Stages of ATP production in glycolysis
Glycolysis takes place in the cell cytosol and includes reactions that ensure the synthesis of high-energy compounds. At the energy synthesis stage, two main pathways of ATP molecule production are distinguished:
- Substrate-level phosphorylation. This is the direct transfer of a phosphate group from a high-energy substrate to ADP. In glycolysis, this mechanism is implemented twice:
- During the conversion of 1,3-bisphosphoglycerate to 3-phosphoglycerate (mediated by the enzyme phosphoglycerate kinase).
- During the conversion of phosphoenolpyruvate to pyruvate (catalyzed by pyruvate kinase).
- Oxidative phosphorylation. Energy is synthesized in mitochondria via the transfer of electrons and protons along the electron transport chain (ETC). In the cytosol, the enzyme glyceraldehyde-3-phosphate dehydrogenase oxidizes the substrate, producing NADH. Reducing equivalents are transferred to the mitochondrial matrix for subsequent energy generation.
Malate-aspartate shuttle system
The mitochondrial membrane is impermeable to NADH produced in the cytosol. To transfer hydrogen atoms into the respiratory chain, a specialized mechanism is used — the malate-aspartate shuttle.
The process works as follows:
- The cytosolic enzyme malate dehydrogenase reduces oxaloacetate to malate, consuming NADH.
- Carrier proteins (translocases) transport malate into the mitochondrion.
- In the matrix, the mitochondrial isoform of the enzyme oxidizes malate back to oxaloacetate, reducing NAD+ to NADH.
- Oxaloacetate is converted into aspartate via transaminases and returned to the cytosol.
The net result of the shuttle's operation is the delivery of reduced NADH to the ETC, yielding an energy output of 3 ATP molecules.
Energy balance of complete breakdown
Complete aerobic catabolism of glucose to carbon dioxide and water consists of two major stages. Efficiency is calculated using classical coefficients ($NADH = 3$ ATP, $FADH_2 = 2$ ATP).
- Aerobic glycolysis. The breakdown of one glucose molecule yields 2 pyruvate molecules. The net yield is 2 ATP, while another 2 NADH molecules transported via the shuttle system yield 6 ATP. In total — 8 mol ATP.
- Pyruvate oxidation in the Common Catabolic Pathway (CCP). Oxidative decarboxylation of one pyruvate molecule to Acetyl-CoA yields 1 NADH (3 ATP). Subsequent combustion of Acetyl-CoA in the Krebs cycle yields 12 ATP. This totals 15 ATP per pyruvate. Since two pyruvates are formed from glucose, this stage yields 30 mol ATP.
The total energy yield is 38 mol ATP per each oxidized glucose molecule.
Lactate metabolism and regulation
Under anaerobic conditions, the end product of breakdown is lactic acid — lactate. It is formed from phosphoenolpyruvate (via pyruvate) with the participation of enzymes and NADH. Lactate leaves the tissues where it was synthesized and is transported to the liver or myocardium.
In cardiac muscle, lactic acid is converted back into pyruvate, which is burned in the CCP, providing energy and maintaining normal blood pH. At rest, blood lactate levels are about 1 mmol/L, but during heavy physical exertion, they can exceed 15 mmol/L. Pathological accumulation of lactate leads to a dangerous condition — lactic acidosis.
The rate of glycolysis is closely linked to the state of the citric acid cycle (TCA) and the ETC. If the cell has an abundance of energy (ATP and NADH accumulate), the rates of the citrate cycle and glycolysis decrease. The main rate-limiting enzyme — phospho-fructokinase-1 — is inhibited by high concentrations of ATP and citrate, and activated by ADP, AMP, and fructose-2,6-bisphosphate.
Briefly: answers to topical questions
What shuttle systems, other than the malate-aspartate system, are used to transport cytosolic NADH into mitochondria?
In addition to the malate-aspartate system, the glycerophosphate shuttle system is used to transport cytosolic NADH into mitochondria.
- Glycerophosphate shuttle system is a mechanism for hydrogen transport across the membrane using dihydroxyacetone phosphate and glycerol-3-phosphate.
In the cytosol, glycerol-3-phosphate dehydrogenase uses NADH to reduce dihydroxyacetone phosphate to glycerol-3-phosphate. A translocase transports it across the inner mitochondrial membrane. Within the mitochondrion, an isoenzyme oxidizes glycerol-3-phosphate back, reducing the coenzyme FAD to $FADH_2$. The resulting $FADH_2$ is oxidized in the electron transport chain, providing an energy yield of 2 ATP molecules.
Which enzymes catalyze the three irreversible reactions of glycolysis?
The three irreversible reactions of glycolysis are catalyzed by hexokinase, phosphofructokinase-1, and pyruvate kinase.
- Hexokinase (or glucokinase) catalyzes the first phosphorylation reaction of glucose to form glucose-6-phosphate.
- Phosphofructokinase-1 catalyzes the second phosphorylation (conversion of fructose-6-phosphate to fructose-1,6-bisphosphate).
- Pyruvate kinase catalyzes the substrate-level phosphorylation reaction of phosphoenolpyruvate to form pyruvate.
These specific enzymes serve as key regulatory checkpoints targeted by factors that alter the rate of glycolysis.
How is lactic acid utilized in the liver as part of the Cori cycle?
Lactic acid is utilized in the liver by being oxidized to pyruvate and subsequently used in gluconeogenesis for glucose synthesis.
Lactate enters hepatocytes from the bloodstream. Since the NADH/NAD⁺ ratio in the liver is lower than in muscles, the lactate dehydrogenase reaction shifts: lactate is converted into pyruvate. The resulting pyruvate enters gluconeogenesis, where it is synthesized into glucose. Glucose then enters the bloodstream and returns to working muscles and erythrocytes, completing the Cori cycle.
Class and exam questions
How much ATP is produced during aerobic glycolysis?
During the breakdown of a glucose molecule into two pyruvate molecules under aerobic conditions, 8 mol of ATP are formed.
Which enzymes are involved in substrate-level phosphorylation?
In glycolysis, these are phosphoglycerate kinase and pyruvate kinase. They synthesize ATP directly by transferring a phosphate from a substrate to ADP.
How does cytosolic NADH enter the mitochondrion?
NADH itself does not cross the membrane. Hydrogen is transported via shuttle systems, such as the malate-aspartate system, where malate acts as the carrier.
What else to study on this topic
- Features of the regulation of glycolytic enzymes in the liver
- Mechanisms of oxidative decarboxylation of pyruvate
- Allosteric regulation of Krebs cycle enzymes
- Functions of coenzymes and redox systems (NADPH, glutathione)
- Interrelation of glycolysis with fatty acid synthesis
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