Gluconeogenesis

Gluconeogenesis

synthesis (breakdown) of glucose

Step 1: Gluconeogenesis

Pyruvate —>PEP

Requires two enzymes: pyruvate carboxylase (biotin) and PEP carboxykinase

1. Pyruvate carboxylase catalyzes the ATP-driven formation of oxaloacetate intermediate from pyruvate

2. PEP carboxykinase converts oxaloacetate to PEP in a reaction that uses GTP as a phosphoryl-group donor (CO2 is eliminated in this step)

Biotin

-a prosthetic group

-acts as a CO2 carrier

Pyruvate carboxylase: phase I

synthesis of carboxy-biotin

• ATP and bicarbonate form carboxyphosphate

• CO2 is generated

• ATP is cleaved to form ADP and dehydrates bicarbonate by forming a high-energy carboxyphosphate intermediate

• Carboxylation of biotin by CO2 is exergonic

Pyruvate carboxylase: phase II

The activated carboxyl group is transferred from carboxybiotin to pyruvate in a three-step reaction to form oxaloacetate.

• CO2 is produced at the active site via the elimination of the biotinyl-enzyme

• pyruvate donates a proton to the biotinyl group to generate pyruvate enolate

• nucleophilic attack by the enolate on CO2 to form oxaloacetate

In species with cytosolic PEPCK, oxaloacetate must first be converted either to aspartate or to malate

Results in the transport of reducing equivalents from the mitochondria to the cytosol, since it uses mitochondrial NADH to produce cytosolic NADH

There is an oxidation of NADH followed by the cytosolic reduction of NAD+

Steps 2-10: Gluconeogenesis

• Step 8: FBP is hydrolyzed by FBPase to form F6P

• F6P is isomerized to G6P

• Step 10: G6P is then hydrolyzed by G6Pase to form Glucose

• These reactions release Pi

• These reactions can only occur in the liver and kidney

Glycolysis regulation

• increase in AMP/ADP

• increase in F2,6P

• decrease in ATP

• decrease in Citrate

• decrease in PEP

Gluconeogenesis regulation

• decrease in AMP/ADP

• decrease in F2,6P

• increase in ATP

• increase in citrate

• increase in PEP

Why does a decrease in F2,6P initiate gluconeogenesis?

• When glucose is low, glucagon stimulates the production of cAMP

• this activates PKA to phosphorylate an enzyme on a Ser residue

• This inactivates the enzymes PFK-2 activity

• This results in a decrease in F2,6P