1. Gluconeogenesis

2. Role of gluconeogenesis in metabolism
Synthesis of glucose from non carbohydrate sources. Requires (i) E from metabolism & (ii) source of carbons
Essential for maintaining blood glucose concentrations
Meets the bodys’ demands for glucose when carbohydrate stores are limited
e.g. fasting & starvation
Occurs primarily in the liver (90%) and < kidney (10%)
Liver and Kidney have G-6-phosphatase activity
Allows release of glucose into blood stream

3. Substrates for GNG Lactate (produced by anaerobic
glycolysis e.g. in RBC’s and exercising
skeletal muscle)
Glucogenic aminoacids
Glycerol
TCA intermediates

4. Lactate can act as a substrate for GNG
Lactate from exercising muscle diffuses into the blood stream
In the liver lactate is converted to pyruvate by lactate dehydrogenase
Produces NADH in the cytoplasm
NADH

5. Glycerol from breakdown of Triglycerides can act as substrate for GNG
ATP
DHAP

6. Some amino acids can acts as substrates for GNG
Amino acids can undergo transamination reactions - amino group transferred to -ketoglutarate
End product is pyruvate or TCA intermediates

7. Alanine, cysteine, glycine, serine, threonine pyruvate
Aspartate & asparganine
oxaloacetate
Phenylalanine & tyrosine
fumarate
Isoleucine, valine & methionine
Succinyl Co A
Arginine, glutamate, glutamine, histidine
a-ketoglutarate
Leucine, lysine, phenylalanine, tryptophan, tyrosine
Acetoacetate and
Acetyl Co A
TCA
No gluconeogenesis from fats

8. Pyruvate Glucose ’ aspartate & asparagine phenylalanine arginine,
& tyrosine
arginine,
glutamate,
glutamine,
histidine
isoleucine,
valine, &
methionine

9. Lactate & some
aminio acids
Some amino
acids
Glycerol

10. Gluconeogenesis is not the reversal of Glycolysis
2 pyruvate + 4 ATP + 2 GTP + 2 NADH + 2 H+ + 4 H20
 glucose + 4 ADP + 2 GDP + 2 NAD+ + 6 Pi
Glycolysis
Glucose + 2 ADP + 2 Pi + 2 NAD+
 pyruvate + 2 ATP + 2 H+ + 2 NADH + 2 H20

11. Bypass Reactions Pyruvate carboxylase & PEP carboxykinase
bypass pyruvate kinase step
Fructose 1,6 bisphosphatase bypasses
Phosphofructokinase step
3. Glucose 6 phosphatase bypasses
hexokinase step
These provide for a spontaneous pathway in
the direction of glucose synthesis
-∆G is in the direction of sugar synthesis

12. 1st bypass: Pyruvate is first converted to oxaloacetate
Carboxylation reaction requiring E
CO2 is added by Pyruvate carboxylase (mitochondrial enzyme)
Most enzymes for GNG are cytoplasmic – only exception
pyruvate + ATP + CO2 + H2O  oxaloacetate + ADP + Pi + 2H+

13. Oxaloacetate is shuttled into the cytosol and converted to (PEP)
Oxaloacetate is synthesised in the mitochondria
Oxaloacetate cannot diffuse out of the mitcohondria
Converted to Malate and shuttled into the cytoplasm
Uses a specific malate transport system

14. pyruvate
oxaloacetate
ATP
CO2
ADP + Pi
malate
NADH
NAD+
matrix
cytosol
phosphoenolpyruvate
PEP carboxykinase
Pyruvate
carboxylase
GTP
GDP
Malate
dehydrogenase
pyruvate(c)

15. 2nd bypass: Fructose 1,6 bisphosphatase bypasses phosphofructokinase step
F-6-P
F-1,6-BP Pi
Fructose 1, 6,
bisphosphatase
Phosphofructose
kinase

16. 3rd bypass: Glucose 6 phosphatase bypasses hexokinase step
Glucose-6-P + H20  glucose + Pi
Blood stream
glucose Pi
Glucose-6-
phosphatase
Hexokinase
G-6-P

17. 3rd bypass: Glucose 6 phosphatase bypasses the hexokinase step
G-6-Pase is primarily an enzyme of liver (and kidneys)
In hepatocytes the glucose-6-phosphatase reactions allows the liver to supply the blood with free glucose
Muscle cells lack G-6-Pase and direct G-6-P to glycogen synthesis

18. G-6-P
glucose Pi
G-6-Pase
Endoplasmic
reticulum
G-6-Pase is located on the membrane of the ER
Hydrolysis of G-6-P releases glucose into the
lumen of the ER
Glucose is packaged into vesicles for transport

19. Reciprocal regulation of GNG and glycolysis
Gluconeogenesis expends 6 P bonds of ATP and GTP
2 pyruvate + 4 ATP + 2 GTP + 2 NADH + 2 H+ + 4 H20
 glucose + 4 ADP + 2 GDP + 2 NAD+ + 6 Pi
Glycolysis yields 2 P bonds of ATP
Glucose + 2 ADP + 2 Pi + 2 NAD+
 pyruvate + 2 ATP + 2 H+ + 2 NADH + 2 H20
If two pathways runs concurrently becomes a futile cycle; must be regulated

20. Reciprocal regulation of GNG and glycolysis cont’d.
When gluconeogenesis is on, glycolysis should be off
When energy stores are high, glycolysis should be off
When energy stores are low, glucose should be rapidly degraded to provide energy
Regulation occurs at the sites of the irreversible reactions

21. Irreversible reactions provide regulation-1
Glucose-6-P + H2O  glucose + Pi
G-6-P inhibits hexokinase (glycolysis)
G-6-phosphatase activity (gluconeogenesis) is dependant on [G-6-P]

22. Irreversible reactions provide regulation – 2 cont’d
F-6-P to F-1-6-BP : Phosphofructokinase (glycolysis)
Enzyme Inhibited by ATP, citrate
Stimulated by AMP
F-1,6-BP to F-6-P : fructose 1,6-bisphosphatase (gluconeogenesis)
Inhibited by AMP and stimulated by citrate.

23. Irreversible reactions provide regulation - 2
Fructose 2,6-bisphosphate reciprocally controls
these two enzymes
levels are controlled by glucagon and insulin
levels are low during starvation – stimulates GNG
levels are high during the fed state accelerates glycolysis.

24. Regulation of gluconeogenesis
G-6-P
Glucose enters liver after meal + -
F-2,6-BP
F-6-P
PFK-2 P
F-1,6-BPase
Gluconeogenesis is inhibited X
Insulin mediates dephosphorylation of PFK-2
PFK-1
F-1,6-BP
PEP

25. Irreversible reactions provide regulation - 3
PEP to pyruvate : pyruvate kinase (Glycolysis)
Enzyme inhibited by acetyl-CoA, ATP and alanine,
signals that energy levels are high.
Also controlled by phosphorylation by glucagon and insulin
pyruvate kinase is inhibited during starvation

26. Irreversible reactions provide regulation – 3 cont’d
Pyruavte to oxaloacetate: pyruavte carboxlyase (gluconeogeneis)
stimulated by acetyl-CoA and inhibited by ADP
more gluconeogenesis when energy levels are high.
Phosphoenolpyruvate carboxykinase is similarly inhibited by ADP levels.