Lecture 26 Flashcards
Regulation of PPP:
• Flux of pathway depends on the need of the cells.
- Examples:
- When NADPH is being used NADP+ drives the oxidative phase
- When ribose is not needed carbons are diverted to glycolysis.
Note that pentoses can be made even without running the oxidative phase of PPP.
(Ribose-5-phosphate is a precursor for Nucleotides needed to make DNA and RNA.)
Importance of PPP for detoxification
The cellular weapon against damaging reactive oxygen species (e.g. peroxides) is glutathione:
NADPH generated in PPP is used to regenerate (reduce) glutathione.
People with mutations that decrease Glc-6-P-DH activity are hypersensitive to oxidative stress.
Glucose-6-Phosphate Dehydrogenase Deficiency
- 400 million people worldwide have G6PDH deficiency
- They are more resistant than normal people to malaria.
- But they also are more sensitive to oxidizing agents because they can’t make enough NADPH to keep Glutathione reduced.
- In erythrocytes reduced glutathione helps keep Fe in the Fe2+ oxidation state.
- Fe3+ hemoglobin does not bind O2 and it causes erythrocytes to change shape.
- Erythrocytes get broken down - causing anemia.
Gluconeogenesis why
To provide glucose for brain (120g/day) and erythrocytes (red blood cells, 160 g/day).
To convert lactate back into glucose during exercise.
Gluconeogenesis where
In liver , NOT in muscle, fat or brain
Mostly in cytosol
Gluconeogenesis when
starvation - (precursor- amino acids)
exercuse (precursor- lactate)
Anabolic pathway, where does the energy come from?
From fatty acid oxidation - Here we are burning fat!
Fatty acids are NOT a precursor for glucose because we can NOT convert acetyl CoA into pyruvate. Plants can do it!
Gluconeogenesis summary
2pyruvate +4ATP +2GTP +2NADH +2H+ +6H2O —->
glucose +4ADP +2GDP +6Pi +2NAD+
ΔG numbers in glycolysis*
hexokinase
phosphofructokinase
pyruvate kinase
h=-27.2
pfk= -25.9
pk= -13.9
*ΔG, NOT ΔG0
From pyruvate to PEP: Two enzymes and a shuttle
Pyruvate Carboxylase uses Biotin (Vitamin B7)
Required activator:Acetyl-CoA
pyruvate +pyruvate carboxylase –> oxaloacetate
oxaloacetate –(PEPCK)–> phosphoenol-pyruvate (PEP)
Required activator: Acetyl-CoA
how is oxaloacetate shuttled from mitochondrial matrix into cytoplasm
Oxaloacetate is shuttled as malate from the mitochondrial matrix into cytoplasm.
This also shuttles reducing power to the cytoplasm.
two ATP equivalents
what is used to reverse the pyruvate kinase step?
Regulation of glycolysis and gluconeogenesis in muscle and liver
It is important to avoid “Futile Cycles”
that just hydrolyze ATP without accomplishing
anything.
- Phosphofructokinase 1
- Phosphofructokinase 2
- Pyruvate Kinase
Fructose bis phosphatase-1 converts
F1,6BP into F6P
Activation of PFK-1 by F-2,6BP
F-2,6BP is an mportant allosteric effector in liver that shifts the inactive state to the active state for PFK-1.
F-2,6BP is made in liver cells only when blood sugar is high.
Note that Vmax increases (a little) and Km decreases for the F-6-P substrate.
regulation steps
- Phosphofructokinase-1 and Fructose -1,6 bisphosphatase
- Phosphofructokinase-2 and Fructose-2,6 bisphosphatase
- Allosteric regulation of PFK-2 / FBPase-2
- Hormonal regulation of PFK-2 / FBPase
- Pyruvate kinase
- 1 allosteric regulation
- 2 hormonal regulation of pyruvate kinase (only in liver)
- Phosphofructokinase-2 and Fructose-2,6 bisphosphatase
PFK-2 and FBPase-2 are unique in that both catalytic domains reside on one polypeptide chain (bifunctional enzyme).
2.1. Allosteric regulation of PFK-2 / FBPase-2
Consider PFK-2 a sensor for Fructose-6-phosphate
In summary, this mechanism is a feed forward activation of PFK-1 by its substrate, fructose-6-phosphate.
What other enzyme in glycolysis is regulated by feed forward activation?
2.2. Hormonal regulation of PFK-2 / FBPase
(only in liver – muscle PFK-2 has no phosphorylation site)
Why are both activities, PFK-2 and FBPase-2, located on one protein?
Phosphorylation can now function like a switch:
ON = phosphatase OFF = kinase
summary
low blood glucose increased glucagon secretion increased [cAMP] increased enzyme phosporylation activation of FBPase-2 and inactivation of PKF-2 decreased [F2,6P] inhibition of PFK and activation of FBPase increased gluconeogenesis
what is a precursor for glucose in gluconeogenesis
alanine
what is made in the liver and released into blood
Glucose made in the liver by gluconeogenesis is released into the blood by Glucose-6-Phosphatase
why can only the liver make glucose
because only the liver has Glucose-6-Phosphatase
what released glucose made in the liver
Glucose-6-Phosphatase releases glucose made in the liver either from Glycogen Breakdown or from Gluconeogenesis.
what has a high km and why
Hexokinase in the liver has a very high Km so it won’t rephosphorylate the glucose.
The Cori-Cycle
- When MUSCLES are very active they cannot oxidize pyruvate as fast as they make it by glycolysis.
- They release LACTATE.
- The LIVER “recycles” the LACTATE to GLUCOSE to support more glycolysis in the muscle.
what supplies carbons for gluconeogenesis
pyruvate and amino acids
what provides energy for gluconeogenesis
fatty acids
carbons from acetyl-CoA ____ be used to increase carbohydrates
cannot
Key Concepts for Gluconeogenesis
- Occurs primarily in liver (and kidney)
- Provides glucose under starvation conditions.
- Carbons come from lactate and amino acids, ATP comes from fat oxidation.
• Exergonic reactions from glycolysis require specific types of reactions to be reversed.
– Pyruvate carboxylase, PEPCK, F1,6 Bisphosphatase, Glucose-6-Phosphatase
– Regulation PFK2, PK regulation
• CoriCycle
