Gluconeogenesis

Dr. Tarek A Salem

Objectives of this lecture

• Definition of gluconeogenesis

• The steps of gluconeogenesis

• Site of occurrence and importance of gluconeogenesis.

• The sources of carbon atoms used in gluconeogenesis

Definition

• Glucneogenesis is making a new glucose from non-carbohydrate precursors

• In other words:– Create new glucose from the products of its

breakdown

Site of gluconeogenesis• Major site of gluconeogenesis: Liver (90%)• Secondary site: Kidney cortex and in small

intestine under some conditions. (10 %)• It takes place in the mitochondria and cytoplasm.

• The production of glucose is necessary for use as a fuel source by the brain, testes, erythrocytes, kidney medulla, lens and cornea of the eye and exercising muscle.

Overview

Synthesis of glucose from pyruvate utilizes many of the same enzymes as Glycolysis.

Three Glycolysis reactions have such a large negative ΔG that they are essentially irreversible Hexokinase (or Glucokinase) Phosphofructokinase Pyruvate Kinase.

These steps must be bypassed in Gluconeogenesis.

First Bypass Reaction: Convervsion of Pyruvate to Phosphoenolpyruvate

• Enzymes involved:

Pyruvate carboxylase

PEP carboxykinase

C

C

CH 2

O O

O PO 32

C

C

CH 3

O O

O

A T P A D P + P i C

CH 2

C

C

O

O O

O O

HC O 3

G T P G D P

CO 2

p y r u v a te o x a lo a c e ta te P E P

P y ru v a te C a rb o x y la s e P E P C a rb o x y k in a s e

In Mitochondria• Pyruvate carboxylase (PC) presents in the mitochondria

of liver and kidney but absent in muscle• ATP, biotin, Mn++ and CO2 are required.

Transport of Oxaloacetate into cytosol as Malate

In cytosol

Summary of the first bypass

The overall equation for this set of bypass reactions

• Pyruvate + ATP + GTP + HCO3-

• phosphoenolpyruvate + ADP + GDP + Pi + H+ + CO2

• Thus the synthesis of one molecule of PEP requires an investment of 1 ATP and 1 GTP.

• Note: when either pyruvate or the ATP/ADP ratio is high, the reaction is pushed toward the right (i.e., in the direction of biosynthesis).

Second Bypass Reaction: Conversion of Fructose 1,6- bisphosphate to Fructose 6-phosphate

• The second glycolytic reaction (phosphorylation of fructose 6-phosphate by PFK) is irreversible.

• Hence, for gluconeogenesis fructose 6-phosphate must be generated from fructose 1,6-bisphosphate by a different enzyme: Fructose 1,6-bisphosphatase.

• Fructose 1,6-bisphosphatse presents in liver and kidney.• This reaction is also irreversible.

Fructose 1,6-bisphosphate + H2O fructose 6-phosphate + Pi

Phosphofructokinase (In Glycolysis): fructose-6-P + ATP fructose-1,6-bisP + ADP

Fructose-1,6-bisphosphatase (In Gluconeogenesis):fructose-1,6-bisP + H2O fructose-6-P + Pi

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

Phosphofructokinase CH2OPO3

2

OH

CH2OH

H

OH H

H HO

O6

5

4 3

2

1 CH2OPO32

OH

CH2OPO32

H

OH H

H HO

O6

5

4 3

2

1ATP ADP

Pi H2O

Fructose-1,6-biosphosphatase

Third Bypass Reaction: Glucose 6-phosphate to Glucose

• Because the hexokinase reaction is irreversible, the final reaction of gluconeogenesis is catalyzed by Glucose 6-phosphatase.

Glucose 6-phosphate + H2O glucose + Pi

• Glucose 6-phosphatase is present in the liver, kidney and

small intestine but absent in brain and muscle. Thus,

glucose produced by gluconeogenesis in the liver, is

delivered by the bloodstream to brain and muscle.

Hexokinase or Glucokinase (In Glycolysis):glucose + ATP glucose-6-phosphate + ADP

Glucose-6-Phosphatase (In Gluconeogenesis): glucose-6-phosphate + H2O glucose + Pi

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

Glyceraldehyde-3-phosphate Dehydrogenase

Phosphoglycerate Kinase

Enolase

PEP Carboxykinase

glyceraldehyde-3-phosphate

NAD+ + Pi

NADH + H+

1,3-bisphosphoglycerate

ADP

ATP

3-phosphoglycerate

Phosphoglycerate Mutase

2-phosphoglycerate H2O

phosphoenolpyruvate

CO2 + GDP

GTP oxaloacetate

Pi + ADP

HCO3 + ATP

pyruvate

Pyruvate Carboxylase

Gluconeogenesis

Summary of Gluconeogenesis Pathway:

Gluconeogenesis enzyme names in red.

Glycolysis enzyme names in blue.

Glucose-6-phosphatase

Fructose-1,6-bisphosphatase

glucose Gluconeogenesis

Pi

H2O glucose-6-phosphate

Phosphoglucose Isomerase

fructose-6-phosphate

Pi

H2O fructose-1,6-bisphosphate

Aldolase

glyceraldehyde-3-phosphate + dihydroxyacetone-phosphate

Triosephosphate Isomerase (continued)

Carbon sources for gluconeogenesis

• Glycerol: is released during hydrolysis of triacylglycerols in adipose tissue and is delivered by the blood to the liver.

Glycerol kinase

• Glycerol Glycerol 3-P G3P dehydrogenase

• Glycerol 3-P DHAP

• DHAP is converted into glyceraldehyde 3-P

Carbon sources for gluconeogenesis

• Lactate (Lactic acid): In vigorous skeletal muscle activity, large amount of lactic acid produced pass to liver through blood stream converted into pyruvic and lastly to glucose reach muscle again through blood stream to provide energy. (This called Cori cycle).

Lactate produced from pyruvate passes via the blood to the liver, where it may be converted to glucose.

The glucose may travel back to the muscle to fuel Glycolysis.

Cori Cycle

Liver Blood Muscle Glucose Glucose 2 NAD+ 2 NAD+

2 NADH 2 NADH 6 ~P 2 ~P 2 Pyruvate 2 Pyruvate 2 NADH 2 NADH 2 NAD+ 2 NAD+ 2 Lactate 2 Lactate

Carbon sources for gluconeogenesis

• Propionic acid: product of odd number fatty acid degradation.

• It is converted into succinyl CoA which converted into oxaloactic acid that can form glucose.

Carbon sources for gluconeogenesis

• Glucogenic amino acids: amino acids by deamination can be converted into keto acids as pyruvic, α-ketoglutaric and oxaloacetic acid.

• Proteins are considered as one of the main sources of blood glucose especially after 18 hr due to depletion of liver glycogen.

Carbon sources for gluconeogenesis

• Glucose-alanine cycle: During starvation, alanine is formed from protein catabolism.

• Alanine is converted into pyruvic acid in liver which can give glucose.

Acetyl CoA can not produce glucose

• Acetyl CoA cannot give rise to a net synthesis of glucose. This is due to the irreversible nature of the pyruvate dehydrogenase reaction, which converts pyruvate to acetyl CoA.

Pyruvate dehydrogenase

• Pyruvate acetyl CoA + CO2

NAD+ NADH+H+

Importance of gluconeogenesis

1- Maintenance of blood glucose during starvation, fasting and prolonged exercise.

2- Removal of lactic acid.

3- Removal of glycerol produced by lipolysis.

Regulation of gluconeogenesis

• In liver: ATP PK PC glycolysis is inhibited and gluconeogenesis is activited

• During starvation, the priority is to conserve blood glucose for the brain and muscle. Thus, under these conditions, PK in the liver is switched off. This occurs because the hormone glucagon is secreted into the blood stream and activates a cAMP cascade that leads to the phosphorylation and inhibition of this enzyme.

Regulation of gluconeogenesis

• In liver: AMP PK PC glycolysis is activated and gluconeogenesis is stopped.

• In case of Acetyl CoA PK PC

When acetyl CoA is abundant, gluconeogenesis is activated