Glycolysis is the central pathway for the breakdown of glucose, a six-carbon sugar, into two molecules of pyruvate, a three-carbon compound. This process occurs in the cytoplasm of cells and represents the first stage of cellular respiration. Glycolysis involves a series of enzymatic reactions, and each step is crucial for the eventual production of ATP and reduced coenzymes. Let's go through glycolysis with chemical reactions:
1. Glucose Priming:
- Glucose is phosphorylated to form glucose-6-phosphate using one ATP molecule. This reaction is catalyzed by hexokinase.
- Chemical reaction:
- Glucose + ATP → Glucose-6-phosphate + ADP
2. Isomerization:
- Glucose-6-phosphate is converted to fructose-6-phosphate through an isomerization reaction. This step is catalyzed by the enzyme phosphoglucose isomerase.
- Chemical reaction:
- Glucose-6-phosphate ⇌ Fructose-6-phosphate
3. Phosphorylation:
- Fructose-6-phosphate is phosphorylated using another ATP molecule to form fructose-1,6-bisphosphate. This reaction is catalyzed by phosphofructokinase.
- Chemical reaction:
- Fructose-6-phosphate + ATP → Fructose-1,6-bisphosphate + ADP
4. Cleavage:
- Fructose-1,6-bisphosphate is cleaved into two three-carbon molecules: dihydroxyacetone phosphate (DHAP) and glyceraldehyde-3-phosphate (G3P). The enzyme aldolase catalyzes this reaction.
- Chemical reaction:
- Fructose-1,6-bisphosphate → DHAP + G3P
5. Isomerization (Interconversion):
- DHAP is isomerized to form another molecule of G3P. The enzyme triose phosphate isomerase catalyzes this reaction.
- Chemical reaction:
- DHAP ⇌ G3P
Now, we have two molecules of glyceraldehyde-3-phosphate (G3P) moving forward in the glycolytic pathway.
6. Oxidation and ATP Formation:
- Each G3P molecule is oxidized, and NAD⁺ is reduced to NADH. Simultaneously, a phosphate group is added to each G3P, forming 1,3-bisphosphoglycerate. This reaction is catalyzed by glyceraldehyde-3-phosphate dehydrogenase.
- Chemical reaction (for one G3P molecule):
- G3P + NAD⁺ + Pi + ADP → 1,3-bisphosphoglycerate + NADH + ATP
7. ATP Generation:
- 1,3-bisphosphoglycerate is converted into 3-phosphoglycerate, with the concomitant transfer of a high-energy phosphate group to ADP, forming ATP. This reaction is catalyzed by phosphoglycerate kinase.
- Chemical reaction:
- 1,3-bisphosphoglycerate + ADP → 3-phosphoglycerate + ATP
8. Substrate-Level Phosphorylation:
- 3-phosphoglycerate is converted into 2-phosphoglycerate, and then into phosphoenolpyruvate (PEP). The conversion of PEP to pyruvate leads to the transfer of another high-energy phosphate group to ADP, forming ATP. These reactions are catalyzed by phosphoglycerate mutase and enolase, respectively.
- Chemical reactions:
- 3-phosphoglycerate ⇌ 2-phosphoglycerate
- 2-phosphoglycerate → Phosphoenolpyruvate (PEP)
- PEP + ADP → Pyruvate + ATP
At the end of glycolysis, one glucose molecule has been converted into two molecules of pyruvate, and a net of two ATP molecules has been generated (two ATP molecules were consumed in the early steps, and four ATP molecules were produced later, resulting in a net gain of two ATP molecules per glucose molecule). Additionally, two molecules of NAD⁺ have been reduced to two molecules of NADH. The pyruvate produced in glycolysis can then enter the next stages of cellular respiration, including the Krebs Cycle and oxidative phosphorylation.