Problem Set 10, Problem 3: Gluconeogenesis

TL;DR

This analysis discusses the spontaneous nature of gluconeogenesis, focusing on the aldolase reaction and the GAP dehydrogenase reaction. It also explains the key differences between glycolysis and gluconeogenesis.

Transcript

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Key Insights

• 🚱 Gluconeogenesis is the reverse process of glycolysis, where non-sugar precursors are used to produce glucose.
• ❓ The aldolase reaction in gluconeogenesis involves the covalent binding of dihydroxyacetone phosphate and glyceraldehyde 3-phosphate to regenerate fructose 1,6 bisphosphate.
• ❓ The GAP dehydrogenase reaction in gluconeogenesis requires NADH as a cofactor to convert 1,3 bisphosphoglycerate to glyceraldehyde 3-phosphate.
• 💁 Gluconeogenesis is a spontaneous pathway, but it requires an energy cost in the form of ATP to run in reverse.
• ❓ Glycolysis is also a spontaneous process that generates ATP, providing energy for cellular processes.
• 😒 Gluconeogenesis uses partially reverse steps from glycolysis, along with alternate enzymatic reactions, to convert pyruvate back to glucose.

Questions & Answers

Q: How does the aldolase reaction in gluconeogenesis differ from glycolysis?

In gluconeogenesis, the aldolase reaction proceeds in reverse, starting with dihydroxyacetone phosphate and glyceraldehyde 3-phosphate to regenerate fructose 1,6 bisphosphate. This is different from glycolysis, where fructose 1,6 bisphosphate is broken down into dihydroxyacetone phosphate and glyceraldehyde 3-phosphate.

Q: What is the role of NADH in the GAP dehydrogenase reaction in gluconeogenesis?

In gluconeogenesis, NADH is used as a cofactor to reduce 1,3 bisphosphoglycerate to glyceraldehyde 3-phosphate. The hydride (H-) from NADH acts as a nucleophile, attacking the thioester of 1,3 bisphosphoglycerate and forming a tetrahedral intermediate.

Q: How does gluconeogenesis differ from glycolysis in terms of spontaneity?

Glycolysis is a spontaneous process that generates ATP, while gluconeogenesis requires ATP and is also spontaneous but involves an energy cost. Glycolysis proceeds from glucose to pyruvate, while gluconeogenesis reverses the steps of glycolysis to convert pyruvate back to glucose.

Q: Are all the steps in gluconeogenesis the reverse of the steps in glycolysis?

While most of the steps in gluconeogenesis are the reverse of the steps in glycolysis, some steps, such as the conversion of pyruvate to phosphoenolpyruvate, involve alternate enzymatic reactions to make the process spontaneous. Phosphatases are also used in gluconeogenesis to remove phosphates without generating ATP.

Summary & Key Takeaways

• The analysis explores the mechanism of the aldolase reaction in gluconeogenesis, highlighting the role of an active site lysine residue in covalently binding substrates.

• It also examines the mechanism of the GAP dehydrogenase reaction in gluconeogenesis, involving the formation of a covalent intermediate and the role of NADH.

• The analysis concludes by discussing the spontaneity of gluconeogenesis as a whole pathway, comparing it to glycolysis and emphasizing the energy cost associated with gluconeogenesis.