General characteristics of glycolysis
Glycolysis is the sequential breakdown of one glucose molecule (6 carbon atoms) into two pyruvate molecules (3 carbon atoms each). The process occurs in the cytoplasm and includes 10 enzymatic reactions. α-D-glucose is used as the substrate.
The ultimate fate of pyruvate determines the type of glycolysis:
- Aerobic glycolysis — in the presence of oxygen, pyruvate is transported into mitochondria, where it undergoes oxidative decarboxylation to acetyl-CoA, then enters the Krebs cycle, while the reduced coenzymes NADH and FADH₂ are oxidized in the electron transport chain (ETC).
- Anaerobic glycolysis — in the absence of oxygen, pyruvate is reduced to lactate (lactic fermentation). Alcoholic fermentation is not considered in human biochemistry.
Preparatory phase (reactions 1–5)
The first phase requires energy investment: 2 ATP molecules are consumed. The goal is to produce two symmetrical phosphotrioses.
- Phosphorylation of glucose at the 6th carbon atom to form glucose-6-phosphate. The enzyme is hexokinase (in the liver — glucokinase). 1 ATP is consumed. Phosphorylation traps glucose inside the cell.
- Isomerization of glucose-6-phosphate (aldose) into fructose-6-phosphate (ketose). The enzyme is phosphoglucoisomerase. Only the type of sugar changes.
- Second phosphorylation to form fructose-1,6-bisphosphate. The enzyme is phosphofructokinase-1 (a key regulatory enzyme). 1 ATP is consumed.
- Aldolytic cleavage of the molecule into two phosphotrioses — dihydroxyacetone phosphate and glyceraldehyde-3-phosphate. The enzyme is aldolase.
- Isomerization of dihydroxyacetone phosphate into glyceraldehyde-3-phosphate. The enzyme is triosephosphate isomerase. Two molecules of glyceraldehyde-3-phosphate enter the subsequent reactions.
Payoff phase (reactions 6–10)
In the second phase, ATP is synthesized via substrate-level phosphorylation.
- Oxidative phosphorylation of glyceraldehyde-3-phosphate involving NAD⁺ and inorganic phosphate. The enzyme is glyceraldehyde-3-phosphate dehydrogenase. 1,3-bisphosphoglycerate and NADH are formed.
- Transfer of the phosphate group from 1,3-bisphosphoglycerate to ADP. The enzyme is phosphoglycerate kinase. ATP and 3-phosphoglycerate are formed.
- Intramolecular phosphate transfer from the 3rd to the 2nd position. The enzyme is phosphoglycerate mutase (the substrate name does not change, the position of the group changes).
- Dehydration of 2-phosphoglycerate to form phosphoenolpyruvate. The enzyme is enolase.
- Transfer of the phosphate group to ADP, yielding pyruvate and ATP. The enzyme is pyruvate kinase.
Differences between aerobic and anaerobic pathways
Under aerobic conditions, NADH generated in the 6th reaction transfers protons and electrons into mitochondria via shuttle mechanisms and yields 2.5 ATP in the ETC (3 ATP according to the old scheme).
Under anaerobic conditions, the ETC does not function, and NADH must be reoxidized in the cytoplasm; otherwise, the 6th reaction of glycolysis will stop. To achieve this, the 11th reaction is added: reduction of pyruvate to lactate by lactate dehydrogenase using NADH. NAD⁺ required for the continuation of glycolysis is regenerated.
Calculation of energy yield
Aerobic glycolysis (glucose → 2 pyruvate):
- Preparatory phase costs: −2 ATP.
- Per each phosphotriose: 2.5 ATP (NADH in ETC) + 1 ATP (phosphoglycerate kinase) + 1 ATP (pyruvate kinase) = 4.5 ATP.
- For two trioses: 4.5 × 2 = +9 ATP.
- Total: 9 − 2 = +7 ATP (+8 ATP according to the old scheme).
Anaerobic glycolysis (glucose → 2 lactate):
- Costs: −2 ATP.
- NADH is consumed in the reduction of pyruvate and does not participate in ATP synthesis.
- Substrate-level phosphorylation: 2 × (1 + 1) = +4 ATP.
- Total: 4 − 2 = +2 ATP.
Key points
- Glycolysis consists of 10 reactions and occurs in the cytoplasm of all body cells.
- The preparatory phase requires an expenditure of 2 ATP to activate glucose and form fructose-1,6-bisphosphate.
- The regulatory enzymes of glycolysis are hexokinase, phosphofructokinase-1, and pyruvate kinase.
- The difference between aerobic and anaerobic glycolysis is determined by the fate of pyruvate and NADH, rather than the glucose cleavage reactions.
- Anaerobic glycolysis includes the 11th reaction — the reduction of pyruvate to lactate with the regeneration of NAD⁺.
- The energy yield of aerobic glycolysis up to pyruvate is +7 ATP, and of anaerobic glycolysis, +2 ATP.
- Under aerobic conditions, pyruvate is further oxidized to acetyl-CoA and enters the Krebs cycle, generating NADH and FADH₂ for the ETC.
- Substrate-level phosphorylation occurs in the phosphoglycerate kinase and pyruvate kinase reactions.