BIOL 111 Figure 9.9
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Glycolysis
Energy Investment Phase
1. Glucose enters the cell and is phosphorylated by the enzyme hexokinase, which transfers a phosphate group from ATP to the sugar. The charge of the phosphate group traps the sugar in the cell because the plasma membrane is impermeable to large ions. Phosphorylation also makes glucose more chemically reactive. In this diagram, the transfer of a phosphate group or pair of electrons from one reactant to another is indicated by coupled arrows.
2. Glucose-6-phosphate is converted to its isomer, fructose-6-phosphate.
3. Phosphofructokinase transfers a phosphate group from ATP to the sugar, investing another molecule of ATP in glycolysis. So far, 2 ATP have been used. With phosphate groups on opposite ends, the sugar is now ready to be split in half, this is a key step for regulation of glycolysis; phosphofructokinase is allosterically regulated by ATP and its products.
4. This reaction is where glycolysis gets its name. Aldolase cleaves the sugar molecule into two different 3-carbon sugars: dihydroxyacetone phosphate (DHAP) and glyceraldehyde-3-phosphate (G3P). These two sugars are isomers of each other.
5. Isomerase catalyzes the reversible conversion between the two 3-carbon sugars. This reaction never reaches equilibrium in the cell because the next enzyme in glycolysis uses only G3P as its substrate (and not DHAP). This pulls the equilibrium in the direction of G3P, which is removed as fast as it forms. Thus the net result of steps 4 and 5 is cleavage of a 6-carbon sugar into two molecules of G3P; each will progress through the remaining steps of glycolysis.
Energy Payoff Phase
6. Triose phosphate dehydrogenase catalyzes two sequential reactions while it holds G3P in its active site: 1) the sugar is oxidized by the transfer of electrons and H+ to NAD+, forming NADH (a redox reaction). This reaction is very exergonic, and the enzyme uses the released energy to 2) attach a phosphate group to the oxidized substrate, making a product of very high potential energy. The source of the phosphates is the pool of inorganic phosphate ions that are always present in the cytosol. Notice that the coefficient 2 precedes all molecules in the energy payoff phase; these steps occur after glucose has been split into two 3-carbon sugars (step 4).
7. Glycolysis produces some ATP by substrate-level phosphorylation. The phosphate group added in the previous step is transferred to ADP in an exergonic reaction. For each glucose molecule that began glycolysis, this step produces 2 ATP, since every product after the sugar-splitting step (step 4) is doubled. Recall that 2 ATP were invested to get sugar ready for splitting; this ATP debt has now been repaid. Glucose has been converted to two molecules of 3-phosphoglycerate, which is not a sugar. The carbonyl group (—COO–), the hallmark of an organic acid. The sugar was oxidized in step 6, and now the energy made available by that oxidation has been used to make ATP.
8. Posphoglyceromutase relocates the remaining phosphate group, preparing the substrate for the next reaction.
9. Enolase causes a double bond to form in the substrate by extracting a water molecule, yielding phosphoenolpyruvate (PEP). The electrons of the substrate are rearranged in such a way that the resulting phosphorylated compound has a very high potential energy, allowing step 10 to occur.
10. The last reaction of glycolysis produces more ATP by transferring the phosphate group from PEP to ADP, a second instance of substrate-level phosphorylation. Since this step occurs twice for each glucose molecule, 2 ATP are produced. Overall, glycolysis has used 2 ATP in the energy investment phase (steps 1 and 3) and produced 4 ATP in the energy payoff phase (steps 7 and 10), for a net gain of 2 ATP. Glycolysis has repaid the ATP investment with 100% interest :). Additional energy was stored by step 6 in NADH, which can be used to make ATP by oxidative phosphorylation if oxygen is present. Glucose has been broken down and oxidized to two molecules of pyruvate, the end product of the glycolytic pathway. If oxygen is present, the chemical energy can be extracted by the citric acid cycle. If oxygen is not present, fermentation may occur; this will be described later.