Synthesis of "new glucose" from common metabolites

• Humans use ~160 g of glucose per day

• 75% is used by the brain

• Body fluids contain only 20 g of glucose

• Glycogen stores yield 180-200 g of glucose

• So the body must be able to make its own glucose

• 90% of gluconeogenesis occurs in the liver and kidneys

Gluconeogenesis

Figure 18.1 The Glycolysis Pathway

Figure 18.1 The Glycolysis Pathway

Why is gluconeogenesis not just the reverse of glycolysis?

The reverse of glycolysis is2 Pyruvate + 2ATP + 2 NADH + 2H+ + 2H20 a

glucose +2ADP +2Pi + 2 NAD + (DG = +74 kJ/mol)This is thermodynamically unfavorable, so energetically unfavorable

steps in the reverse glyolysis reaction are replaced and energy is added in the form of GTP and ATP to give:

The actual equation for gluconeogenesis of2Pyruvate + 4ATP +2GTP+ 2NADH + 2H+ + 6H20 a

glucose +4ADP +2GDP +6Pi + 2 NAD + (DG = -38 kJ/mol)

Notice the extra ATPs and GTPs drive the process

Glycolysis vs Gluconeogenesis

GlycolosisGlucose (6C) to 2 pyruvates (3C)Creates energy 2ATPReduces 2 NAD+ to 2 NADHActive when energy in cell low10 steps from glucose to pyruvate Pyruvate to AcCoA before Krebs

Gluconeogenesis2 pyruvates (3C) to Glucose (6C)Consumes energy 4ATP+2GTPOxidizes 2NADH to 2 NAD+Active when energy in cell high11 steps from pyruvate to glucoseAcCoA isn’t used in gluconeogenesis

Gluconeogenesis uses 7 of the 10 enzymatic reactions of glycolysis but in the reverse direction. The 3 not used are the ones requiring ATP in glycolysis.

The pyruvate carboxylase reaction.

First Reaction of Gluconeogenesis

- recall that pyruvate is the final product of glycolysis.

(Simplified)

Biotin is an essential cofactor in most carboxylation reactions.

It is an essential vitamin in the human diet, but deficiencies are rare.

Avidin, a protein found in egg white binds tightly to biotin and excessive consumption of raw egg white can lead to biotin deficiency.

ATP

Carbonyl phosphate

oxaloacetate

Oxaloacetate cannot be transported directly across the mitochondrial membrane so it is converted to malate, then transported, then oxidized back to oxaloacetate.

Pyruvate is converted to oxaloacetate in the mitochondria

The PEP carboxykinase reaction.

Nucleotide diphosphate kinases

Both glycolysis and Oxidative phosphorylation produce ATP with its high energy phoshoanhydride bonds: How does GTP get made from GDP?

Directly from a single step in the Krebs cycle AND from the following reaction

GDP + ATP → GTP + ADPThis is carried out in the cell by an enzyme called Nucleotide diphosphate kinase which carries out the

general reactionNDP + ATP → NTP + ADP (where N is T, G, or C)

Fig. 18-26, p. 595

Enolase Reaction

gluconeogenesis

glycolysis

Fig. 18-23, p. 594

The Phosphoglycerate Mutase Reaction

gluconeogenesis

glycolysis

Isomerase: An enzyme that catalyzes the transformation of compounds into their positional isomers. In the case of sugars this usually involves the interconversion of an aldose into a ketose, or vice versa.

Kinase: An enzyme that catalyzes the phosphorylation (or dephosphorylation) of a molecule using ATP (or ADP).

Mutase: An enzyme that catalyzes the transposition of functional groups, such as phosphates, sulfates, etc.

Fig. 18-20, p. 593

Phospoglycerate kinase

glycolysis

gluconeogenesis

The glyceraldehyde-3-phosphate dehydrogenase reaction

glycolysis

gluconeogenesis

Fig. 18-14, p. 589

Triose phosphate isomerase

glycolysis

gluconeogenesis

Aldolase4th reaction of glycolysis (7th reaction of gluconeogenesis).

Reversible reaction also used in gluconeogenesis.

An aldol cleavage reaction (the reverse of an aldol condensation).

glycolysis

gluconeogenesis

Fig. 18-4, p. 584

- enzyme unique to liver and kidney allowing them to supply glucose to other tissues. Found in ER

Glucose-6-phosphatase

The Cori Cycle

Regulation of Gluconeogenesis

Glucose-6-phosphatase is subject to substrate level control.

- at higher G6P concentrations reaction rate increases

- recall, this happens in the liver. Other tissues do not hydrolyze their G6P, thereby trapping it in the cells.

Glycolysis and gluconeogenesis are reciprocally regulated.

- regulatory molecules that inhibit gluconeogenesis often activate glycolysis, and vise versa.

A potent allosteric regulatory molecule.

- activates phosphofructokinase.

- inhibits fructose-1,6-bisphosphatase.

- its synthesis and degradation are catalyzed by the same bifunctional enzyme.

Fructose-2,6-bisphosphate activates glycolysis and inhibits gluconeogenesis, so its level is very important.

F2,6 BP

ATPADP

Pi

F2,6 BPPFK-1

PFK-2

INHIBITS

F2,6 BP

STIMULATES

6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase

6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase

P

High glucagon

Increased phosphorylation

Phosphorylation of the enzyme results in the inactivation of the phosphofructokinase-2 activity and activation of the fructose-2,6-bisphosphatase activity. This results in a down regulation of glycolysis and increased gluconeogenesis.

Low glucose

Substrates for gluconeogenesis:

Not substrates for gluconeogenesis:

PyruvateLactateTCA cycle intermediatesMost amino acids

Acetyl-CoAFatty acidsLysineLeucine

Plants and bacteria can make glucose from acetate.