PPP

Question 1

What pathways can Glucose-6-phosphate enter?

Answer 1

• Pentose phosphate pathway → Ribose-5-phosphate
• Glycolysis // Gluconeogenesis
• Glycogen synthesis // Glycogen breakdown

Question 2

What is the committing enzyme of the PPP?
What is that reaction?

Answer 2

Glucose-6-phosphate dehydrogenase (G6PDH)

G6P → {G6PDH} → Ru5P + CO2 + NADPH

Question 3

What are the outputs of the PPP?

Where is the PPP most present?

Answer 3

• Ribose-5-Phosphate
• NADPH (not NADH!!!)

Pathway mostly present in:
- Liver (mostly!!, ~30% of G6P)
- Adipose tissue
- Red blood cells
*largely absent in muscle

Question 4

What are the differences between NAD and NADPH?
Generated from…
Metabolic uses
[Cytosolic ratio]

Answer 4

*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

Question 5

What metabolic process requires NADPH? Briefly, How?

Answer 5

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

Question 6

What is Ribose-5-phosphate a precursor for?

Answer 6

End-product of PPP, but precursor/main substrate of Nucleotide Biosynthesis → ATP, TTP, GTP, CTP

Question 7

Do cancer cells activate or inhibit PPP?

Answer 7

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

Question 8

Why is NADPH required for red blood cells?

Answer 8

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)

Question 9

What is the consequence of a defficiency in G6PDH for RBC?

Answer 9

Defficiency in G6PDH → Low NADPH → lesser ability to keep GSH high → can’t handle extra oxidants → damage to cell membranes → cell lysis

Question 10

What is the rate limiting / committing step of PPP?
(what is it regulated by?)

Answer 10

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

Question 11

What is the stoechiometric equation of the oxidative reaction of the Pentose Phosphate Pathway?

Answer 11

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

Question 12

A loss-of-function mutation in which enzymes would cause fructosemia (fructose intolerance)?

Answer 12

Fructokinase
Fructose-1-Phosphate Aldolase
*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

Question 13

Why is fructose stored as fatty acids in fed state?

Answer 13

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

Question 14

What determines the fate of G6P?

Answer 14

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

Question 15

What does an increase in bilirubin in the blood mean when trying to diagnose a patient?

Answer 15

Accumulation of bilirubin comes from Liver issue

Question 16

What can explain a low Hb concentration in the blood or a decreased cells count?

Answer 16

G6PDH deficiency → low NADPH → more cell lysis

Question 17

What is the definition of Gluconeogenesis?
Where/when does it occur?

Answer 17

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)

Question 18

What are different substrates of Gluconeogenesis?

Answer 18

• 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

Question 19

What is the starting point of gluconeogenesis?

Answer 19

Oxaloacetate (an intermediate of CAC)

Question 20

What are the 4 enzymes required for the irreversible steps of gluconeogenesis?

Answer 20

1. Pyruvate carboxylase
2. PEPCK (phosphoenolpyruvate carboxykinase)
3. FBPase
4. Glucose-6-phosphatase

*The reversible steps are shared with glycolysis

Question 21

When starting with pyruvate, what is the first step of gluconeogenesis?

Answer 21

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

Question 22

Explain the 2nd irreversible step of gluconeogenesis.
What is it regulated by?

Answer 22

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

Question 23

How does hormonal regulation of PFK2 influence gluconeogenesis?

Answer 23

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

Question 24

Where/When does the Cori cycle occur?

Answer 24

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

Question 25

What reactions are involved in the Cori Cycle?

Answer 25

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