CH2OHOOHOHOHHOglucoseCH2OH+OOHOHOHHOglucose 6-phosphate + + + hexokinase CH2OOOHOHOHHOglucose 6-phosphate fructose 6-phosphate (ring form) (ring form) (open-chain form) 1 1 2 2 3 4 5 6 6 OHCCHOHCHOHCHOHCHOHCH2O3 4 5 (open-chain form) 1 1 2 2 6 CCHOHCHOHCHOHCH2OCH2OH334455phosphoglucoseisomeraseOH2CCH2OHOHOOHOH6+H+++phosphofructokinaseOH2CCH2OHOHOOHOHOH2COHOOHOHGlucose is phosphorylated by ATP to form a sugar phosphate. The negative charge of the phosphate prevents passage of the sugar phosphate through the plasma membrane, trapping glucose inside the cell. The six-carbon sugar is cleaved to produce two three-carbon molecules. Only the glyceraldehyde 3-phosphate can proceed immediately through glycolysis. A readily reversible rearrangement of the chemical structure (isomerization) moves the carbonyl oxygen from carbon 1 to carbon 2, forming a ketose from an aldose sugar. (See Panel 2–3, pp. 70–71.) The new hydroxyl group on carbon 1 is phosphorylated by ATP, in preparation for the formation of two three-carbon sugar phosphates. The entry of sugars into glycolysis is controlled at this step, through regulation of the enzyme phosphofructokinase. fructose 6-phosphate fructose 1,6-bisphosphate + (ring form) OHCCHOHaldolase (open-chain form) CCHOHCHOHCHOHCH2OCH2OOCCHOHHCH2OCH2OOOH2CCH2OOHOOHOHfructose 1,6-bisphosphate dihydroxyacetone phosphate glyceraldehyde 3-phosphate OFor each step, the part of the molecule that undergoes a change is shadowed in blue, and the name of the enzyme that catalyzes the reaction is in a yellow box. ATPATPADPADPP P P P P PP P P P P P P P CH2OStep 1 Step 2 Step 3 Step 4 104 PaNel 2–8: Details of the 10 Steps of Glycolysis + + enolase phosphoglycerate mutase + OO–CCHOHCH2O3-phosphoglycerate OO–CCHOCH2OH2-phosphoglycerate OO–CCHOCH2OH2-phosphoglycerate OO–CCOCH2H2Ophosphoenolpyruvate OO–CCOCH2phosphoenolpyruvate OO–CCOCH3pyruvate + phosphoglycerate kinaseOCCHOHCH2O1,3-bisphosphoglycerate + OO–CCHOHCH2O3-phosphoglycerate The two molecules of glyceraldehyde 3-phosphate are oxidized. The energy-generation phase of glycolysis begins, as NADH and a new high-energy anhydride linkage to phosphate are formed (see Figure 13–5). H++ ++ glyceraldehyde 3-phosphate dehydrogenaseOHCCHOHCH2Oglyceraldehyde 3-phosphate OOCCHOHCH2O1,3-bisphosphoglycerate H++ The transfer to ADP of the high-energy phosphate group that was generated in step 6 forms ATP. The remaining phosphate ester linkage in 3-phosphoglycerate, which has a relatively low free energy of hydrolysis, is moved from carbon 3 to carbon 2 to form 2-phosphoglycerate. The removal of water from 2-phosphoglycerate creates a high-energy enol phosphate linkage. The transfer to ADP of the high-energy phosphate group that was generated in step 9 forms ATP, completing glycolysis. + pyruvate kinase 1 2 3 ATPADPATPADPNADHNAD+Pi P P P P P P P P P P P OStep 6 Step 7 Step 8 Step 9 Step 10 105