1. Initial Phosphorylation:
Glucose is first phosphorylated by the enzyme hexokinase, using one molecule of ATP. This forms glucose-6-phosphate (G6P).
2. Isomerization:
G6P is converted into its isomer, fructose-6-phosphate (F6P) by the enzyme phosphoglucose isomerase.
3. Second Phosphorylation:
F6P undergoes a second phosphorylation by phosphofructokinase-1 (PFK-1). This reaction, which uses another molecule of ATP, results in the formation of fructose-1,6-bisphosphate (FBP).
4. Fructose Cleavage:
The six-carbon FBP is then split into two three-carbon molecules: glyceraldehyde-3-phosphate (G3P) and dihydroxyacetone phosphate (DHAP) by the enzyme aldolase.
5. Isomerization:
DHAP is readily isomerized to G3P by the enzyme triose phosphate isomerase.
6. Oxidation:
The G3P molecules undergo oxidative reactions to form 1,3-bisphosphoglycerate (BPG) by the enzymes glyceraldehyde-3-phosphate dehydrogenase. This process also generates two molecules of NADH (nicotinamide adenine dinucleotide) for every glucose molecule.
7. Transfer of Phosphate:
The high-energy phosphate group from BPG is then transferred to ADP, forming ATP, through a substrate-level phosphorylation catalyzed by phosphoglycerate kinase. This step generates two molecules of ATP for each glucose molecule.
8. Isomerization:
The 3-phosphoglycerate (3-PGA) molecules produced in the previous step are isomerized into 2-phosphoglycerate (2-PGA) by phosphoglyceromutase.
9. Dehydration:
The enzyme enolase removes water from 2-PGA to form phosphoenolpyruvate (PEP), generating two molecules of water in the process.
10. Transfer of Phosphate:
PEP subsequently donates its phosphate group to ADP, forming a third molecule of ATP for each glucose molecule. This step is catalyzed by pyruvate kinase, resulting in the production of pyruvate.
In brief, ten reactions occur in glycolysis, including phosphorylation, isomerizations, cleavages, oxidations, and substrate-level phosphorylations. This process allows the conversion of one glucose molecule into two pyruvate molecules, while also generating a net of two ATP molecules and two NADH molecules, which serve as electron carriers in cellular respiration. The NADH and ATP produced during glycolysis will play crucial roles in subsequent metabolic pathways, such as the citric acid cycle (Krebs cycle) and oxidative phosphorylation.
