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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
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• Lactate (produced by anaerobic glycolysis e.g. in RBC’s and exercising skeletal muscle)
• Glucogenic aminoacids
• Glycerol
• TCA intermediates
Substrates for GNG
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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
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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
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Alanine, cysteine, glycine, serine, threonine
pyruvate
Aspartate & asparganine
oxaloacetate
Phenylalanine & tyrosine
fumarate
Isoleucine, valine & methionine
Succinyl Co A
Arginine, glutamate, glutamine, histidine
-ketoglutarate
Leucine, lysine, phenylalanine, tryptophan, tyrosine
Acetoacetate and Acetyl Co A
TCA
No gluconeogenesis from fats
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aspartate & asparagine
phenylalanine & tyrosine
isoleucine, valine, & methionine
arginine, glutamate, glutamine, histidine
’
Pyruvate
Glucose
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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
Glucose + 2 ADP + 2 Pi + 2 NAD+
pyruvate + 2 ATP + 2 H+ + 2 NADH + 2 H20
Gluconeogenesis
Glycolysis
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1. Pyruvate carboxylase & PEP carboxykinase bypass pyruvate kinase step
2. 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
Bypass Reactions
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• 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+
1st bypass: Pyruvate is first converted to oxaloacetate
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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
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pyruvate
oxaloacetate
ATP CO2
ADP + Pi
malate
NADH
NAD+
malate
oxaloacetate
NAD+
NADH
matrix
cytosol
phosphoenolpyruvate
PEP carboxykinase
Pyruvate carboxylase
GTP
GDPCO2
Malatedehydrogenase
Malatedehydrogenase
pyruvate(c)
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2nd bypass: Fructose 1,6 bisphosphatase bypasses phosphofructokinase step
Fructose 1, 6, bisphosphatase
Phosphofructose kinase
F-6-P
F-1,6-BP
Pi
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3rd bypass: Glucose 6 phosphatase bypasses
hexokinase step
• Glucose-6-P + H20 glucose + Pi
Hexokinase
glucose
G-6-P
Pi
Blood stream
Glucose-6-phosphatase
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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
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G-6-Pase
G-6-P glucosePi
•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
Endoplasmic reticulum
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Reciprocal regulation of GNG and glycolysis
•Glycolysis yields 2 P bonds of ATP
•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
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
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• 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
Reciprocal regulation of GNG and glycolysis cont’d.
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Glucose-6-P + H2O glucose + Pi
• G-6-P inhibits hexokinase (glycolysis)• G-6-phosphatase activity (gluconeogenesis) is
dependant on [G-6-P]
Irreversible reactions provide regulation-1
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Irreversible reactions provide regulation – 2
cont’dF-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.
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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.
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Regulation of gluconeogenesis
G
G-6-P
PEP
PFK-1F-1,6-BPase
PFK-2 F-2,6-BP
P
Insulin mediates dephosphorylation of PFK-2
Glucose enters liver after meal
+-
F-1,6-BP
F-6-P
Gluconeogenesis is inhibited
X
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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