Oxidative Phosphorylation in Eukaryotes Takes Place in Mitochondria
II. Transducing and Storing Energy 18. Oxidative Phosphorylation Mitochondria, Stained Green, Form a Network Inside a Fibroblast Cell (Left). Mitochondria oxidize carbon fuels to form cellular energy. This transformation requires electron transfer through several large protein complexes (above), some of which pump protons, forming a proton gradient that powers the synthesis of ATP. [(Left) Courtesy of Michael P. Yaffee, Dept. of Biology, University of California at San Diego.] II. Transducing and Storing Energy 18. Oxidative Phosphorylation 18.1. Oxidative Phosphorylation in Eukaryotes Takes Place in Mitochondria Mitochondria are oval-shaped organelles, typically about 2 µm in length and 0.5 µm in diameter, about the size of a bacterium. Eugene Kennedy and Albert Lehninger discovered a half-century ago that mitochondria contain the respiratory assembly, the enzymes of the citric acid cycle, and the enzymes of fatty acid oxidation. 18.1.1. Mitochondria Are Bounded by a Double Membrane Electron microscopic studies by George Palade and Fritjof Sjöstrand revealed that mitochondria have two membrane systems: an outer membrane and an extensive, highly folded inner membrane. The inner membrane is folded into a series of internal ridges called cristae. Hence, there are two compartments in mitochondria: (1) the intermembrane space between the outer and the inner membranes and (2) the matrix, which is bounded by the inner membrane (Figure 18.3). Oxidative phosphorylation takes place in the inner mitochondrial membrane, in contrast with most of the reactions of the citric acid cycle and fatty acid oxidation, which take place in the matrix. The outer membrane is quite permeable to most small molecules and ions because it contains many copies of mitochondrial porin, a 30 35 kd poreforming protein also known as VDAC, for voltage-dependent anion channel. VDAC plays a role in the regulated flux of metabolites usually anionic species such as phosphate, chloride, organic anions, and the adenine nucleotides across the outer membrane. VDAC appears to form an open β -barrel structure similar to that of the bacterial porins (Section 12.5.2), although mitochondrial porins and bacterial porins may have evolved independently. Some cytoplasmic kinases bind to VDAC, thereby obtaining preferential access to the exported ATP. In contrast, the inner membrane is intrinsically impermeable to nearly all ions and polar molecules. A large family of transporters shuttles metabolites such as ATP, pyruvate, and citrate across the inner mitochondrial membrane. The two faces of this membrane will be referred to as the matrix side and the cytosolic side (the latter because it is freely accessible to most small molecules in the cytosol). They are also called the N and P sides, respectively, because the membrane potential is negative on the matrix side and positive on the cytosolic side. In prokaryotes, the electron-driven proton pumps and ATP-synthesizing complex are located in the cytoplasmic membrane, the inner of two membranes. The outer membrane of bacteria, like that of mitochondria, is permeable to most small metabolites because of the presence of porins. 18.1.2. Mitochondria Are the Result of an Endosymbiotic Event Mitochondria are semiautonomous organelles that live in an endosymbiotic relation with the host cell. These organelles contain their own DNA, which encodes a variety of different proteins and RNAs. The genomes of mitochondrial range broadly in size across species. The mitochondrial genome of the protist Plasmodium falciparum consists of fewer than 6000 base pairs (6 kbp), whereas those of some land plants comprise more than 200 kbp (Figure 18.4). Human mitochondrial DNA comprises 16,569 bp and encodes 13 respiratory-chain proteins as well as the small and large ribosomal RNAs and enough tRNAs to translate all codons. However, mitochondria also contain many proteins encoded by nuclear DNA. Cells that contain mitochondria depend on these organelles for oxidative phosphorylation, and the mitochondria in turn depend on the cell for their very existence. How did this intimate symbiotic relation come to exist? An endosymbiotic event is thought to have occurred whereby a freeliving organism capable of oxidative phosphorylation was engulfed by another cell. The double membrane, circular DNA (with some exceptions), and mitochondrial-specific transcription and translation machinery all point to this conclusion. Thanks to the rapid accumulation of sequence data for mitochondrial and bacterial genomes, it is now possible to speculate on the origin of the "original" mitochondrion with some authority. The most mitochondrial-like bacterial genome is that of Rickettsia prowazekii, the cause of louseborne typhus. The genome for this organism is more than 1 million base pairs in size and contains 834 protein-encoding genes. Sequence data suggest that all extant mitochondria are derived from an ancestor of R. prowazekii as the result of a single endosymbiotic event. The evidence that modern mitochondria result from a single event comes from examination of the most bacteria-like mitochondrial genome, that of the protozoan Reclinomonas americana. Its genome contains 97 genes, of which 62 specify proteins that include all of the protein-coding genes found in all of the sequenced mitochondrial genomes (Figure 18.5). Yet, this genome encodes less than 2% of the protein-coding genes in the bacterium E. coli. It seems unlikely that mitochondrial genomes resulting from several endosymbiotic events could have been independently reduced to the same set of genes found in R. americana. Note that transient engulfment of prokaryotic cells by larger cells is not uncommon in the microbial world. In regard to mitochondria, such a transient relation became permanent as the bacterial cell lost DNA, making it incapable of independent living, and the host cell became dependent on the ATP generated by its tenant. II. Transducing and Storing Energy 18. Oxidative Phosphorylation 18.1. Oxidative Phosphorylation in Eukaryotes Takes Place in Mitochondria Figure 18.3. Diagram of a Mitochondrion. [After Biology of the Cell by Stephen L. Wolfe. © 1972 by Wadsworth Publishing Company, Inc., Belmost, California 94002. Adapted by permission of the publisher.] II. Transducing and Storing Energy 18. Oxidative Phosphorylation 18.1. Oxidative Phosphorylation in Eukaryotes Takes Place in Mitochondria Figure 18.4. Sizes of Mitochondrial Genomes. The sizes of three mitochondrial genomes compared with the genome of Rickettsia, a relative of the the presumed ancestor of all mitochondria. For genomes of more than 60 kbp, the DNA coding region for genes with known function is shown in red.