Gluconeogenesis and the Reciprocal Regulation of Opposing Pathways
Gluconeogenesis bypasses glycolysis's three effectively irreversible reactions using different enzymes and energy-coupled chemistry. Reciprocal regulation can limit wasteful same-cell substrate cycling. Different tissues can carry out glucose breakdown and glucose production simultaneously.
What a learner can do afterwards
- Matches the three effectively irreversible glycolytic reactions to their gluconeogenic bypass enzymes and explains why alternative chemistry is needed.
- Explains reciprocal PFK-1 and fructose-1,6-bisphosphatase regulation in liver cells.
- Calculates ATP consumption for a defined equal-flux PFK-1/fructose-1,6-bisphosphatase cycle.
1 · Read
Direct reversal of glycolysis's three far-from-equilibrium reactions is unfavourable under usual cell conditions. Gluconeogenesis uses alternative reactions, including energy-coupled steps, instead. Many other reactions use the same enzymes in reverse. The liver uses gluconeogenesis during fasting to help maintain blood glucose. Separate bypass enzymes also allow separate regulation.
Glucose-6-phosphatase (G6Pase) converts glucose-6-phosphate to free glucose, bypassing hexokinase/glucokinase. Fructose-1,6-bisphosphatase (FBPase) converts fructose-1,6-bisphosphate to fructose-6-phosphate, bypassing PFK-1. Pyruvate carboxylase (PC) makes oxaloacetate from pyruvate; PEP carboxykinase (PEPCK) then makes phosphoenolpyruvate (PEP). Together, PC and PEPCK bypass the one pyruvate kinase reaction.
Lactate, glycerol and glucogenic amino acids can supply glucose carbon. Triglyceride glycerol can enter gluconeogenesis. Carbon supplied only as acetyl CoA from even-chain fatty acids cannot give net glucose in humans. Odd-chain fatty acids also produce a propionyl-CoA component that can supply glucogenic carbon. Fat is therefore not a single metabolic category.
In the Cori cycle, muscle converts glucose to lactate. Blood carries lactate to the liver, which can make glucose and return it to the circulation. Breakdown and synthesis can thus occur simultaneously in different tissues. This coordinated, energy-consuming cycle differs from unregulated opposing reactions within one cell.
PFK-1 uses one ATP to convert fructose-6-phosphate to fructose-1,6-bisphosphate. FBPase returns it to fructose-6-phosphate and releases inorganic phosphate. One paired turn consumes one ATP with no net sugar conversion. Equal opposing flux gives such cycling. Multiply paired turns by one ATP: 4 turns consume 4 ATP.
In liver cells, fructose 2,6-bisphosphate activates PFK-1 and inhibits FBPase. This reciprocal control limits wasteful cycling rather than eliminating every opposing flux. Hormones can rapidly change existing enzyme activity through phosphorylation. They can also change enzyme production over longer times. Check the tissue, enzyme and timescale when interpreting a signal.
Alternative reactions bypass unfavourable glycolytic reversals. Reciprocal regulation limits same-cell cycling, while different tissues can coordinate glucose use and production.
Lesson, questions and flashcards written for this dataset.
Readings and flashcards written for this dataset.
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