Explain the steps of glycolysis. How is ATP generated during glycolysis?

Glycolysis is the metabolic pathway that converts glucose into pyruvate, generating ATP and NADH in the process. The steps of glycolysis are as follows:

1. Glucose is phosphorylated by the enzyme hexokinase to form glucose-6-phosphate.
2. Glucose-6-phosphate is converted to fructose-6-phosphate by the enzyme phosphoglucose isomerase.
3. Fructose-6-phosphate is phosphorylated by ATP to form fructose-1,6-bisphosphate, with the help of the enzyme phosphofructokinase.
4. Fructose-1,6-bisphosphate is split into two molecules of glyceraldehyde-3-phosphate by the enzyme aldolase.
5. Each molecule of glyceraldehyde-3-phosphate is oxidized and phosphorylated by NAD+ and inorganic phosphate to form 1,3-bisphosphoglycerate, generating NADH in the process. This reaction is catalyzed by the enzyme glyceraldehyde-3-phosphate dehydrogenase.
6. 1,3-bisphosphoglycerate is converted to 3-phosphoglycerate by the enzyme phosphoglycerate kinase, generating ATP in the process.
7. 3-phosphoglycerate is converted to 2-phosphoglycerate by the enzyme phosphoglycerate mutase.
8. 2-phosphoglycerate is converted to phosphoenolpyruvate by the enzyme enolase.
9. Phosphoenolpyruvate is converted to pyruvate by the enzyme pyruvate kinase, generating ATP in the process.

Overall, glycolysis generates a net of 2 ATP molecules per glucose molecule, as well as 2 molecules of NADH. The ATP is generated through substrate-level phosphorylation, where a phosphate group is transferred from a high-energy molecule to ADP to form ATP.

Steps of Glycolysis:

1. Phosphorylation of Glucose: Glucose is phosphorylated by hexokinase, utilizing ATP to form glucose-6-phosphate (G6P). This step traps glucose within the cell.

2. Isomerization of G6P: G6P is converted to fructose-6-phosphate (F6P) by glucose phosphate isomerase.

3. Second Phosphorylation: F6P is phosphorylated by phosphofructokinase-1 (PFK-1), utilizing another ATP to form fructose-1,6-bisphosphate (F1,6BP). This is the rate-limiting step of glycolysis.

4. Cleavage of F1,6BP: F1,6BP is cleaved by aldolase into two 3-carbon fragments: glyceraldehyde-3-phosphate (G3P) and dihydroxyacetone phosphate (DHAP).

5. Isomerization of DHAP: DHAP is converted to G3P by triose phosphate isomerase.

6. Oxidation and Phosphorylation of G3P: G3P is oxidized and phosphorylated by glyceraldehyde-3-phosphate dehydrogenase (GAPDH) to form 1,3-bisphosphoglycerate (1,3BPG), releasing two molecules of NADH and two molecules of ATP (substrate-level phosphorylation).

7. Isomerization of 1,3BPG: 1,3BPG is converted to 3-phosphoglycerate (3PG) by phosphoglycerate kinase, transferring one phosphate group to ADP, forming ATP (substrate-level phosphorylation).

8. Dehydration of 3PG: 3PG is dehydrated by phosphoglycerate mutase, forming 2-phosphoglycerate (2PG).

9. Kinase Reaction: 2PG is phosphorylated by enolase, utilizing phosphoenolpyruvate (PEP), forming PEP and H2O.

10. Pyruvate Kinase Reaction: PEP is transferred to ADP, forming ATP (substrate-level phosphorylation), and pyruvate is generated.

ATP Generation during Glycolysis:

- Direct generation:
- 2 molecules of ATP are generated in steps 6 and 7 through substrate-level phosphorylation.
- 2 molecules of ATP are generated in step 10 from PEP.

- Indirect generation:
- 2 molecules of NADH are produced in step 6, which can be used in the electron transport chain to generate up to 6 molecules of ATP.