Figure 14–34 The ADP/ATP carrier protein. (a) the aDp/atp carrier protein is a small membrane protein that carries the atp produced on the matrix side of the inner membrane to the intermembrane space, and the aDp that is needed for atp synthesis into the matrix. (B) In the aDp/atp carrier, six transmembrane α helices define a cavity that binds either aDp or atp. In this x-ray structure, the substrate is replaced by a tightly bound inhibitor instead (colored). When aDp binds from outside the inner membrane, it triggers a conformational change and is released into the matrix. In exchange, a molecule of atp quickly binds to the matrix side of the carrier and is transported to the intermembrane space. From there the atp diffuses through the outer mitochondrial membrane to the cytoplasm, where it powers the energy-requiring processes in the cell. (B, pDB code: 1OKC.) mitochondrial membrane contains about 20 related carrier proteins exchanging various other metabolites, including the phosphate that is required along with ADP for ATP synthesis.
In some specialized fat cells, mitochondrial respiration is uncoupled from ATP synthesis by the uncoupling protein, another member of the mitochondrial carrier family. In these cells, known as brown fat cells, most of the energy of oxidation is dissipated as heat rather than being converted into ATP. In the inner membranes of the large mitochondria in these cells, the uncoupling protein allows protons to move down their electrochemical gradient without passing through ATP synthase. This process is switched on when heat generation is required, causing the cells to oxidize their fat stores at a rapid rate and produce heat rather than ATP. Tissues containing brown fat serve as “heating pads,” helping to revive hibernating animals and to protect newborn human babies from the cold.
Bacteria use enormously diverse energy sources. Some, like animal cells, are aerobic; they synthesize ATP from sugars they oxidize to CO2 and H2O by glycolysis, the citric acid cycle, and a respiratory chain in their plasma membrane that is similar to the one in the inner mitochondrial membrane. Others are strict anaerobes, deriving their energy either from glycolysis alone (by fermentation, see Figure 2–47) or from an electron-transport chain that employs a molecule other than oxygen as the final electron acceptor. The alternative electron acceptor can be a nitrogen compound (nitrate or nitrite), a sulfur compound (sulfate or sulfite), or a carbon compound (fumarate or carbonate), for example. A series of electron carriers in the plasma membrane that are comparable to those in mitochondrial respiratory chains transfers the electrons to these acceptors.
Despite this diversity, the plasma membrane of the vast majority of bacteria contains an ATP synthase that is very similar to the one in mitochondria. In bacteria that use an electron-transport chain to harvest energy, the electron-transport chain pumps H+ out of the cell and thereby establishes a proton-motive force across the plasma membrane that drives the ATP synthase to make ATP. In other bacteria, the ATP synthase works in reverse, using the ATP produced by glycolysis to pump H+ and establish a proton gradient across the plasma membrane.