Glycolysis: General Characteristics
Glycolysis is a specific pathway of glucose catabolism that occurs entirely in the cell cytosol. Depending on conditions, the end product is either pyruvate (aerobic glycolysis) or lactate (anaerobic glycolysis). The process is divided into two phases: the preparatory phase (consuming ATP) and the ATP synthesis phase.
Preparatory Phase Reactions
The first part of glycolysis proceeds with the consumption of two ATP molecules and concludes with the cleavage of a hexose into two trioses.
- Phosphorylation of glucose by hexokinase to form glucose-6-phosphate.
- Isomerization to fructose-6-phosphate mediated by glucose-6-phosphate isomerase.
- Phosphorylation by phosphofructokinase-1 to fructose-1,6-bisphosphate — the key regulatory and rate-limiting reaction of glycolysis.
- Cleavage by fructose-1,6-bisphosphate aldolase into glyceraldehyde-3-phosphate and dihydroxyacetone phosphate.
- Isomerization of dihydroxyacetone phosphate to glyceraldehyde-3-phosphate (triosephosphate isomerase).
Oxidation and ATP Synthesis Reactions
The sixth reaction is the only oxidation-reduction step: glyceraldehyde-3-phosphate dehydrogenase catalyzes the oxidation of the substrate with the incorporation of inorganic phosphate and the reduction of NAD⁺ to NADH. This yields 1,3-bisphosphoglycerate containing a high-energy bond.
- 1,3-bisphosphoglycerate → 3-phosphoglycerate: substrate-level phosphorylation, ATP synthesis (phosphoglycerate kinase).
- 3-phosphoglycerate → 2-phosphoglycerate (phosphoglycerate mutase).
- 2-phosphoglycerate → phosphoenolpyruvate (enolase, dehydration).
- Phosphoenolpyruvate → pyruvate: second substrate-level phosphorylation mediated by pyruvate kinase.
Shuttle Systems and the Fate of NADH
Since glycolysis occurs in the cytosol and NADH does not cross the mitochondrial membrane, its reducing equivalents are transferred via shuttle systems:
- Malate-aspartate shuttle: oxaloacetate is reduced to malate, which is transported into the matrix; yield is 3 ATP per NADH molecule.
- Glycerol phosphate shuttle: hydrogen is transferred to FAD in the inner membrane; yield is 2 ATP.
Under anaerobic conditions the shuttles do not operate, NAD⁺ is regenerated differently: lactate dehydrogenase reduces pyruvate to lactate using NADH. This allows glycolysis to continue during oxygen deficiency.
Energy Balance
- Anaerobic glycolysis: 2 × 2 ATP (substrate-level phosphorylation) − 2 ATP (investment) = 2 ATP.
- Aerobic glycolysis to pyruvate: 2 × (2 + 3) − 2 = 8 ATP (accounting for the malate-aspartate shuttle).
- Complete aerobic oxidation of glucose to CO₂ and H₂O: 8 ATP (glycolysis) + 2 × 15 ATP (oxidative decarboxylation of pyruvate + TCA cycle) = 38 ATP.
Six dehydrogenation reactions occur during complete catabolism: one in glycolysis, one in the oxidative decarboxylation of pyruvate, and four in the tricarboxylic acid cycle.
Regulation of glycolysis
Regulation takes place at three irreversible steps:
- Muscle hexokinase is inhibited by glucose-6-phosphate (its product).
- Phosphofructokinase-1 is the key regulator; inhibited by ATP and NADH (energy excess), activated by AMP and ADP.
- Pyruvate kinase is inhibited by ATP and activated when the cellular energy charge is low.
The principle is uniform: when energy is in excess, glycolysis (along with the TCA cycle and pyruvate dehydrogenase complex) is inhibited; during energy deficit, it is activated.
Anabolic Significance of Glycolytic Metabolites
- Fructose-6-phosphate and glyceraldehyde-3-phosphate → synthesis of ribose-5-phosphate for nucleotides.
- 3-phosphoglycerate → synthesis of non-essential amino acids (serine, glycine, cysteine).
- Dihydroxyacetone phosphate → synthesis of triacylglycerols.
- Acetyl-CoA (from pyruvate) → synthesis of fatty acids, cholesterol, ketone bodies.
- Erythrocytes utilize the alternative bisphosphoglycerate mutase pathway: 1,3-BPG → 2,3-bisphosphoglycerate, an allosteric regulator of hemoglobin that decreases its oxygen affinity.
Pentose phosphate pathway
The pentose phosphate pathway is an alternative pathway for glucose-6-phosphate oxidation in the cytosol. It does not yield ATP, but provides the cell with NADPH and ribose-5-phosphate. It is active in the liver, adipose tissue, adrenal glands, erythrocytes, and lactating mammary glands.
Oxidative Phase:
- Glucose-6-phosphate → 6-phosphogluconolactone via glucose-6-phosphate dehydrogenase (a regulatory enzyme induced by insulin); NADP⁺ is reduced.
- Hydrolysis of the lactone by gluconolactonase to 6-phosphogluconate.
- Oxidative decarboxylation of 6-phosphogluconate by 6-phosphogluconate dehydrogenase yielding ribulose-5-phosphate and a second NADPH molecule.
The non-oxidative phase involves reversible reactions catalyzed by transketolase (cofactor: TPP, a vitamin B₁ derivative) and transaldolase, which transfer two- and three-carbon units. This allows the interconversion of pentoses and glycolytic metabolites (fructose-6-phosphate, glyceraldehyde-3-phosphate) according to cellular demands.
Glutathione and Erythrocyte Antioxidant Defense
In erythrocytes, NADPH is required to maintain glutathione in its reduced form. Glutathione is a tripeptide (γ-glutamylcysteinylglycine) containing a cysteine SH group.
- Glutathione peroxidase uses reduced glutathione (G-SH) to neutralize H₂O₂, converting it to water while oxidizing glutathione to G-S-S-G.
- Glutathione reductase regenerates G-SH using NADPH.
- Reduced glutathione protects the SH groups of hemoglobin cysteine residues from oxidation.
In hereditary glucose-6-phosphate dehydrogenase deficiency, NADPH production drops, leading to decreased levels of reduced glutathione. Disulfide bonds form in hemoglobin, and denatured protomers aggregate into Heinz bodies, resulting in erythrocyte hemolysis (hemolytic anemia).
Key points
- Glycolysis occurs in the cytosol; the aerobic pathway yields pyruvate, while the anaerobic pathway yields lactate to regenerate NAD⁺.
- The main regulatory enzyme is phosphofructokinase-1, controlled by the ATP/AMP ratio.
- Energy yield: 2 ATP (anaerobic), 8 ATP (aerobic to pyruvate), 38 ATP (complete oxidation to CO₂ and H₂O).
- Cytosolic NADH enters mitochondria via the malate-aspartate (3 ATP) or glycerol phosphate (2 ATP) shuttles.
- Glycolytic metabolites serve as precursors for amino acids, nucleotides, and lipids; 2,3-BPG regulates hemoglobin oxygen affinity.
- The pentose phosphate pathway supplies NADPH for biosyntheses and antioxidant defense, alongside ribose-5-phosphate for nucleotides.
- The key regulatory enzyme of the pentose phosphate pathway is glucose-6-phosphate dehydrogenase, which is induced by insulin.
- G6PD deficiency impairs glutathione regeneration, causing the formation of Heinz bodies and erythrocyte hemolysis.