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The free energy released in this process is used to form the high-energy molecules adenosine triphosphate (ATP) and reduced nicotinamide adenine dinucleotide (NADH). [1] Glycolysis is a sequence of ten reactions catalyzed by enzymes. Summary of the 10 reactions of the glycolysis pathway
2 NADH+H + from glycolysis: 2 × 1.5 ATP (if glycerol phosphate shuttle transfers hydrogen atoms) or 2 × 2.5 ATP (malate-aspartate shuttle) 2 NADH+H + from the oxidative decarboxylation of pyruvate and 6 from Krebs cycle: 8 × 2.5 ATP; 2 FADH 2 from the Krebs cycle: 2 × 1.5 ATP
Combining the above reaction with the ones occurring in the course of glycolysis, the following overall glucose oxidation reaction (excluding reactions in the respiratory chain) is obtained: Glucose + 10 NAD + + 2 FAD + 2 ADP + 2 GDP + 4 P i + 2 H 2 O: → 10 NADH + 2 FADH 2 + 10 H + + 2 ATP + 2 GTP + 6 CO 2
The second electron and proton atom are transferred to the carbon atom adjacent to the N atom. The midpoint potential of the NAD + /NADH redox pair is −0.32 volts, which makes NADH a moderately strong reducing agent. [7] The reaction is easily reversible, when NADH reduces another molecule and is re-oxidized to NAD +.
The net effect of the malate–aspartate shuttle is purely redox: NADH in the cytosol is oxidized to NAD +, and NAD + in the matrix is reduced to NADH. The NAD + in the cytosol can then be reduced again by another round of glycolysis, and the NADH in the matrix can be used to pass electrons to the electron transport chain so ATP can be synthesized.
The glycerol-3-phosphate shuttle is a mechanism used in skeletal muscle and the brain [1] that regenerates NAD + from NADH, a by-product of glycolysis. NADH is a reducing equivalent that stores electrons generated in the cytoplasm during glycolysis. NADH must be transported into the mitochondria to enter the oxidative phosphorylation pathway.
When sufficient oxygen is not present in the muscle cells for further oxidation of pyruvate and NADH produced in glycolysis, NAD+ is regenerated from NADH by reduction of pyruvate to lactate. [4] Lactate is converted to pyruvate by the enzyme lactate dehydrogenase. [3] The standard free energy change of the reaction is -25.1 kJ/mol. [6]
The reaction may be simplified as: Pyruvate + NAD + + CoA → Acetyl-CoA + NADH + CO 2. Pyruvate oxidation is the step that connects glycolysis and the Krebs cycle. [4] In glycolysis, a single glucose molecule (6 carbons) is split into 2 pyruvates (3 carbons each).