GLYCOLYSIS

  • means "sugar splitting"
  • during glycolysis, glucose ( a six-carbon sugar) is split into two three-carbon sugars
  • the smaller sugars are then oxidized and their remaining atoms rearranged to form two molecules of pyruvate
  • glycolysis can be divided into two phases: energy investment and energy payoff
    • during energy investment phase the cell spends ATP
    • during energy payoff phase this investment is repaid when ATP is produced and NAD+ is reduced to NADH
  • the net energy yield from glycolysis per glucose molecule is 2 ATP plaus 2 NADH
  • all of the carbon originally present in glucose is accounted for in the 2 molecules of pyruvate - no CO2 is released in glycolysis
  • glycolysis occurs whether or not O2 is present, but if O2 is present then the chemical energy stored in pyruvate and NADH can be extracted by the citric acid cycle and oxidative phosphorylation

ENERGY INVESTMENT PHASE

external image 09_09aGlycolysisInvest.jpg
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 the other is indicated by curved arrows.
2. Glucose-6 phosphate is converted to its isomer, fructose-6-phosphate
3. This enzyme transfers a phosphate group from ATP to the sugar, investing another molecule of ATP in glycolysis. So far, 2 ATP molecules have been used. With phosphate groups on its 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 is the reaction from which glycolysis gets its name. the enzyme cleaves the sugar molecule into two different three-molecule carbon sugars: dihydroxyacetone phosphate and glyceraldehyde-3-phosphate. These two sugars are isomers of each other.
5. Isomerase catalyzes the reversible conversion between the two three-carbon sugars. This reaction never reaches equilibrium in the cell because the next enzyme in glycolysis uses only glyceraldehyde-3-phosphate as its substrate (and not dihydroxyacetone phosphate). This pulls the equilibrium in the direction of glyceraldehyde-3-phosphate, which is removed as fast as it forms. Thus, the net result of steps 4 and 5 is cleavage of a six-carbon sugar into two molecules of gyceraldehyde-3-phosphate; each will progress through the remaining steps of glycolysis.


ENERGY PAYOFF PHASE

external image 09_09bGlycolysisPayoff.jpg
6. This enzyme catalyzes two sequential reactions while it holds glyceraldehyde-3-phosphate in its active site. First, the sugar is oxidized by the transfer of electrons and H+ to NAD+, forming NADH. This reaction is very exergonic and the enzyme uses the released energy to attatch 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 three-carbon sugars.
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, step 7 produces 2 ATP, since every product after the sugar-splitting step 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-phophoglycerate, which is not a sugar. The carbonyl group that characterizes a sugar has been oxidized to a carboxyl group, 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. This enzyme relocates the remaining phosphate group, preparing the substrate for the next reaction
9.This enzyme 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 and produced 4 ATP in the energy payoff phase, 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 oxiative 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 in pyruvate can be extracted by the citric acid cycle. If oxygen is not present, fermentation may occur

NEXT: CITRIC ACID CYCLE