Mitochondria structural organization

Figure 7.9 Structural organization of the mitochondrion. Figure reproduced from [1] with permission.

The synthesis of heme (Fig. 1) is completed in the mitochondrion by insertion of iron into the protoporphyrin IX framework by ferrochelatase. Ferrochelatases from various organisms have been crystallized and their structures determined. The human enzyme contains one 2Fe-2S cluster in each of the two subunits of the functional dimer (22), possibly as a mechanism to link heme synthesis to iron availability. Erythropoietic protoporphyria, which is characterized by cutaneous photosensitivity, is caused by mutations in the ferrochelatase gene (23). 

Each mitochondrion (plural mitochondria) is bounded by two membranes (Figure 2.24a). The smooth outer membrane is relatively porous, because it is permeable to most molecules with masses less than 10,000 D. The inner membrane, which is impermeable to ions and a variety of organic molecules, projects inward into folds that are called cristae (singular crista). Embedded in this membrane are structures composed of molecular complexes and called respiratory assemblies (described in Chapter 10) that are responsible for the synthesis of ATP. Also present are a series of proteins that are responsible for the transport of specific molecules and ions. 

The degree of conservation, in terms of subunit composition and protein sequence, between mammalian respiratory chain complexes and those characterized from fungi and other organisms depends on the subunit and complex being considered (detailed in specific sections below), but in general, those subunits which are known to have a central role in electron transport are well conserved in terms of protein sequence and, where known, tertiary structure. For these subunits, a dear relationship to bacterial respiratory chain components can also be seen, which leads to the condusion that the mitochondrial respiratory chain complexes have evolved and adapted from those of the symbiotic bacterial ancestor of the mitochondrion [23]. Mitochondrial complexes have in most cases acquired many additional subunits whose function remains obscure. 

The inner membrane of the mitochondrion accounts for 80-95% of the protein found in mitochondrial membranes and over 90% of the lipid. It is the site of the respiratory chain and the synthesis of ATP. It is this membrane, in conjunction with studies on transport through the plasma membrane, that has contributed most forcefully both to the viewpoint of the anisotropic organization of membrane structural elements and of biochemical events carried out by or in membranes. As regards mitochondria, the interaction of the inner membrane components in carrying out electron transport and oxidative phosphorylation is the focal investigative question both for mitochondrial function and for the organization of vectorial events in general. 

---