a. Pyruvate carboxylase (an anaplerotic reaction that we have seen before)

pyruvate + HCO3- + ATP ----> oxaloacetate + ADP + Pi

Pyruvate carboxylase is completely inactive in absence of acetyl-CoA, which acts as a positive allosteric modulator. This type of regulation makes sense because high levels of Ac-CoA signal the need for more oxaloacetate.

This oxaloacetate is formed inside the mitochondrion, and passes into the cytoplasm as malate:

NADH

OAAm ----> malatem ----> malatec ----> OAAc

For this reaction to occur, the mitochondrial levels of NADH must be high (this would occur if energy levels were also high).

Acetate, itself, is not a precursor to glucose in animals, because they have no glyoxylate cycle.

b. Phosphoenolpyruvate carboxykinase

A second reaction completes the conversion of pyruvate (now oxaloacetate) to phosphoenolpyruvate:

oxaloacetate + GTP ----> phosphoenolpyruvate + GDP + CO2

The sum of the two reactions is

pyruvate + ATP + GTP <===> PEP + ADP + GDP + Pi

The DG' is - 25 kJ/mol (under cellular conditions), and will only proceed when ATP/ADP is high (this means that the cell can afford to make glucose).

c. Fructose bisphosphatase

F 6-P + ATP ===> F 1,6-DP + ADP

This glycolytic reaction is catalyzed by phosphofructokinase, which is activated by AMP, inhibited by citrate.

In gluconeogenesis, the reverse reaction is catalyzed by fructose bisphosphatase, which is a cytosolic enzyme.

F 1,6-DP + H2O <===> F 6-P + Pi DGo' = - 16.3 kJ/mol

This enzyme is inhibited by AMP (i.e. it requires a high energy state to be active), and it is stimulated by 3-phosphoglycerate and citrate (TCA cycle is proceeding slowly because there is no need for new ATP). Notice that the regulation of glycolysis and gluconeogenesis is complimentary.

The liver expresses the gene for this enzyme, but muscle does not. Hence the liver can release glucose and the muscle can not.

G-6-P ----> releases free glucose (goes to bloodstream and then to the brain)

Comparison of overall reactions

Gluconeogenesis:

2 pyruvate + 4 ATP + 2 GTP + 2 NADH + 2 H2O ---->

glucose + 4 ADP + 2 GDP + 2 NAD+ + 6 Pi DG = - 37.6 kJ/mole

Glycolysis:

glucose + 2 ADP + 2 Pi + 2 NAD+ ----> 2 pyruvate + 2 ATP + 2 NADH + 2 H2O

DG = -83.7 kJ/mole = glycolysis or +83.7 kJ/mole if gluconeogenesis were the reverse of glycolysis (clearly, this can't happen)

There is a loss of 4 moles of ATP/mole of glucosemade by gluconeogenesis.

Review of Energy Physiology

a. the brain requires glucose (it can use ketone bodies during starvation)

b. muscles, when at rest, use fatty acids; when exercising they use glycogen and can produce lactate when oxygen levels are limiting.

c. liver (the glucose buffer) converts lactate to glucose

d. adipose tissue needs glucose for triglyceride synthesis; low glucose leads to release of fatty acids.

The Cori Cycle (named after Carl and Gerti Cori)

Cori cycle: no phosphatase has G-6-Phosphatase

skeletal muscle blood liver

<---- glucose <---- glucose

lactate ----> lactate ----> lactate

Ethanol and gluconeogenesis:

Extreme alcoholics (winos) have very clean arteries (low risk of heart disease, stroke) but their livers are like stone (due to scarring). They are also very gaunt, due to loss of muscle mass (glucogenic amino acids are being converted to glucose, since ethanol can't participate in gluconeogenesis). Ethanol is metabolized in the human body via the enzyme alcohol dehydrogenase; the reaction sequence is as follows:

ethanol ----> acetaldehyde ----> acetate

Acetaldehyde is similar to formaldehyde, which is used as pickling agent. It builds up in this metabolic sequence because the second reaction is slower than the first (i.e. it is rate-limiting).

Regulation of gluconeogenesis:

cAMP ---> stimulates the production of F-2,6-BP ---> slows gluconeogenesis

glucagon ---> breakdown of glycogen ---> release of glucose