GluconeogenesisGluconeogenesis(de novo synthesis of glucose)( y g )

Gluconeogenesisg• Gluconeogenesis is the biosynthesis of new glucose.

h f l h • The main purpose of gluconeogenesis is to maintain the constant blood Glc concentration.

• Gluconeogenesis occurs mainly (90%) in the liver and in the cortex of kidney (10%).

The major noncarbohydrate precursors:j y p• lactate

i id• amino acids• glycerolg y

Rections of gluconeogenesis

• Seven of the reactions of glycolysis are reversible and are used in the synthesis of Glc from lactate or pyruvatethe synthesis of Glc from lactate or pyruvate.

• 3 of the reactions, catalyzed by hexokinase, phosphofructokinase and pyruvate kinase are irreversibleand must be circumvented by alternate reactions:and must be circumvented by alternate reactions:

- Carboxylation of pyruvate. - Decarboxylation and phosphorylation of cytosolic OA. - Dephosphorylation of F1.6 bisP. - Dephosphorylation of G-6-P- Dephosphorylation of G-6-P.

Start

1. Carboxylation of pyruvate(Pyruvate to Phosphoenolpyruvate (PEP))(Pyruvate to Phosphoenolpyruvate (PEP))

• Conversion of pyruvate to pyPEP requires the action of two mitochondrial enzymes. The first is an ATP-requiring reaction catalyzed by pyruvate carboxylasepyruvate carboxylase, (PC). Pyruvate carboxylasecontains biotin that is covalently bound tocovalently bound to apoenzyme.

• PEP carboxykinase(PEPCK) requires GTP in(PEPCK) requires GTP in the decarboxylation of OA to yield PEP.

Human cells contain almost equal amounts of mitochondrial and cytosolic PEPCK. y

2. Fructose-1,6-bisphosphate to Fructose-6-phosphatephosphate

(Dephosphorylation of F1,6bisP)

• The reaction is catalyzed by fructose-1,6-bisphosphatase (F1,6BPase).

• F1,6BPase reaction is a major point of control of gluconeogenesis.

3. Dephosphorylation of Glucose-6-phosphate to Glucose

• G6P is converted to glucose through the action of glucose-6-phosphatase (G6Pase).

The net equation of gluconeogenesishe net equation of gluconeogenesis

2Pyruvate + 4ATP + 2GTP + 2NADH + H+ +2Pyruvate 4ATP 2GTP 2NADH H 6H2O

Glucose + 2NAD+ + 4ADP + 2GDP + 6Pi + 6H+6H+

Substrates for GluconeogenesisLactate:Lactate:• Lactate is released into blood by cells that

lack mitochondria, such as red blood cells and exercising skeletal muscle. g

• Lactate diffuses out of active skeletal muscle into the blood and is carried to the liver.

• In the liver lactate is oxidized to pyruvate.• Pyruvate is converted into Glc by the

gluconeogenic pathway in liver. Glc t th bl d d i t k benters the blood and is taken up by

skeletal muscle. • In skeletal muscles Glc is converted to

lactate by the glycolysis pathwaylactate by the glycolysis pathway. • This relationship between glycolysis in

muscles and gluconeogenesis in liver is called the Cori cyclecalled the Cori cycle.

Glycerol:Glycerol:

Amino Acids:• All 20 of the amino acids,

except leucine and lysine, can be degraded to TCA cycle intermediatesintermediates.

Regulation of Gluconeogenesis

• Regulation by energy levels within the cell: F-1 6-bisphosphatase is inhibited byF-1.6-bisphosphatase is inhibited by elevated levels of AMP. Conversely, high levels of ATP and low of AMP stimulate gluconeogenesis.g g

• Allosteric regulation: Pyruvate carboxylase is an allosteric enzymePyruvate carboxylase is an allosteric enzyme activated by acetyl-CoA.Fructose 1,6-bisphosphatase is an allosteric enzyme inhibited by fructose 2,6-bisphosphate.y y , p p

• Regulation through the amount of enzymes:PEP carboxykinase, fructose 1,6-bisphosphatase and glucose 6-phosphatase are inducible enzymes whose amounts are increased in respose toamounts are increased in respose to hormones (e.g. glucagon and glucocorticoids – hormones of adrenal cortex).

Pentose phosphate pathway(PPP)

Functions of PPPFunctions of PPP

• Provides NADPH as a reductant• Provides ribose 5-phosphate needed forProvides ribose 5 phosphate needed for

synthesis of nucleotides (e.g. AMP) and nucleic acidsnucleic acids.

• Converts glucose into other sugars.

LocationLocation

• In tissues, the pathway is most active in the liver, mamary glands, adipose tissue and the adrenal cortexcortex.

• Within the cell the enzymes of pathway are located in• Within the cell, the enzymes of pathway are located in the cytoplasm.

Stages of the PPPStages of the PPP

Glucose 6-PdehydrogenaseHexokinase 6-Phosphogluconate

dehydrogenase

OXIDATIVE BRANCH

HB

RA

NC

HA

TIV

E B

N-O

XID

AN

ON

The stoichiometry of the pentose phosphate pathway:pathway:

3 G6P + 6 NADP+ 2 F6P + 3 CO2 + G3P + 6 NADPH + 6 H+

RegulationRegulation

• The cellular concentration of NADP+ is the major factor in regulating flux through the pathway.

• Its availability regulates the rate-limiting G6PD reaction.

To read at home:

Metabolism of fructose and galactose

To read at home:

Metabolism of fructose and galactose