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Glycolysis is the metabolic pathway that converts glucose (C 6 H 12 O 6) into pyruvate and, in most organisms, occurs in the liquid part of cells (the cytosol). 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 ]
Anaerobic glycolysis is the transformation of glucose to lactate when limited amounts of oxygen (O 2) are available. [1] This occurs in health as in exercising and in disease as in sepsis and hemorrhagic shock. [1] providing energy for a period ranging from 10 seconds to 2 minutes.
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.
"The metabolic pathway of glycolysis converts glucose to pyruvate via a series of intermediate metabolites. Each chemical modification (red box) is performed by a different enzyme. Steps 1 and 3 consume ATP (blue) and steps 7 and 10 produce ATP (yellow). Since steps 6-10 occur twice per glucose molecule, this leads to a net production of energy."
Most enzymes of glycolysis also participate in gluconeogenesis, as it is mostly the reverse metabolic pathway of glycolysis; a deficiency of these liver enzymes will therefore impact both glycolysis and gluconeogenesis. (Note: gluconeogenesis is taking place only in the liver and not in other cells like e.g. muscle cells.)
The first reaction is the oxidation of glyceraldehyde 3-phosphate (G3P) at the position-1 (in the diagram it is shown as the 4th carbon from glycolysis), in which an aldehyde is converted into a carboxylic acid (ΔG°'=-50 kJ/mol (−12kcal/mol)) and NAD+ is simultaneously reduced endergonically to NADH.
Glucose-6-phosphate dehydrogenase is also an enzyme in the Entner–Doudoroff pathway, a type of glycolysis. Clinically, an X-linked genetic deficiency of G6PD makes a human prone to non-immune hemolytic anemia .
The loss of a high-energy phosphate bond and the substrate for the rest of glycolysis makes formation of methylglyoxal inefficient. Studies suggest that a lysine close to the active site (at position 12) is also crucial for enzyme function. The lysine, protonated at physiological pH, may help neutralize the negative charge of the phosphate group.