Gluconeogenesis Notes

Understand the three steps in glycolysis that must be bypassed in gluconeogenesis.
Identify the major noncarbohydrate precursors for gluconeogenesis.
Explain the role of the malate shuttle.
Describe the generation of free glucose in the liver.
Understand the reciprocal regulation of gluconeogenesis and glycolysis.
Integrate the relationship between glycolysis and gluconeogenesis.
Describe the Cori Cycle.

Gluconeogenesis is the process of making glucose from pyruvate or other non-carbohydrate molecules.
- "Gluco-" refers to glucose.
- "Neo-" means new.
- "Genesis" means making.
It mainly occurs in the liver.
Gluconeogenesis supplies glucose to the brain and red blood cells.
It also replenishes glucose in muscles during intense exercise.

The brain relies on glucose as its main fuel, using approximately $120g$ per day.
Normally, glucose is obtained from the diet and the breakdown of glycogen stores.
During starvation or intense exercise, glucose reserves may be depleted, with an overall reserve of approximately $190g$ .
Glucose can then be synthesized from:
- Lactate (a product of anaerobic respiration).
- Amino acids (from broken-down protein in muscles).
- Glycerol (from triacylglycerol fat stores).

Pyruvate can be formed from muscle-derived lactate in the liver by lactate dehydrogenase.
Carbon skeletons of some amino acids can be converted into gluconeogenic intermediates.
Glycerol, derived from the hydrolysis of triacylglycerols, can be converted into dihydroxyacetone phosphate, which can be processed by gluconeogenesis or glycolysis.
Glycerol can enter either the gluconeogenic or the glycolytic pathway.

The formation of phosphoenolpyruvate (PEP) from pyruvate requires two enzymes: pyruvate carboxylase and phosphoenolpyruvate carboxykinase.
This reaction occurs in the mitochondria.
Pyruvate carboxylase requires the vitamin biotin as a prosthetic group.
- Biotin is covalently bound to the $ε$ -aminogroup of a lysine residue.

Oxaloacetate is reduced to malate by malate dehydrogenase and transported into the cytoplasm, where it is reoxidized to oxaloacetate with the generation of NADH.
PEP is then synthesized from oxaloacetate by phosphoenolpyruvate carboxykinase (PEPCK).
The phosphoryl donor is GTP.

Fructose 1,6-bisphosphatase is an allosteric enzyme (regulated by the binding of molecules).
Unlike in glycolysis, ATP is not involved; the phosphate is released as Pi (inorganic phosphate).

In most tissues, gluconeogenesis stops at the level of glucose 6-phosphate, where it is converted into glycogen (storage form of glucose).
The liver converts glucose 6-phosphate into glucose.
The liver is the primary organ responsible for maintaining adequate levels of glucose in the blood.

Glucose is not formed in the cytoplasm.
Glucose 6-phosphate is transported into the lumen of the endoplasmic reticulum (ER).
Glucose 6-phosphatase is a membrane protein with the active site in the ER lumen.
Glucose 6-phosphate is first transported into the ER, then glucose and Pi are transported back out.

Gluconeogenesis requires 4 more NTPs to convert pyruvate to glucose than glucose to pyruvate.
The reverse of glycolysis is unfavorable ( $+84 kJ \cdot mol^{-1}$ ).
This must be coupled to a favorable hydrolysis of ATP/GTP so that, overall, the $\Delta G$ is negative ( $-38 kJ \cdot mol^{-1}$ ).

During intense exercise, muscles respire anaerobically, producing lactate.
Lactate is transported to the liver, where it is converted back into glucose by gluconeogenesis.
This cycle allows muscles to continue gaining ATP even when they run out of $O_2$ . Emphasizes cooperation between tissues.

Gluconeogenesis and glycolysis are regulated so that within a cell, one pathway is relatively inactive while the other is highly active.
The interconversion of fructose 1,6-bisphosphate and fructose 6-phosphate is a key regulatory site.
Additionally, glycolysis and gluconeogenesis are reciprocally regulated at the interconversion of phosphoenolpyruvate and pyruvate.
If ATP is required, glycolysis predominates. If glucose is required, gluconeogenesis is favored.

Glycolysis and the citric acid cycle provide building blocks needed for biosynthesis.
Citrate and alanine act as indicators of the presence of these building blocks.
When [citrate] and [alanine] are high, it indicates the cell has plenty of energy and can synthesize other molecules (amino acids, nucleotides etc.).

High blood glucose activates glycolysis and inhibits gluconeogenesis.
This regulation is mediated by levels of fructose 2,6-bisphosphate (F-2,6-BP).
F-2,6-BP is produced by phosphofructokinase 2 (PFK2) and hydrolyzed by fructose bisphosphatase 2 (FBPase 2).
PFK2 and FBPase 2 are different domains of the same protein, and their activity is reciprocally regulated by phosphorylation.

The PFK2/FBPase2 protein can be phosphorylated on a serine residue.
Phosphorylation switches PFK2 off and FBPase2 on.
The hormone glucagon signals low blood sugar and activates protein kinase A (PKA), which phosphorylates PFK2/FBPase2.

Type 2 diabetes is a disease in which blood glucose levels are poorly controlled.
The hormone insulin signals high blood glucose levels.
Insulin normally switches off the expression of enzymes of gluconeogenesis.
In Type 2 diabetes, cells do not respond to insulin in this way, so gluconeogenesis occurs even when glucose is high.
This can lead to dangerously high glucose levels - hyperglycemia.

Requires 3 steps different from glycolysis.
Requires 6 ATP/GTPs.
Activated by:
- Citrate and Acetyl CoA (biosynthetic building blocks)
Inhibited by:
- AMP/ADP (low energy)
- F-2,6-BP (high glucose)
Key Steps:
- Pyruvate to oxaloacetate in the mitochondrial matrix.
- Glucose 6P to Glucose in the ER lumen.

Involves 3 steps different from gluconeogenesis.
Makes 2 ATPs.
Activated by:
- AMP (low energy)
- F-2,6-BP (high glucose)
Inhibited by:
- ATP (high energy)
- Citrate/alanine (biosynthetic building blocks)
All reactions occur in the cytoplasm.