is the commitment step in the oxidative phase of PPP?
When Glucose-6 phosphate dehydrogenase catalyzes the irreversible reaction of Glucose-6-P to 6-Phosphogluconolactone using up a NADP+
Describe the initial steps for the non-oxidative phase of PPP.
AKA Carbon Shuffle Reactions
6 Ribulose-5-P are converted into 4 F6P and 2 GAP using carbon shuffle reactions
Some of the carbon shuffle reactions and enzymes used here are similar to those seen in the Regeneration stage of the Calvin Cycle.
What is the net outcome of the initial steps of the non-oxidative phase of PPP?
6 Ru-5-P generated in the oxidative phase are converted to 4 molecules of F6P and 2 molecules of GAP (G3P)
What does Transketolases do in the non-oxidative phase of PPP?
Transketolases catalyze transfer of a 2C group from a donor to an acceptor
What does Transaldolases do in the non-oxidative phase of PPP?
Transaldolases catalyze transfer of a 3C group from a donor to acceptor
What are specific to the non-oxidative phase of PPP?
Ribose-5-P isomerase, Ribulose-5-P epimerase, Transketolase and Transaldolase
Desribe the final steps for the non-oxidative phase of PPP.
G6P is regenerated from F6P and GAP by enzymes in the glycolytic and/or gluconeogenic pathway.
Overall, the non-oxidative phase of the PPP converts 6 molecules of ribulose-5-P to 5 molecules of glucose-6-P
6x5 = 30 molecules can be made, 5 glucose can be made.
REMEMBER REACTION!!! WILL BE ON TEST
What enzymes of the non-oxidative phase are components of glycolytic pathway and the gluconeogenic pathway?
Phosphoglycoisomerase
Triose phosphate isomerase
Aldolase
What is unique to the gluconeogenic pathway?
Fructose-1,6-bisphosphate-1 (FBPase-1)
Which part of the PPP is favorable?
Oxidative part:
deltaG’* «_space;0
VERY favorable
Non-oxidative part:
deltaG’* =~ 0 (approx. 0)
What is the rate limiting step in the PPP?
G6PDH
Enzyme that limits the rate
How is G6PDH activity regulated?
G6DPH activity is allosterically regulated by [NADP+]-to-[NADPH] ratio in the cell
Allosteric activator = NADP+, there is high amounts of NADP+
Allosteric inactivator = NADPH, there is high amounts of NADPH
What happens if there is excess amounts of NADPH?
If there is an excess amount of NADPH, you shouldn’t be doing PPP and instead be doing glycolysis creating ATP.
NADPH is used in _______ as well as _______ pathways
used in biosynthetic as well as detoxification pathways
