Gluconeogenesis

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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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Glycerol from breakdown of Triglycerides can act as

substrate for GNG

ATP

DHAP

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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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Glycerol

Lactate & some aminio acids

Some amino acids

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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

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Irreversible reactions provide regulation – 3

cont’dPyruavte 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.