Mitochondria chain components

Structure of the mitochondrion The components of the electron transport chain are located in the inner membrane. Although the outer membrane contains special pores, making it freely perme-... 

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. 

Mitochondria have their own DNA (mtDNA) and genetic continuity. This DNA only encodes 13 peptide subunits synthesized in the matrix that are components of complexes I, III, IV, and V of the respiratory chain. Most mitochondrial proteins are synthesized on cytoplasmic ribosomes and imported by specific mechanisms to their specific locations in the mitochondrion (see below). 

Not all the cellular DNA is in the nucleus some is found in the mitochondria. In addition, mitochondria contain RNA as well as several enzymes used for protein synthesis. Interestingly, mitochond-rial RNA and DNA bear a closer resemblance to the nucleic acid of bacterial cells than they do to animal cells. For example, the rather small DNA molecule of the mitochondrion is circular and does not form nucleosomes. Its information is contained in approximately 16,500 nucleotides that func-tion in the synthesis of two ribosomal and 22 transfer RNAs (tRNAs). In addition, mitochondrial DNA codes for the synthesis of 13 proteins, all components of the respiratory chain and the oxidative phosphorylation system. Still, mitochondrial DNA does not contain sufficient information for the synthesis of all mitochondrial proteins most are coded by nuclear genes. Most mitochondrial proteins are synthesized in the cytosol from nuclear-derived messenger RNAs (mRNAs) and then transported into the mito-chondria, where they contribute to both the structural and the functional elements of this organelle. Because mitochondria are inherited cytoplasmically, an individual does not necessarily receive mitochondrial nucleic acid equally from each parent. In fact, mito-chondria are inherited maternally. 

The sequence of the carriers in the chain is shown in Figure 9.6. Each of the components of the chain reduces the next, in sequence, according to the redox potential (Table 9.3). The enzymes and their prosthetic groups are organised into complexes, which can be isolated by gentle disruption of the whole mitochondrion or its inner membrane. Ubiqui-... 

Because of the difficulty of isolating the electron transport chain from the rest of the mitochondrion, it is easiest to measure ratios of components (Table 18-3). Cytochromes a, a3, b, cv and c vary from a 1 1 to a 3 1 ratio while flavins, ubiquinone, and nonheme iron occur in relatively larger amounts. The much larger... 

These organelles are the sites of energy production of aerobic cells and contain the enzymes of the tricarboxylic acid cycle, the respiratory chain, and the fatty acid oxidation system. The mitochondrion is bounded by a pair of specialized membranes that define the separate mitochondrial compartments, the internal matrix space and an intermembrane space. Molecules of 10,000 daltons or less can penetrate the outer membrane, but most of these molecules cannot pass the selectively permeable inner membrane. By a series of infoldings, the internal membrane forms cristae in the matrix space. The components of the respiratory chain and the enzyme complex that makes ATP are embedded in the inner membrane as well as a number of transport proteins that make it selectively permeable to small molecules that are metabolized by the enzymes in the matrix space. Matrix enzymes include those of the tricarboxylic acid cycle, the fatty acid oxidation system, and others. 

A quick review of some aspects of mitochondrial structure is in order here because we shall want to describe the exact location of each of the components of the citric acid cycle and the electron transport chain. Recall from Chapter 1 that a mitochondrion has an inner and an outer membrane (Figure 19.2). The region enclosed by the inner membrane is called the mitochondrial matrix, and an intermembrane space exists between the inner and outer membranes. The inner membrane is a tight barrier between the matrix and the cytosol, and very few compounds can cross this barrier without a specific transport protein (Section 8.4). The reactions of the citric acid cycle take place in the matrix, except for the one in which the intermediate electron acceptor is FAD. The enzyme that catalyzes the FAD-linked reaction is an integral part of the inner mitochondrial membrane and is linked direcdy to the electron transport chain (Chapter 20). 

FIGURE 4.1 Schematics of the mitochondrion. Electron transport chain, proton pumps, and ATP synthase are components of oxidative phosphorylation. Major complexes of electron transport chain are NADH dehydrogenase, bcl complex, and cytochrome c oxidase. The quinone, Q, and cytochrome c are intermediates that shuttle electrons between the complexes. Each of the three complexes is a proton pump. Cytochrome c oxidase is the terminal complex of the electron transport chain. 

Four protein complexes, three ofthem function as proton pumps, are embedded in the inner mitochondrial membrane and constitute essential components of the electron transport chain. Every complex consists of a different set of proteins with a variety of redox-active prosthetic groups. AU in all, through oxidation of NADH, a proton gradient between the matrix and the intermembrane space is created, which eventually drives the ATP synthase-complex. The correspondingly released electrons are consumed in the reduction of oxygen to water. Both, the NADH oxidation and the oxygen reduction, as well as the ATP synthesis, take place in the matrix of the mitochondrion (Fig. 8.14). 

In the mitochondrion ApH is about 1.4, thereby giving a value of 0.06 x 1.4 = 0.084V for the chemical potential component, and Ai ) is about 0.14V thus the PMF has a value at 30 °C of about 0.224V. Ibis corresponds to a AG value of about 21.6 kJ per mole of H transported, as can be calculated by insertion of the ApH and Atti values into equation (vii). Hius it can be deduced that at least two moles of H have to be transported from the mitochondrial matrix to the intermembrane space by exergonic flow of electrons down the electron transport chain to drive the ender-gonic generation of one mole of ATP (ADP + P -> ATP + HjO AG" = + 30.5 kj.mol ). 

The ETP can be further divided by appropriate chemical treatment into even smaller particles that contain all the components of the electron transport chain (except cytochrome c) in concentrations greater than that found in the intact mitochondrion. These particles are referred to as elementary particles. 

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. 

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