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Chapter 14b (Summary)
Takusagawa’s note
Summary of Chapter 14b
1. Gluconeogenesis
- is the biosynthesis of glucose from non-carbohydrate precursors at liver and kidney (minor).
- Glycogen stored in liver is only a half day supply of glucose to brain which uses only glucose as fuel.
- Initially, glycolysis products (pyruvate & lactate), citric acid cycle intermediates, and carbon skeletons of most
amino acids are converted to oxaloacetate.
- Gluconeogenesis uses the reversed glycolysis pathway, except for 3 steps which have large positive ∆G.
Those and alternated pathways are:
1. “Pyruvate + ATP → PEP + ADP” by pyruvate kinase is replaced with:
-
HCO3 + ATP
ADP + P i
GTP
GDP + CO2
Pyruvate
Oxaloacetate
Phosphoenol pyruvate (PEP)
Pyruvate carboxylase
PEP carboxykinase
2. “FBP + ADP → F6P + ATP” by phosphofructokinase is replaced with:
H2 O
Pi
Fructose-1,6-bisphosphate
Fructose-6-phosphate
Fructose bisphosphatase
3. “G6P + ADP → Glucose + ATP” by hexokinase is replaced with:
Pi
H2O
Glucose-6-phosphate
Glucose
Glucose-6-phosphatase
2. As shown above, gluconeogenesis is accomplished by avoiding three energetically unfavorable reverse
reactions and by expensing hydrolysis of 4ATP and 2GTP per glucose.
Glucose + 2NAD+ + 2ADP + 2Pi
→ 2(Pyruvate) + 2NADH + 4H+ + 2ATP + 2H2O
Glucose + 2NAD+ + 4ADP + 6Pi + 2GDP ← 2(Pyruvate) + 2NADH + 4H+ + 4ATP + 6H2O + 2GTP
Overall (glycolysis & gluconeogenesis): 2ADP + 4Pi + 2GDP ↔ 2ATP + 4H2O + 2GTP
3. Pyruvate carboxylase has a biotin prosthetic group which is covalently bound to the enzyme by amide linkage
between its -COO- and H3N+-(CH2)4- of Lys residue. Biotin is at the end of 14 Å long flexible arm. This is
quite similar to the lipoic acid attached to Lys residue in pyruvate dehydrogenase multienzyme complex.
- Biotin is a coenzyme and CO2 carrier, and is an essential human nutrient.
4. Acetyl-CoA is a powerful allosteric activator of pyruvate carboxylase since Acetyl-CoA requires oxaloacetate
to continue the citric acid cycle pathway.
5. Gluconeogenesis only occurs when the citric acid cycle is inhibited by excess of ATP and/or NADH.
6. PEP in mitochondrion is transported through the specific transport protein whereas oxaloacetate is transported
by malate-aspartate shuttle since oxaloacetate does not have specific transport proteins.
- Oxaloacetate is reduced to malate by mitochondrial NADH. The transported malate reduces the cytosolic
NAD+ to NADH and becomes oxaloacetate. The produced NADH is utilized at 1,3-BPG → GAP.
7. Regulation of gluconeogenesis
- Glycolysis and gluconeogenesis pathways are reciprocally regulated at three independent pathways:
1. Glucose ↔ G6P [HK / glucose-6-phosphatase]
2. F6P ↔ FBP [PFK / FBPase]
3. PEP ↔ Pyruvate [PK / pyruvate carboxylase-PEPCK]
- Dominant mechanisms are:
1. Allosteric regulations
- Fructose-2,6-bisphosphate (F2,6P) is a potent activator of glycolysis (activate PFK, inhibit FBPase).
- F2,6P is synthesized and hydrolyzed by two enzyme activities (PFK-2 and FBPase-2).
F6P → F2,6P → F6P
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Chapter 14b (Summary)
Takusagawa’s note
PFK-2
FBPase-2
2. cAMP-dependent covalent modifications (initiated by glucagon and other hormonal activity).
- Phosphorylation of liver {PFK-2/FBPase-2} by cAPK inhibits PFK-2 and activates FBPase-2 activities.
- Thus, phosphorylation of {PFK-2/FBPase-2} by cAPK decreases PFK activity and increases FBPase
activity, i.e., decreases glycolysis and increase gluconeogenesis in liver.
Note: Heart {PFK-2/FBPase-2} responds oppositely. However, gluconeogenesis takes place in
liver.
- Liver pyruvate kinase (PK) is inhibited allosterically by Ala (precursor of pyruvate), and inactivated
by phosphorylation.
8. Cori cycle --- In muscle {glucose → pyruvate → lactate} via blood to liver {lactate → pyruvate →
glucose} via blood to muscle {glucose → pyruvate → lactate}. This cycle process looks ATP/GTP
consuming “futile cycle”, but does not take place in the same cell.
9. Glyoxylate pathway (Only plants)
- Acetyl-CoA cannot enter gluconeogenesis pathway in animals, but in plants, Acetyl-CoA can enter
gluconeogenesis via glyoxylate pathway. Glyoxylate + acetyl-CoA → malate, catalyzed by malate synthase.
Glyoxysome
Mitochondrion
Aspartate
Oxaloacetate
Oxaloacetate
Aspartate
C.A.C
+ Acetyl-CoA
Succinate
Isocitrate
Glyoxylate
Glucose
+ Acetyl-CoA
Gluconeogenesis
Oxaloacetate
Malate
Malate
Citric acid cycle:
Acetyl-CoA → 2CO2 + 2NADH + GTP → Oxidative phosphorylation → ATP
- Glyoxylate pathway: Acetyl-CoA → Glyoxylate → Gluconeogenesis → Glucose
10. Biosysnthesis of oligosaccharides and glycoproteins
- Glycosyl donors are nucleotide-sugars such as UDP-glucose, and oligosaccharide synthses are catalyzed by
glycosyl transferases.
- Glycoproteins are classified into three groups.
1. N-linked oligosaccharides.
- The oligosaccharides are constructed on dolichol in the lumen of the rough endoplasmic reticulum.
- A common 14-residue core oligosaccharide is transfered to the newly synthesized polypeptide at an Asn.
2. O-linked oligosaccharides.
- A monosaccharide is attached to Ser or Thr with a-O-glycosidic bond mostly, and then additional
oligosaccharides are attached one by one on a completed polypeptide chain in Golgi apparatus.
3. GPI-linked proteins.
- GPI core is synthesized in ER from phosphatidylinositol, UDP-GlcNAc and Dolichol-P-mannose.
11. Pentose phosphate pathway
- produces two important biomolecules (NADPH and ribose-5-phosphate (R5P)) from G6P.
- ~30% of glucose oxidation in liver occurs via the pentose phosphate pathway.
- Although NADH and NADPH are chemically similar, those are not metabolically interchangeable.
- NADPH is the reducing power currency in cell. For example, H2O2 and R-O-O-H produced in cell are very
toxic compounds, and thus must be reduced to H2O and R-OH, respectively. Glutathione (GSH) is mainly
used as reducing agent in cell. The oxidized glutathione (GSSG) is subsequently regenerated by NADPH.
- In cell, [NAD+]/[NADH] ≈ 1000 vs. [NADP+]/[NADPH] ≈ 0.01. What does this mean?
- Pentose phosphate pathway has three stages:
1. 3G6P + 6NADP+ + 3H2O → 3Ru5P + 6NADPH + 6H+ + 3CO2
2. 3Ru5P → R5P + 2Xu5P
3. R5P + 2Xu5P → 2F6P + GAP
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Chapter 14b (Summary)
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Takusagawa’s note
Overall reaction: 3G6P +6NADP + 3H2O → 2F6P + GAP + 6NADPH + 6H+ + 3CO2
Pentoses cannot directly enter the glycolysis, gluconeogenesis or pentose phosphate pathway, thus they are
converted to hexoses (F6P) and triose (GAP). Two enzymes are involved.
1. Transketolase that adds and/or subtracts C2 units: C5 + C5 ↔ C3 + C7
2. Transaldolase that adds and/or subtracts C3 units: C3 + C7 ↔ C6 + C4
Transketolase has a TPP co-factor which carries the C2-unit from one sugar to another sugar.
Transaldolase forms a Schiff base with the substrates, which carries the C3-unit from one sugar to another
sugar.
One G6P can produce 12 NADPH by 6 cycles of pentose phosphate pathway and gluconeogenesis.
Pentose phosphate pathway (NADPH production) is controlled by the rate of glucose-6-phosphate
dehydrogenase (first committed step with ∆G = -17.6 kJ/mol in liver).
+
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