PPP Flashcards
What pathways can Glucose-6-phosphate enter?
- Pentose phosphate pathway → Ribose-5-phosphate
- Glycolysis // Gluconeogenesis
- Glycogen synthesis // Glycogen breakdown
What is the committing enzyme of the PPP?
What is that reaction?
Glucose-6-phosphate dehydrogenase (G6PDH)
G6P → {G6PDH} → Ru5P + CO2 + NADPH
What are the outputs of the PPP?
What tissues is the PPP most present in?
- Ribose-5-Phosphate
- NADPH (not NADH!!!)
Pathway mostly present in:
- Liver (mostly!!, ~30% of G6P)
- Adipose tissue
- Red blood cells
*largely absent in muscle
What are the differences between NAD and NADPH?
Generated from…
Metabolic uses
[Cytosolic ratio]
*NOT interchangeable
Generated from:
NAD+/NADH → Glycolysis, CAC
NADP+/NADPH → PPP
Metabolic uses:
NAD+/NADH → ATP production (Ox.Phos), Oxidative reactions
NADP+/NADPH → Fatty acid & cholesterol biosynthesis, Reduction reaction
[Cytosolic] :
[NAD+]/[NADH] = 1000 → 1000x more NAD+ available for glycolysis
[NADP+]/[NADPH] = 0.01 → 100x more NADPH available
What metabolic process requires NADPH? Briefly, How?
Biosynthesis of lipids
Fatty acid synthesis → NADPH acts as electron donor mutliple times until get the molecule of interest
Cholesterol synthesis → also requires NADPH, but only once
What is Ribose-5-phosphate a precursor for?
End-product of PPP, but precursor/main substrate of Nucleotide Biosynthesis → ATP, TTP, GTP, CTP
Do cancer cells activate or inhibit PPP?
Cancer cells activate PPP to have more nucleotides for synthesis of DNA when replicating
Also need lots of NADPH for lipid synthesis to duplicate the membrane when dividing
Why is NADPH required for red blood cells?
cNADPH maintains GSH (glutathione peroxidase) and protexts against hemeolysis
RBC produce hydrogen peroxide (toxic product that reacts with fatty acid residues in cell membrane) → creates organic hydroperoxides
Organic hydroperoxides cleave fatty acid bonds → cell lysis
GSH prevents peroxide-mediated hemolysis of RBCs
2 GSH + ROOH (damaging compound) → {GSH} → GSSG + ROH + H2O
*GSSG → 2GSH done by NADPH (to NADP)
What is the consequence of a defficiency in G6PDH for RBC?
Defficiency in G6PDH → Low NADPH → lesser ability to keep GSH high → can’t handle extra oxidants → damage to cell membranes → cell lysis
How is the PPP regulated?
Step 1 = irreversible (all other steps are reversible) → glucose-6-phosphate dehydrogenase reaction
G6P + NADP+ → {glucose-6-phosphate dehydrogenase} → 6-phospho… + NADPH+H+
- Higly specific to NADP+ (purpose = NADPH production)
- STRONGLY INHIBITED by NADPH
What is the stoechiometric equation of the oxidative reaction of the Pentose Phosphate Pathway?
3 G6P + 6 NADP+ + 3 H2O → 6 NADPH + 6 H+ + 3 CO2 + 3 Ru5P
*In the non-oxidative step, Ru5P can become 2x Xu5P or 1x R5P
A loss-of-function mutation in which enzymes would cause fructosemia (fructose intolerance)?
Fructokinase
Fructose-1-Phosphate Aldolase (Aldolase B)
*Both involved in major digestion of fructose in the liver
A defficiency in Hexokinase and Fructose-1,6-Phosphate Aldolase would not cause fructosemia as they are part of the minor fructose digestion pathway in the muscles, which is only responsible for ~5% of fructose digestion
Why is fructose stored as fatty acids in fed state?
In fed state, the body has enough ATP → signals to stop Ox. Phos. → NADH accumulates → NADH is a strong inhibitor of the CAC at the Isocitrate → a-ketoglutarate step
Citrate will try to go into CAC, but will be blocked, will go back to citrate → go into cytoplasm → activate synthesis of fatty acids
*If you eat fructose in a starvation state, it will be digested to produce ATP, does not automatically → fatty acids
What determines the fate of G6P?
Depends on the activity of:
PFK → inhibited by ATP
G6PDH → inhivited by NADPH
→ Cellular need in ATP, NADPH and Ribose-5-phosphate dictate the fate of G6P
What does an increase in bilirubin in the blood mean when trying to diagnose a patient?
Accumulation of bilirubin comes from Liver issue
What can explain a low Hb concentration in the blood or a decreased cells count?
G6PDH deficiency → low NADPH → more cell lysis
What is the definition of Gluconeogenesis?
Where/when does it occur?
Biosynthesis of new glucose from non-carbohydrate sources
Occurs in the LIVER (major) and in kidney (minor)
Occurs when fasting:
- Dietary glucose is not available
- Exhaustion of intracellular glucose
- Glycogen pool in liver is depleted
- Blood glucose levels are about to go below normal (CNS depends on glucose)
What are different substrates of Gluconeogenesis?
- Lactate
- Pyruvate
- Intermediate of the CAC (coming from amino acid break down → CAC intermediates)
- Amino acids (muscles)
*NOT from fatty acids, fatty acids breakdown to provide the required energy for gluconeogenesis, but are not substrates to form glucose
What is the starting point of gluconeogenesis?
Oxaloacetate (an intermediate of CAC)
What are the 4 enzymes required for the irreversible steps of gluconeogenesis?
- Pyruvate carboxylase
- PEPCK (phosphoenolpyruvate carboxykinase)
- FBPase
- Glucose-6-phosphatase
*The reversible steps are shared with glycolysis
When starting with pyruvate, what is the first step of gluconeogenesis?
Pyruvate → {carboxylation adds CO2 to pyruvate, consumes 1 ATP} Oxaloacetate → {decarboxylation removes CO2, consumes 1 GTP} → PEP
*Have to add an extra step, can’t do Pyruvate → PEP because to much of an energy step
Requires:
- Pyruvate (from AA catabolism)
- Energy (ATP from fatty acids catabolism)
- Acetyl-CoA (from fatty acids catabolism) → allosteric activator of pyruvate carboxylase
→Oxaloacetate is a high energy intermediate
→ Exergonic decarboxylation of oxaloacetate provides free energy for PEP synthesis
Explain the 2nd/3rd irreversible step of gluconeogenesis.
What is it regulated by?
F1,6P + H2O → {F1,6Pase} → F6P + Pi
∆G = -9 kJ/mol
(compared to the reverse with PFK1- → ∆G = -26 kJ/mol)
This step is inhibited by AMP and F2,6P, so in their absence, the breaks are released
How does hormonal regulation of PFK2 influence gluconeogenesis?
ONLY the liver isoenzyme:
Fasting/prolonged exercise → lower blood glucose → release of glucagon by the pancreas → increase in [cAMP] → activates PKA → Kinase off/Phosphatase active → [F2,6P] decreases
Decrease in F2,6P releases the breaks on PFK-1 which favours PFBase for gluconeogenesis
Where/When does the Cori cycle occur?
In the muscles and in the Liver in parallel
*Glycolysis does not require O2, but is requires a fresh supply of NAD+ (that can be generated with the help of O2)
Cori cycle occurs, when the supply in oxygen is not sufficient to produce enough ATP
Cori Cycle = Homolactic fermentation
What reactions are involved in the Cori Cycle?
Pyruvate + NADH ⭤ {Lactate Dehydrogenase} ⭤ L-Lactate + NAD+
In the muscles:
Glucose/Glycogen → {glycogenolysis/glycolysis} → Lactate + ATP
In the liver:
Lactate + ATP + GTP → {gluconeogenesis} → Glucose + ADP + GDP + Pi
