An average person turns over roughly 50 kg of ATP per day. In athletes running a marathon, this figure can go up to several hundred kilograms. The ATP produced in mitochondria is derived from the energy available in the intermediates NADH, FADH2, and GTP. These three energy-rich compounds are produced both by the oxidation of glucose (Table 14–1A), and by the oxidation of fats (Table 14–1B; see also Figure 2–56).
Glycolysis alone can produce only two molecules of ATP for every molecule of glucose that is metabolized, and this is the total energy yield for the fermentation processes that occur in the absence of O2 (discussed in Chapter 2). In oxidative
Figure 14–28 A general model for H+ pumping coupled to electron transport.
this mechanism for h+ pumping by a transmembrane protein is thought to be used by NaDh dehydrogenase and cytochrome c oxidase, and by many other proton pumps. the protein is driven through a cycle of three conformations. In one of these conformations, the protein has a high affinity for h+, causing it to pick up an h+ on the inside of the membrane. In another conformation, the protein has a low affinity for h+, causing it to release an h+ on the outside of the membrane. as indicated, the transitions from one conformation to another occur only in one direction, because they are being driven by being allosterically coupled to the energetically favorable process of electron transport (discussed in Chapter 11).
phosphorylation, each pair of electrons donated by the NADH produced in mitochondria can provide energy for the formation of about 2.5 molecules of ATP. Oxidative phosphorylation also produces 1.5 ATP molecules per electron pair from the FADH2 produced by succinate dehydrogenase in the mitochondrial matrix, and from the NADH molecules produced by glycolysis in the cytosol. From the product yields of glycolysis and the citric acid cycle, we can calculate that the complete oxidation of one molecule of glucose—starting with glycolysis and ending with oxidative phosphorylation—gives a net yield of about 30 molecules of ATP. Nearly all this ATP is produced by the mitochondrial ATP synthase.
In Chapter 2, we introduced the concept of free energy (G). The free-energy change for a reaction, ∆G, determines whether that reaction will occur in a cell. We showed on pp. 60–63 that the ∆G for a given reaction can be written as the sum of two parts: the first, called the standard free-energy change, ∆G°, depends only on the intrinsic characters of the reacting molecules; the second depends only on their concentrations. For the simple reaction A →B, [A] where [A] and [B] denote the concentrations of A and B, and ln is the natural logarithm. ∆G° is the standard reference value, which can be seen to be equal to the value of ∆G when the molar concentrations of A and B are equal (since ln 1 = 0). In Chapter 2, we discussed how the large, favorable free-energy change (large negative ∆G) for ATP hydrolysis is used, through coupled reactions, to drive many other chemical reactions in the cell that would otherwise not occur (see pp. 65–66). The ATP hydrolysis reaction produces two products, ADP and Pi; it is therefore of the type A → B + C, where, as demonstrated in Figure 14–29, [B][C]