66 Oxidative Phosphorylation (60/84) -- Biology Part I

66 Oxidative Phosphorylation Learning Objectives By the end of this section, you will be able to do the following: - Describe how electrons move through the electron transport chain and explain what happens to their energy levels during this process - Explain how a proton (H+) gradient is established and maintained by the electron transport chain You have just read about two pathways in glucose catabolism—glycolysis and the citric acid cycle—that generate ATP. Most of the ATP generated during the aerobic catabolism of glucose, however, is not generated directly from these pathways. Instead, it is derived from a process that begins by moving electrons through a series of electron carriers that undergo redox reactions. This process causes hydrogen ions to accumulate within the intermembranous space. Therefore, a concentration gradient forms in which hydrogen ions diffuse out of the intermembranous space into the mitochondrial matrix by passing through ATP synthase. The current of hydrogen ions powers the catalytic action of ATP synthase, which phosphorylates ADP, producing ATP. Electron Transport Chain The electron transport chain (Figure 7.10) is the last component of aerobic respiration and is the only part of glucose metabolism that uses atmospheric oxygen. Oxygen continuously diffuses into plant tissues (typically through stomata), as well as into fungi and bacteria; however, in animals, oxygen enters the body through a variety of respiratory systems. Electron transport is a series of redox reactions that resembles a relay race or bucket brigade, in that electrons are passed rapidly from one component to the next to the endpoint of the chain, where the electrons reduce molecular oxygen and, along with associated protons, produce water. There are four complexes composed of proteins, labeled I through IV in Figure 7.10, and the aggregation of these four complexes, together with associated mobile, accessory electron carriers, is called the electron transport chain. The electron transport chain is present with multiple copies in the inner mitochondrial membrane of eukaryotes and within the plasma membrane of prokaryotes. Complex I First, two electrons are carried to the first complex via NADH. This complex, labeled I, is composed of flavin mononucleotide (FMN) and an iron-sulfur (Fe-S)-containing protein. FMN, which is derived from vitamin B2 (also called riboflavin), is one of several prosthetic groups or cofactors in the electron transport chain. A prosthetic group is a nonprotein molecule required for the activity of a protein. Prosthetic groups are organic or inorganic, nonpeptide molecules bound to a protein that facilitate its function. Prosthetic groups include coenzymes, which are the prosthetic groups of enzymes. The enzyme in complex I is NADH dehydrogenase and is composed of 44 separate polypeptide chains. Complex I can pump four hydrogen ions across the membrane from the matrix into the intermembrane space, and it is in this way that th