GluconeogenesisDr.S.Chakravarty MD

Maintenance of blood glucose during various states

Gluconeogenesis is the process of synthesizing glucose or glycogen from non-carbohydrate precursors.

Biomedical Importance• Gluconeogenesis meets the needs of the body for glucose when insufficient carbohydrate is

available from the diet or glycogen reserves.

• A supply of glucose is necessary especially for the nervous system and erythrocytes.

• Glucose is also important in maintaining the level of intermediates of the citric acid cycle even when fatty acids are the main source of acetyl-CoA in the tissues.

• Gluconeogenesis clears lactate produced by muscle and erythrocytes and glycerol produced by adipose tissue.

• In ruminants, propionate is a product of rumen metabolism of carbohydrates, and is a major substrate for gluconeogenesis.

• Failure of gluconeogenesis is usually fatal. Hypoglycemia causes brain dysfunction, which can lead to coma and death.

• Tissues :- Liver and Kidney are the major gluconeogenic tissues, but the small intestine may also be a source of glucose in the fasting state.

• Subcellular site :- Partly mitochondrial and partly cytosolic.

Substrates for Gluconeogenesis

LactateGlucogenic amino acids

GlycerolPropionate

Glucogenic amino acids

α-ketoglutarate Glutamate

Alanine Pyruvate

α-ketoglutarate Glutamate

Aspartate Oxaloacetate

Alanine transaminase (ALT)

Aspartate transaminase (ALT)

Entry points of Glucogenic amino acids

Lactate

NAD+ NADH + H+

Lactate Pyruvate Lactate Dehydrogenase (LDH)

USMLE!

Remember :-Conversion of Lactate to pyruvate requires NAD and not NADH

Glycerol

ATP ADP

Glycerol Glycerol 3 –P

This enzyme is absent in adipose tissue .

Glycerol kinase

Conversion of Glycerol to Glucose:Triglycerides

Glycerol Fatty acids Beta oxidation

Acetyl Co-A

Liver

Glycerol 3- PO4

Dihydroxyacetone phosphate

Glycerol kinase

Glycerol-3-po4 dehydrogenase

NAD+

NADH

FASTING OR LOW GLUCOSE

Propionate – From oxidation of odd chain fatty acids

The pathway

• Thermodynamic Barriers Prevent a Simple Reversal of Glycolysis.

• Three nonequilibrium reactions in glycolysis catalyzed by hexokinase, phosphofructokinase and pyruvate kinase, prevent simple reversal of glycolysis for glucose synthesis.

Pyruvate

Oxaloacetate

Phosphoenol pyruvate

ATP

GTP

ADP

GDP

Pyruvate carboxylase

Phosphoenolpyruvate carboxykinase

Energy derived from fatty acid oxidation

GTP derived from succinate thoikinase

Step 1:

(Mitochondria)

(cytosol)

CO2

CO2

Problem --Mitochondrial membrane is impermeable to OAA!!

USMLE CONCEPT!!!

ABC carboxylase

BIOTIN

Oxaloacetate formed in the mitochondria enters cytosol through Malate

Oxaloacetate

Malate

Malate

Oxaloacetate

Mitochondria

cytosol

Malate dehydrogenase

Malate dehydrogenase

NAD

NADH

NAD

NADH

The next few steps are reversal of Glycolysis till Fructose 1,6 bisphosphate is formed.

Fructose -6-PO4

Fructose 1,6 Bisphosphate

PFK -1 Fructose 1,6 bisphosphatase

ADP, AMP

(+)

ATP

(-)

Fructose 2,6 Bisphosphate

(+)

Fructose 2,6 Bisphosphate

Step 2: Conversion of fructose 1,6 bisphosphate to fructose 6-PO-4

(+)

GluconeogenesisGlycolysis

Glucose

Pyruvate

Step 3: Conversion to Glucose

Glucose

Glucose-6-po4

Glucose-6-phosphatase

Glucokinase

Glycolysis Gluconeogenesis

ATP

ADP

PO4

1.Pyruvate & Phosphoenolpyruvate

Fructose 1,6-Bisphosphate & Fructose 6-Phosphate

Glucose 6-Phosphate & Glucose& Glucose to glycogen(not shown)

G L U C O N E O G E N E S I S

Regulation

• Glycolysis and Gluconeogenesis are regulated reciprocally.

Regulation

Induction /Repression

Covalent modification

Allosteric

• Induction & Repression of Key Enzymes Requires Several Hours.

Table 20.1

Harper 29th page 190

Covalent Modification by Reversible Phosphorylation Is Rapid

Glucagon and epinephrine inhibit glycolysis and stimulate gluconeogenesis in the liver by increasing the concentration of cAMP.

cAMP cAMP-dependent protein kinase

phosphorylation and inactivation of pyruvate kinase.

They also affect the concentration of fructose 2,6-bisphosphate which is the most potent positive allosteric effector of of Phosphofructokinase -1 and inhibitor of Fructose 1,6 bisphosphatase .

Fructose -6-po4

Fructose -1,6- Bisphosphate

Fructose -2,6- Bisphosphate

PFK-1

PFK-2

Insulin

Glucagon

Regulation of PFK -1 :

USMLE concept!!!

Allosteric Regulation is Instantaneous !!

Pyruvate

Acetyl Co-AOxaloacetate

Pyruvate dehydrogenase

Pyruvate Carboxylase

(-)(+)

Acetyl Co-A is the allosteric activator of pyruvate carboxylase

What is the source of acetyl Co-A during starvation ?

Glucagon (+) Source of pyruvate ?

= allosteric regulation

Allosteric Modification Is Instantaneous

• Acetyl -CoA as an is an allosteric activator of Pyruvate carboxylase.

• Phosphofructokinase (phosphofructokinase-1 )is inhibited by

citrate and by normal intracellular concentrations of ATP and is activated by 5'AMP. (5'AMP acts as an indicator of the energy status of the cell. )

– When ATP is used AMP increases sensitive signal for energy state of the cell.

ATP ADP

A B

Pi H20

NET FLUX OF B = 0 +heat

Substrate cycles allow fine tuning , rapid response & generate heat

At rest the activity of PFK IS 10x greater than F1,6BPhosphatase .During Muscle contraction , PFK activity increases and F1,6BPhosphatase falls so that rate of Glycolysis becomes 1000 fold higher than resting state.

Futile cycle may occur physiologically for generation of heat.It takes place to a great extent in animals undergoing arousal from hibernation, when body temp is much lower .

100

100

120

80

40

Other factors that favor AB

Clinical aspects 1. Pyruvate carboxylase deficiency (A.R)- 1 in 25,000 births –characterized

by Hypoglycemia , lactic acidosis and Mental retardation .

2. Fructose 1,6bisphosphatase deficiency – lactic acidosis and hypoglycemia .Treatment – feed high carb. Diets and avoidance of fasting .

3. Hypoglycemia during pregnancy and in neonates – Increased risk of maternal hypoglycemia if there are long intervals b/w meals .

Premature babies are more susceptible to hypoglycemia. They have immmature non functional enzymes for gluconeogenesis and low adipose tissue mass.

Pyruvate carboxylase deficiency:

• Malfunctioning of citric acid cycle – def of oxaloacetate

• Malfunctioning of gluconeogenesis – def of oxaloacetate - Hypoglycemia

• Malfunctioning of urea cycle – Def of Aspartate• Acetyl Co-A forms ketone bodies-KETOGENESIS

ALCOHOL DEHYDROGENASE

• Alcohol Acetaldehyde Acetate

• Excess NADH – EXCESS LACTATE from PYRUVATE – Excess Malate FROM OAA– Excess Glycerol 3 P from DHAP– No or less Gluconeogenesis!!Hypoglycemia

NAD NADH NAD NADH

• Immediately after completing a 25-mile marathon race, a healthy 24-yr old man was extremely dehydrated and thirsty. He quickly consumed a 6-pack of ice-cold beer and shortly thereafter became very weak and light-headed and nearly fainted. He complained of muscle cramping and pain. What is the most probable cause ?

1. Excess lactate in blood2. Excess Alcohol in blood 3. Excess NADH 4. Dehydration5. Electrolyte imbalance

• Which of the following enzymes can be induced genetically by hormones in a person with prolonged history of fasting?

A. Glucokinase B. Pyruvate Carboxylase C. PFK-1D. Acetyl co-A Carboxylase E. Phosphofructokinase

• In the citric acid cycle, succinate thiokinase catalyzes the cleavage of the succinyl –Co-A to succinate with formation of a high energy compound. This compound can then be used by the body in which of the following biochemical pathways?

– Oxidative phosphorylation– Gluconeogenesis– Formation of creatine phosphate– Cholesterol synthesis– Fatty acid synthesis

Thank you