Glycolysis is a fundamental metabolic pathway that breaks down glucose into pyruvate, generating energy in the form of ATP and NADH.
Overview of Glycolysis
Glycolysis can be broken down into several key steps. According to the reference, the first few steps involve reactions that are irreversible. The next five steps produce energy; specifically, one ATP is made per glyceraldehyde 3-phosphate in reactions 7.
Steps of Glycolysis
While the reference only mentions some of the steps, a complete overview of glycolysis is beneficial to understanding the entire process.
- Glucose Phosphorylation: Glucose is phosphorylated by hexokinase (or glucokinase in the liver) to form glucose-6-phosphate (G6P), consuming one ATP. This step traps glucose inside the cell and destabilizes it for further reactions.
- Isomerization: G6P is isomerized to fructose-6-phosphate (F6P) by phosphoglucose isomerase.
- Second Phosphorylation: F6P is phosphorylated by phosphofructokinase-1 (PFK-1) to form fructose-1,6-bisphosphate (F1,6BP), consuming another ATP. PFK-1 is a key regulatory enzyme in glycolysis.
- Cleavage: F1,6BP is cleaved by aldolase into two 3-carbon molecules: dihydroxyacetone phosphate (DHAP) and glyceraldehyde-3-phosphate (G3P).
- Isomerization of DHAP: DHAP is converted to G3P by triose phosphate isomerase, ensuring that each molecule of glucose yields two molecules of G3P for the subsequent steps.
- Oxidation and Phosphorylation: G3P is oxidized and phosphorylated by glyceraldehyde-3-phosphate dehydrogenase (GAPDH) to form 1,3-bisphosphoglycerate (1,3-BPG). This reaction produces NADH.
- ATP Generation: 1,3-BPG transfers its high-energy phosphate group to ADP, forming ATP and 3-phosphoglycerate (3-PG). This reaction is catalyzed by phosphoglycerate kinase. This is one of the reactions that produces ATP, as mentioned in the reference.
- Phosphate Group Shift: 3-PG is converted to 2-phosphoglycerate (2-PG) by phosphoglycerate mutase.
- Dehydration: 2-PG is dehydrated by enolase to form phosphoenolpyruvate (PEP).
- Pyruvate Formation: PEP transfers its high-energy phosphate group to ADP, forming ATP and pyruvate. This reaction is catalyzed by pyruvate kinase.
Energy Yield
- ATP Consumption: 2 ATP are consumed in the early steps (steps 1 and 3).
- ATP Production: 4 ATP are produced (2 in step 7 and 2 in step 10) per glucose molecule.
- Net ATP Gain: 2 ATP per glucose molecule.
- NADH Production: 2 NADH molecules are produced (in step 6).
Irreversible Steps
The reference highlights that some reactions are irreversible. Key irreversible steps include:
- Hexokinase (or glucokinase) reaction
- Phosphofructokinase-1 (PFK-1) reaction
- Pyruvate kinase reaction
Regulation of Glycolysis
Glycolysis is tightly regulated to meet the energy needs of the cell. Key regulatory enzymes include:
- Hexokinase/Glucokinase: Inhibited by G6P.
- Phosphofructokinase-1 (PFK-1): Activated by AMP and fructose-2,6-bisphosphate; inhibited by ATP and citrate.
- Pyruvate Kinase: Activated by fructose-1,6-bisphosphate; inhibited by ATP and alanine.
