Gluconeogenesis: Glucose Synthesis

• Gluconeogenesis is the synthesis of glucose from non-carbohydrate precursors.
• It maintains blood glucose levels, particularly during starvation or in diabetes mellitus (DM).

Liver's Role

• Plays an important role in blood sugar homeostasis.
• Replenishes blood sugar because glucose-6-phosphatase is present.

Sources of Non-Carbohydrate Precursors

• Precursors such as alanine are major.
• Lactate comes from muscle and forms pyruvate.
• Glucogenic amino acids.
• Glycerol.
• Propionyl CoA.

Site of Synthesis

• Primarily occurs in the liver (90%).
• Secondarily in the kidneys (10%).
• Pathways occur partly in mitochondria and cytoplasm.
• Gluconeogenesis is not a simple reversal of glycolysis.
• It is under reciprocal control, so physiological conditions favoring one, disfavor the other.
• 2 Pyruvate + 2 NADH + 4 ATP + 2 GTP + 6 H2O results in Glucose + 2 NAD+ + 4 ADP + 2 GDP + 6 Pi + 2H+.

Key Enzymes

• Four key enzymes are involved in gluconeogenesis.
• Pyruvate Carboxylase (PC).
• PEP Carboxykinase (PEPCK).
• Fructose 1,6 Bisphosphatase.

Gluconeogenesis vs Glycolysis

• Gluconeogenesis utilizes four unique enzymes.
• These enzymes bypass three irreversible steps of glycolysis.
• The remaining seven steps use the same enzymes as glycolysis.

Bypassed Reactions and Enzymes

• Hexokinase in glycolysis is bypassed by Glucose-6-Phosphatase in gluconeogenesis.
• PFK (phosphofructokinase) in glycolysis is bypassed by Fructose-1,6-Bisphosphatase.
• Pyruvate Kinase in glycolysis is bypassed by Pyruvate Carboxylase and PEP Carboxykinase.

Obstacle One: Pyruvate to Phosphoenolpyruvate

• In glycolysis, PEP turns into pyruvate via the enzyme pyruvate kinase.
• The reverse reaction in gluconeogenesis requires two enzymes to bypass this step.
• The enzymes in gluconeogenesis are Pyruvate Carboxylase and PEP Carboxykinase.

Conversion of Pyruvate to PEP (Part One)

• The enzyme is Pyruvate Carboxylase.
• Carboxylation reactions use CO2 and the vitamin biotin as a cofactor.
• Pyruvate carboxylase is found only in the liver and kidneys, not in muscles.
• Its location is in the mitochondria, requiring 2 ATP.

Reversal of PEP to Pyruvate

• Oxaloacetate is converted to phosphoenolpyruvate by PEP Carboxykinase, consuming GTP and releasing CO2.
• Simultaneously, in the reverse direction, pyruvate is converted to oxaloacetate by pyruvate carboxylase.

GTP-Dependent Decarboxylation

• GTP-dependent decarboxylation of oxaloacetate involves PEP carboxykinase, a cytosolic enzyme.
• The reaction is: oxaloacetate + GTP to phosphoenolpyruvate + CO2 + GDP.
• GTP is used instead of ATP.
• CO2, is added last in this step.

Alternative Fates for Pyruvate

• Pyruvate can be a source of new glucose.
• It can store energy as glycogen.
• It can generate NADPH via the pentose phosphate pathway.
• Pyruvate can be a source of acetyl-CoA.
• It can store energy as body fat.
• It can make ATP via the citric acid cycle.
• Acetyl-CoA stimulates glucose synthesis by activating pyruvate carboxylase.

Obstacle Two: Fructose-1,6-bisphosphate to Fructose-6-phosphate

• The enzyme is Fructose-1,6-bisphosphatase.
• Bypasses phosphofructokinase-I.
• It is a simple hydrolysis.
• It is highly exergonic and irreversible.

Fructose-1,6-bisphosphatase

• Catalyzes the conversion of fructose-1,6-bisphosphate to fructose-6-phosphate.
• The enzyme is found in the liver.

Obstacle Three: Glucose-6-phosphate to Glucose

• The enzyme involved is glucose-6-phosphatase.
• It bypasses hexokinase.
• Highly exergonic and irreversible.
• Absent in muscle.
• G-6-Phosphatase converts G-6p to Glucose in the liver, kidney, and small intestine.

Energy Requirements in Gluconeogenesis

• Requires 6 ATP to generate one glucose molecule.
• 2 Pyruvate to 2 Oxaloacetate consumes 2 ATP.
• 2 Oxaloacetate to PEP, 2GTP=
• (2)3Phospho glycerate to (2)1,2 BPG =2ATP