Gluconeogenesis is the biosynthesis of glucose from non-carbohydrate precursors. It is essentially the reverse of glycolysis, occurring primarily in the liver and kidneys during fasting, starvation, or intense exercise.

How Gluconeogenesis Works

Precursors

The main precursors for gluconeogenesis are lactate (from anaerobic glycolysis in muscle), amino acids (especially alanine from muscle protein breakdown), and glycerol (from fat breakdown). These molecules enter the pathway at various points.

Bypassing Irreversible Steps

Glycolysis has three irreversible steps that must be bypassed in gluconeogenesis, catalyzed by different enzymes. Pyruvate is converted to oxaloacetate by pyruvate carboxylase, then to phosphoenolpyruvate by PEP carboxykinase (bypassing pyruvate kinase). Fructose-1,6-bisphosphate is converted to fructose-6-phosphate by fructose-1,6-bisphosphatase (bypassing PFK-1). Glucose-6-phosphate is converted to glucose by glucose-6-phosphatase (bypassing hexokinase).

Energy Cost

Gluconeogenesis is energetically expensive. Synthesizing one molecule of glucose requires 4 ATP, 2 GTP, and 2 NADH.

Regulation

Gluconeogenesis and glycolysis are reciprocally regulated to prevent a futile cycle. When energy is abundant, glycolysis is inhibited and gluconeogenesis is activated. The hormones glucagon and cortisol stimulate gluconeogenesis, while insulin inhibits it.

The Cori Cycle

During intense exercise, muscles produce lactate through anaerobic glycolysis. The lactate is released into the blood and taken up by the liver, which converts it back to glucose via gluconeogenesis. The glucose returns to the muscles, completing the Cori cycle.