Gluconeogenesis Chapter 10 (117)

BCH 340 lecture 8

What is the Significance of Gluconeogenesis?

Synthesis of glucose from non-carbohydrate precursorsSome tissues absolutely require a constant

supply of glucose.The brain, for example.

When glucose is absent (during fasting) andwhen liver stores of glycogen are depleted,

gluconeogenesis becomes essential.

• Gluconeogenesis is the generation of glucose from non-

carbohydrate sources such as lactate , glycerol and some

amino acids

Gluconeogenesis clear the blood from the waste products

of other tissues as lactate (produced by muscles and

RBCs)

• It happens mostly in the liver (90%); it can also occur in

the kidney (10%)

• It occurs mainly in cytoplasm and partly in mitochondria

• Gluconeogenesis begins with various substrates converted into

pyruvate

• Synthesis of glucose from pyruvate utilizes many of the same

enzymes as Glycolysis

• Almost reverse of glycolysis except for 3 reactions, which are

essentially irreversible

o Hexokinase (or Glucokinase)

o Phosphofructokinase

o Pyruvate Kinase

These steps must be bypassed in Gluconeogenesis

Four enzymes are required to reverse the three irreversible steps of glycolysis:

• Mitochondrial - Pyruvate Carboxylase (liver, kidney but not in muscle)

• Cytoplasmic - Phosphoenolpyruvate (PEP) Carboxykinase

• Cytoplasmic -Fructose-1,6,-Bisphosphatase

• Cytoplasmic -Glucose 6-Phosphatase

• PK removes phosphate group from PEP to form pyruvate

and ATP in glycolysis

Bypassing Pyruvate Kinase reaction

C

C

CH3

O O

O2

3

1

ADP ATPC

C

CH2

O O

OPO32

2

3

1

phosphoenolpyruvate pyruvate

Pyruvate Kinase

In Gluconeogenesis PK bypassed by two reactions:

1. pyruvate is carboxylated to oxaloacetate by pyruvate Carboxylase in mitochondria

2. oxaloacetate is decarboxylated and phosphorylated to yield PEP, this reaction is catalyzed by PEP Carboxykinase in cytosol

Cost 2 ATP per pyruvate (x2)

Bypassing Pyruvate Kinase reaction

Transport of OAA to the Cytosol

Malate Dehydrogenase

Malate DehydrogenasePEP

Pyruvate Carboxylase

ATP ADP+Pi

OAA

HCO3-

GDP GTP CO2

pyruvate

Mitochondria

Cytosol

PEP Carboxykinase

Bypassing PFK reaction

• In glycolysis PFK phosphorylates Fructose 6-phosphate to form Fructose 1,6-bisphosphate

• In Gluconeogenesis PFK bypassed by Fructose 1,6 -bisphosphatase reaction (Removes phosphate group)

fructose-1,6-bisP + H2O fructose-6-P + Pi

fructose-6-phosphate fructose-1,6-bisphosphate

Phosphofructokinase

CH2OPO32

OH

CH2OH

H

OH H

H HO

O

6

5

4 3

2

1 CH2OPO32

OH

CH2OPO32

H

OH H

H HO

O

6

5

4 3

2

1

ATP ADP

Pi H2O

Fructose-1,6-bisphosphatase

In glycolysis HK/GK phosphorylates glucose to form glucose 6-phosphate

glucose + ATP glucose-6-phosphate + ADP

In Gluconeogenesis this reaction bypassed by glucose 6-phosphatase reaction:

Removes phosphate group Free glucose is formed by the action of glucose-6-phosphatase in liver

and kidney while it is absent in muscles and adipose tissues glucose can not be formed by these organs

H O

OH

H

OHH

OH

CH2OH

H

OH

HH O

OH

H

OHH

OH

CH2OPO32

H

OH

HH2O

1

6

5

4

3 2

+ Pi

glucose-6-phosphate glucose

Glucose-6-phosphatase

Bypassing HK/GK

reaction

Glycolysis enzymenames in red.

Gluconeogenesis enzyme names in blue.

Total Energy Cost:

6 high energy bonds used per glucose synthesized.

Four more than produced in glycolysis.

These four are needed to convert pyruvate to PEP:

1. 2ATP are utilized by pyruvate carboxylase

2. 2GTP are utilized by PEP carboxyKinase

Cori Cycle• Lactate released by active skeletal

muscle or red blood cells is carried

to the liver where it is converted

to glucose by gluconeogenic

pathway (Cori-cycle) and released

for reuptake by skeletal muscle

Precursors for gluconeogenesis

LDHLDH

Glucogenic amino acids

All amino acids can give glucose except leucine and

lysine

After 18h fasting, proteins are considered as one of the main source of glucose

Glucose-alanine cycle

• Protein broken down in skeletal muscle during exercise

Amino acids converted to alanine and released by skeletal muscle

Taken up by liver and converted to glucose and released for reuptake by skeletal muscle

Glycerol

• Glycerol released from adipocytes and skeletal muscle during

lipolysis

• Glycerol enters gluconeogenic pathway as DHAP

• 2 glycerol required to make one glucose in liver and kidney in

fasting or low CHO diet

GlucoseF1,6BP GLUT2

Blood

Hepatocyte

• Gluconeogenesis is subject to both

• Hormonal control

• Glucagon

• Insulin

• Allosteric regulation of gluconeogenic enzymes

Allosteric regulation

• Glucagon, Cortisol, Adrenaline

• Are secreted during fasting, stress and muscular exercise

• Induce gluconeogenic enzymes

• Represses glycolytic enzymes

Insulin

◦ Secreted after CHO meal

◦ Is repressor for the synthesis of gluconeogenic 4 key enzymes

◦ At the same time it is inducer of the glycolytic key enzymes

◦ Insulin stimulates PFK-2 producing accumulation of F2,6 bisP:

• + PFK-1 + glycolysis• – Fr1,6 Bisphosphatase – gluconeogenesis