Overview of Glycolysis

Summary

This lecture delves into glycolysis, the metabolic pathway that converts glucose into pyruvate, ATP, and NADH. It's a three-stage process occurring in the cytoplasm, with stage one preparing glucose for cleavage, stage two breaking it into two three-carbon molecules, and stage three harvesting energy to form ATP. The lecture emphasizes the purpose of each stage, the enzymes involved, and the energy changes, concluding that glycolysis yields two ATP molecules per glucose molecule.

Takeaways

• 📚 Glycolysis is the metabolic pathway that breaks down glucose into pyruvate, ATP, and NADH molecules, occurring in the cell's cytoplasm.
• 🔑 Glycolysis is divided into three stages for clarity and functional specificity: Stage 1 (3 steps), Stage 2 (2 steps), and Stage 3 (5 steps).
• 🔄 Stage 1 focuses on trapping glucose within the cell and preparing it for cleavage in Stage 2, involving the addition of phosphate groups to destabilize the molecule.
• 🔬 Hexokinase initiates Stage 1 by phosphorylating glucose to form glucose 6-phosphate, using ATP and creating ADP and a hydrogen ion.
• 🔄 Stage 2, the 'cleavage stage,' involves the transformation of fructose 1,6-bisphosphate into two identical three-carbon molecules: DHAP and G3P, facilitated by aldolase.
• 🔄 Stage 3 is the 'energy-harvesting stage,' where ATP is produced and pyruvate is formed, with a focus on transferring high-energy phosphate groups.
• ⚡️ The conversion of G3P to 1,3-BPG in Stage 3 is crucial as it increases the molecule's potential to transfer a high-energy phosphate group.
• 🔋 The net result of glycolysis is the production of two ATP molecules, two NADH molecules, and two pyruvate molecules per glucose molecule, considering the energy invested and harvested.
• ⚖️ The overall process is energy-efficient as it yields a net gain of two ATP molecules after accounting for the two ATP molecules used in the initial phosphorylation steps.
• 🌟 Glycolysis is a fundamental metabolic pathway essential for energy production in cells, even under anaerobic conditions.

Q & A

What is glycolysis and where does it occur in the cell?

Glycolysis is the metabolic process that breaks down glucose into pyruvate molecules, ATP molecules, and NADH molecules. It takes place in the cytoplasm of the cell.

How many stages are there in glycolysis and what is the purpose of each stage?

Glycolysis is typically divided into three stages. Stage 1 prepares the glucose molecule for cleavage, Stage 2 involves the cleavage of glucose into two three-carbon molecules, and Stage 3 harvests energy to form ATP molecules.

What is the role of hexokinase in glycolysis?

Hexokinase is the enzyme that phosphorylates glucose, forming glucose 6-phosphate. This step traps the glucose molecule inside the cell and makes it more reactive for the subsequent steps.

Why is glucose 6-phosphate converted into fructose 6-phosphate in glycolysis?

Glucose 6-phosphate is converted into fructose 6-phosphate to create a symmetrical molecule, which is necessary for the production of two identical three-carbon molecules in Stage 2.

What is the significance of the second phosphorylation step in Stage 1 of glycolysis?

The second phosphorylation step in Stage 1, catalyzed by phosphofructokinase, further destabilizes the glucose molecule, making it more reactive and preparing it for cleavage in Stage 2.

What are the two three-carbon molecules produced from the cleavage of fructose 1,6-bisphosphate?

The two three-carbon molecules produced from the cleavage of fructose 1,6-bisphosphate are dihydroxyacetone phosphate (DHAP) and glyceraldehyde 3-phosphate (G3P).

Why is DHAP converted into G3P in glycolysis?

DHAP is converted into G3P because both are trioses (three-carbon sugars), and this conversion is necessary for the continuation of glycolysis, as G3P is the molecule that participates in the energy-harvesting reactions in Stage 3.

What is the role of GAPDH in the sixth step of glycolysis?

GAPDH (glyceraldehyde 3-phosphate dehydrogenase) catalyzes the reaction that converts G3P into 1,3-BPG (1,3-bisphosphoglycerate), using NAD+ and inorganic phosphate, and producing NADH and ATP.

How does the conversion of 3-phosphoglycerate to 2-phosphoglycerate in glycolysis affect the molecule's stability?

The conversion of 3-phosphoglycerate to 2-phosphoglycerate by phosphoglycerate mutase destabilizes the molecule, making it more reactive and preparing it for the subsequent reaction that forms phosphoenolpyruvate (PEP).

What is the final product of glycolysis and how many ATP molecules are produced per glucose molecule?

The final products of glycolysis are pyruvate molecules and a net gain of 2 ATP molecules per glucose molecule, considering the initial investment of 2 ATP molecules in Stage 1.