Mitochondria structural proteins

Assuming a protein unit of molecular weight 64,000 such as the monomeric unit of mitochondria structural proteins, (Lenaz et al., 1968) the value of R is — 58 A (area = 8900 A ). ... 

Experimental data offered as evidence for hydrophobic bonding of lipids to protein by Green and Tzagaloff (1966) can be readily given interpretations which exclude hydrophobic bond involvement of the protein with lipid chains. For example, (1) the stronger binding to mitochondrion structural protein of lipids with longer acyl chains... 

A lipoprotein is a biochemical assembly that contains both proteins and lipids. The lipids or their derivatives may be covalently or noncovalently bound to the proteins. Many enzymes, transporters, structural proteins, antigens, adhesins, and toxins are lipoproteins. Examples include the high density and low density lipoproteins of the blood, the transmembrane proteins of the mitochondrion and the chloroplast, and bacterial lipoproteins [34]. Lipoproteins in the blood, an aqueous medium, carry fats around the body. The protein particles have hydrophilic groups aimed outward so as to attract water molecules this makes them soluble in the salt water based blood pool. Triglyceride-fats and cholesterol are carried internally, shielded from the water by the protein particle [35]. 

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. 

In spite of the variety of appearances of eukaryotic cells, their intracellular structures are essentially the same. Because of their extensive internal membrane structure, however, the problem of precise protein sorting for eukaryotic cells becomes much more difficult than that for bacteria. Figure 4 schematically illustrates this situation. There are various membrane-bound compartments within the cell. Such compartments are called organelles. Besides the plasma membrane, a typical animal cell has the nucleus, the mitochondrion (which has two membranes see Fig. 6), the peroxisome, the ER, the Golgi apparatus, the lysosome, and the endosome, among others. As for the Golgi apparatus, there are more precise distinctions between the cis, medial, and trans cisternae, and the TGN trans Golgi network) (see Fig. 8). In typical plant cells, the chloroplast (which has three membranes see Fig. 7) and the cell wall are added, and the lysosome is replaced with the vacuole. 

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. 

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). 

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 first electron carrier in the electron transport chain is an enzyme that contains a tightly bound coenzyme. The coenzyme has a structure similar to FAD. The enzyme formed by the combination of this coenzyme with a protein is called flavin mononucleotide (FMN). Two electrons and one ion from NADH plus another H ion from a mitochondrion pass to FMN, then to an iron-sulfur (Fe—S) protein, and then to coenzyme Q (CoQ). CoQ is also the entry point into the electron transport chain for the two electrons and two H ions from FADH2. As NADH and FADH2 release their hydrogen atoms and electrons, NAD and FAD are regenerated for reuse in the citric acid cycle. 

Four of the five remaining electron carriers are cytochromes (abbreviated cyt), which are structurally related proteins that contain an iron group. CoQ passes along the two electrons to two molecules of cytochrome b, and the two H ions are given up to the mitochondrion. The electrons are passed along the chain, and in the final step, an oxygen atom accepts the electrons and combines with two H ions to form water. 

Yolk granule enlargement leads ultimately to the formation of a yolk body with regular geometric shape and an internal structure with well-marked crystal-like periodicity. Ward (1962b) has shown that the yolk body produced in this way consists of protein, but whether the raw materials for yolk formation are synthesized within the mitochondrion itself, or are simply assembled from phosphoproteins synthesized elsewhere and then transported to the oocyte via the bloodstream (Flick-inger and Rounds, 1956 Wallace and Dumont, 1968), is not known. In view of the limited synthetic capacities of mitochondria, it seems likely that the function of the mitochondrion in yolk formation is primarily one of assembly. 

Fig. 9. Hypothesis on the control of hemoglobin synthesis in chick embryo blastoderm by control of the synthesis of ALA-synthetase. In the nucleus a repressor protein (I) blocks transcription, and a 5jS-H steroid acts as a derepressor, permitting the structural gene (II) to code for the mRNA of ALA-synthetase. In the cytoplasm the information in the mRNA is translated into the enzyme ALA-synthetase (E,) which migrates into the mitochondrion where ALA (III) is made and finally converted by other enzymes (E2-E7) to heme (IV). Heme controls the synthesis of globin either by acting at the initiating site or by permitting proper folding of the globin.

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