Enzymes of Glycolysis
By Jennifer McDowall
to view structures of glycolysis enzymes
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Cellular location of energy pathways. Reprinted
by permission from J. Barnett, Yeast
20, J.A. Barnett, A history of research on yeast 6: the main respiratory
pathway, 1015-44 (2003). |
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How do organisms generate energy?
All cells need energy, which they get through ATP, an inherently unstable molecule that must continually be produced. Though ATP can be produced in different ways, nearly all living cells can harness ATP through glycolysis, the stepwise degradation of glucose, and other sugars, obtained from the breakdown of carbohydrates without the need for molecular oxygen (anaerobic). Glycolysis is an ancient, universal pathway that probably developed before there was sufficient oxygen in the atmosphere to sustain more effective methods of energy extraction. When aerobic organisms evolved, they simply added more efficient energy extraction pathways onto glycolysis, breaking down the end products from glycolysis (pyruvate) still further through the tricarboxylic acid cycle. Yet, aerobic cells can still rely predominantly on glycolysis when oxygen is limiting, such as in hard working muscle cells where glycolysis ends in the production of lactate, causing muscle fatigue. The aerobic and anaerobic processes are kept separate in eukaryotic cells, with glycolysis occurring in the cytoplasm, and the aerobic tricarboxylic acid cycle occurring in the mitochondria.
Glycolysis
During glycolysis, glucose is broken down in ten steps to two molecules of pyruvate, which then enters the mitochondria where it is oxidised through the tricarboxylic acid cycle to carbon dioxide and water. Glycolysis can be split into two phases, both of which occur in the cytosol. Phase I involves splitting glucose into two molecules of glyceraldehyde-3-phosphate (G3P) at the expense of 2 ATP molecules, but allows the subsequent energy-producing reactions to be doubled up with a higher net gain of ATP. Phase II converts G3P into pyruvate, with the concomitant generation of 4 ATP molecules, giving a net gain of 2 ATP per glucose molecule. Glycolysis, therefore, provides the cell with a small amount of energy, and, in aerobic cells, provides the starting materials for the complete oxidation of glucose to carbon dioxide and water.
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Phase I: Glucose molecule is split, using 2 ATP Phase II: Energy is extracted in the form of 4 ATP |
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Reprinted by permission from Robert J. Huskey, |
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