are tightly bound. As detailed in Figure 2–48, the first enzyme (glyceraldehyde 3-phosphate dehydrogenase) forms a short-lived covalent bond to the aldehyde through a reactive –SH group on the enzyme, and catalyzes its oxidation by NAD+ in this attached state. The reactive enzyme–substrate bond is then displaced by an inorganic phosphate ion to produce a high-energy phosphate intermediate, which is released from the enzyme. This intermediate binds to the second enzyme (phosphoglycerate kinase), which catalyzes the energetically favorable transfer of the high-energy phosphate just created to ADP, forming ATP and completing the process of oxidizing an aldehyde to a carboxylic acid. Note that the C–H bond oxidation energy in step 6 drives the formation of both NADH and a high-energy phosphate bond. The breakage of the high-energy bond then drives ATP formation.
We have shown this particular oxidation process in some detail because it provides a clear example of enzyme-mediated energy storage through coupled reactions (Figure 2–49). Steps 6 and 7 are the only reactions in glycolysis that create a high-energy phosphate linkage directly from inorganic phosphate. As such, they account for the net yield of two ATP molecules and two NADH molecules per molecule of glucose (see Panel 2–8, pp. 104–105).
As we have just seen, ATP can be formed readily from ADP when a reaction intermediate is formed with a phosphate bond of higher energy than the terminal phosphate bond in ATP. Phosphate bonds can be ordered in energy by comparing the standard free-energy change (∆G°) for the breakage of each bond by hydrolysis. Figure 2–50 compares the high-energy phosphoanhydride bonds in ATP with the energy of some other phosphate bonds, several of which are generated during glycolysis.
All organisms need to maintain a high ATP/ADP ratio to maintain biological order in their cells. Yet animals have only periodic access to food, and plants need to survive overnight without sunlight, when they are unable to produce sugar from photosynthesis. For this reason, both plants and animals convert sugars and fats to special forms for storage (Figure 2–51).
To compensate for long periods of fasting, animals store fatty acids as fat droplets composed of water-insoluble triacylglycerols (also called triglycerides). The triacylglycerols in animals are mostly stored in the cytoplasm of specialized fat cells called adipocytes. For shorter-term storage, sugar is stored as glucose 1,3-bisphosphoglycerate formation of hydrolysis of high-energy bond high-energy bond
TOTAL ENERGY CHANGE for step 6 followed by step 7 is a favorable –12.5 kJ/mole
Figure 2–49 Schematic view of the coupled reactions that form NaDH and aTP in steps 6 and 7 of glycolysis. The C–h bond oxidation energy drives the formation of both naDh and a high-energy phosphate bond. The breakage of the high-energy bond then drives aTp formation.
bond to carbon anhydride bond to phosphate type of phosphate bond –61.9 kJ (see Panel 2–8, pp. 104–105) for example, 1,3-bisphosphoglycerate –49.0 kJ (see Panel 2–8) –40 creatine phosphate (activated carrier that –43.0 kJ stores energy in muscle) for example, ATP when hydrolyzed –30.6 kJ to ADP –20 for example, glucose 6-phosphate –17.5 kJ (see Panel 2–8) specifc examples showing the standard free-energy change (˜G°) for hydrolysis of phosphate bond