Mitochondria protein synthesis

Mitochondria can have different shapes, depending on the kind of cell they are in. The number of mitochondria present in a cell also varies with cell type and may range from a single large mitochondrion to thousands. The region inside the inner membrane is called the matrix. This is where the Krebs cycle that converts pyruvate into C02 and energy takes place, so it contains a lot of enzymes. Mitochondria contain ribosomes, small particles composed of RNA and protein that are the sites of protein synthesis. Mitochondria also contain their own special DNA. 

Pathways are compartmentalized within the cell. Glycolysis, glycogenesis, glycogenolysis, the pentose phosphate pathway, and fipogenesis occur in the cytosol. The mitochondrion contains the enzymes of the citric acid cycle, P-oxidation of fatty acids, and of oxidative phosphorylation. The endoplasmic reticulum also contains the enzymes for many other processes, including protein synthesis, glycerofipid formation, and dmg metabolism. 

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. 

Ered Sanger, a double Nobel Prize winner, sequenced the human mitochondrial genome back in 1981. This genome codes for 13 proteins and the mitochondrion possesses the genetic machinery needed to synthesize them. Thus, the mitochondria are a secondary site for protein synthesis in eukaryotic cells. It turns out that the 13 proteins coded for by the mitochondrial genome and synthesized in the mitochondria are critically important parts of the complexes of the electron transport chain, the site of ATP synthesis. The nuclear DNA codes for the remainder of the mitochondrial proteins and these are synthesized on ribosomes, and subsequently transported to the mitochondria. 

It is important to appreciate that this principle of coupling-in-series underlies all biochemical pathways or processes, e.g. glycolysis, generation of ATP in the mitochondrion, protein synthesis from amino acids or a signal transduction pathway. Indeed, despite the fundamental importance of signalling pathways in biochemistry, a thermodynamic analysis of such a pathway has never been done, but the principles outlined above must apply even to signalling pathways. 

Each mitochondrion contains several molecules of DNA (mtDNA), usually in a closed, circular form, as well as the ribosomes, tRNA molecules, and enzymes needed for protein synthesis.1 23 26 With rare exceptions almost all of the mitochondrial DNA in a human cell is inherited from the mother.6 263 The size of the DNA circles varies from 16-19 kb in animals27 to over 200 kb in many higher plants. Complete sequences of many mitochondrial DNAs are known.28 283 Among these are the 16,569 bp human mtDNA,29 the 16,338 bp bovine mtDNA, the 16,896 bp mtDNA of the wallaroo Macropus robustus,30 and the 17,533 bp mtDNA of the amphibian Xenopus laevis.31 32 The sea urchin Paracentotus lividus has a smaller 15,697 bp genome. However, the order of the genes in this and other invertebrate mtDNA is different from that in mammalian mitochondria.33 Protozoal mtDNAs vary in size from 5900 bp for the... 

Mitochondrion. An organelle, found in eukaryotic cells, in which oxidative phosphorylation takes place. It contains its own genome and unique ribosomes to carry out protein synthesis of only a fraction of the proteins located in this organelle. 

In mammalian cells, some 1% of the total cellular DNA is found in the mitochondria. This DNA is double stranded, circular, and small, with a molecular weight of about 10 million, which is in the same range as that of viral DNAs. Some four to ten molecules of DNA per mitochondrion, along with some ribosomes, are found in the matrix space. DNA replication, transcription, and synthesis of some mitochondrial proteins take place in the matrix space. This protein synthesis very much resembles that of bacteria. The mitochondrial genetic code differs from the "universal" genetic code (Chapter 12) used for nuclearly encoded proteins and bacteria. The reasons for this are unknown. 

Chloramphenicol blocks translation in bacteria by inhibiting peptidyltransferase of the large ribosomal subunit. It does not interfere with peptidyltransferase in the large subunit of eukaryotic ribosomes. However, the mitochondrion of animal cells contains ribosomes that are similar to bacterial ribosomes, and chloramphenicol can block protein synthesis in this organelle. This could contribute to the side effects of this drug when used in the treatment of animals. 

There are also inhibitors that affect enzyme synthesis. Inhibitors of transcription (e.g., dibromothymoquinone [DBMIB]) and inhibitors of translation (e.g., cyclo-heximide [CHX]) are available but these are not specific to particular enzymes. In addition, because protein synthesis takes place within the chloroplast and mitochondrion in eukaryotes, prokaryotic protein synthesis inhibitors (e.g., chloramphenicol [CAP]) may be necessary to distinguish prokaryotic versus eukaryotic activity (e.g., Segovia and Berges, 2005). 

As far as Is known, all RNA transcripts of mtDNA and their translation products remain In the mitochondrion, and all mtDNA-encoded proteins are synthesized on mitochondrial ribosomes. Mitochondria encode the rRNAs that form mitochondrial ribosomes, although all but one or two of the rl-bosomal proteins (depending on the species) are imported from the cytosol. In most eukaryotes, all the tRNAs used for protein synthesis in mitochondria are encoded by mtDNAs. However, In wheat, in the parasitic protozoan Trypanosoma brucei (the cause of African sleeping sickness), and in ciliated... 

The mitochondrion is an organelle where aerobic respiration occurs. Mitochondria consist of a donble membrane that is the location of the conversion of pyruvate (a metabolic componnd) and the tricarboxylic acid cycle. The nucleus stores the cell DNA and is delineated by a lipid membrane that envelopes the nucleus and is similar to the plasma membrane. The cell uses mRNA to transfer the information out into the cytoplasm for use in protein synthesis (White Zainasheff, 2010). 

Normally, the activity of ALA-synthetase in mitochondria is very low, and in the cytosol it is not detectable. When ALA-synthetase is induced in an animal the activity rises ten to forty times, and the enzyme is then found also outside the mitochondria [Granick and Urata, 35]. Studies by Hayashi et al. [33] have shown that as much as 35% of the enzyme may be obtained in the supernatant from an induced rat liver homogenate after centrifugation at 77,000 g for 2 hours. The soluble enzyme may represent newly synthesized enzyme on the way to being incorporated into the mitochondrion, as suggested by studies with marker enzymes of the mitochondria. When the activities were determined in vivo using inhibitors of protein synthesis, the soluble enzyme was estimated to have a half-life of 20 minutes, compared to 1 hour for the mitochondrial enzyme. ALA-synthetase is relatively more stable in the isolated mitochondrion [35] than in vivo. Whether its first-order decay in vivo occurs inside or outside the mitochondrion is unknown, nor is it known whether the enzyme, once transported into the mitochondrion, can be transported out again. More recently Beattie and Stuchell [35a] have shown that after induction with AIA, ALA synthetase activity in the rat liver first accumulates in the cytosol and then is transferred into the mitochondria. 

GTP has also been found to stimulate protein synthesis in intact mitochondria (13, 14). Because mitochondria are thought not to transport GTP, it is suggested that GTP is acting by binding to the outer surface of the mitochondrion. 

It can be seen from the above that all aspects of control of protein synthesis are influenced by GTP including initiation, elongation, termination, phosphorylation control of inhibition, and synthesis within an intact organelle (mitochondrion). In each case GTP stimulates protein synthesis. 

Mitochondrial DNA is small and codes for relatively few mitochondrial proteins. Although mitochondria contain their own protein synthesis machinery, the majority of the hundreds of mitochondrial proteins are coded for by nuclear genes. These proteins are synthesized in the cytoplasm and imported into the mitochondria. Plastid DNA is somewhat larger than that of the mitochondrion and contains the genetic information for more chloroplast proteins. However, as is the case for mitochondria, most of the proteins in a chloroplast are coded by nuclear genes... 

Within the cytoplasm are specialized strnctnres called organelles that carry out specific functions in the cell. The ribosomes are the sites of protein synthesis. The mitochondria are the energy-prodncing factories of the cells. A mitochondrion has an outer and an inner membrane, with an intermembrane space between them. The fluid section surrounded by the inner membrane is called the matrix. Enzymes located in the matrix and along the inner membrane catalyze the oxidation of carbohydrates, fats, and amino acids. All these oxidation pathways eventnally produce CO2, H2O, and energy, which is used to form energy-rich componnds. Table 18.1 summarizes some of the functions of the components in animal cells. 

Some insights into these questions may be gained by considering current knowledge about the biosynthesis of the ATPase and cytochrome oxidase complexes of mitochondria. Each of these two enzymes is composed of multiple subunits belonging to both the intrinsic and extrinsic classes of proteins. It has been shown that the extrinsic components (Fi and OSCP) are synthesized on cytoplasmic ribosomes and are imported into the mitochondrion, while the intrinsic subunits are synthesized in the mitochondrion. The respiratory complex, cytochrome oxidase, has also been found to be dually derived from cytoplasmic and mitochondrial protein synthesis (refs. 15, 16, Chapter 5). This enzyme consists of seven nonidentical polypeptides, three of which are made in mitochondria and four in cytoplasmic ribosomes. The cytoplasmic products are extrinsic proteins by the aforementioned criteria, whereas the mitochondrial products are markedly hydrophobic. 

These facts bring us to one of the major outstanding problems in mitochondrial biogenesis what is the mechanism whereby the vast majority of mitochondrial proteins, coded for by nuclear DNA and synthesized on cytoplasmic ribosomes, traverse the mitochondrial membrane barriers and enter the closed organelle Since the mitochondrion is impermeable to proteins of even relatively low molecular weight, a number of models have been proposed to account for the transport of products of cytoplasmic protein synthesis into mitochondria, but convincing experimental support for these is not yet available. 

Energy-linked transhydrogenase, a protein in the inner mitochondrial membrane, couples the passage of protons down the electrochemical gradient from outside to inside the mitochondrion with the transfer of H from intramitochondrial NADH to NADPH for intramitochondrial enzymes such as glutamate dehydrogenase and hydroxylases involved in steroid synthesis. 

Mitochondrial DNA is inherited maternally. What makes mitochondrial diseases particularly interesting from a genetic point of view is that the mitochondrion has its own DNA (mtDNA) and its own transcription and translation processes. The mtDNA encodes only 13 polypeptides nuclear DNA (nDNA) controls the synthesis of 90-95% of all mitochondrial proteins. All known mito-chondrially encoded polypeptides are located in the inner mitochondrial membrane as subunits of the respiratory chain complexes (Fig. 42-3), including seven subunits of complex I the apoprotein of cytochrome b the three larger subunits of cytochrome c oxidase, also termed complex IV and two subunits of ATPase, also termed complex V. 

Eugene Kennedy and Albert Lehninger showed in 1948 that, in eulcaiyotes, the entire set of reactions of the citric acid cycle takes place in mitochondria. Isolated mitochondria were found to contain not only all the enzymes and coenzymes required for the citric acid cycle, but also all the enzymes and proteins necessaiy for the last stage of respiration—electron transfer and ATP synthesis by oxidative phosphoiylation. As we shall see in later chapters, mitochondria also contain the enzymes for the oxidation of fatty acids and some amino acids to acetyl-CoA, and the oxidative degradation of other amino acids to a-ketoglutarate, succinyl-CoA, or oxaloacetate. Thus, in nonphotosynthetic eulcaiyotes, the mitochondrion is the site of most energy-yielding... 

Matrix of the mitochondrion This gel-like solution in the interior of mitochondria is fifty percent protein. These molecules include the enzymes responsible for the oxidation of pyruvate, amino acids, fatty acids (by p-oxidation), and those of the tricarboxylic acid (TCA) cycle. The synthesis of urea and heme occur partially in the matrix of mitochondria. In addition, the matrix contains NAD+and FAD (the oxidized forms of the two coenzymes that are required as hydrogen acceptors) and ADP and Pj, which are used to produce ATP. [Note The matrix also contains mitochondrial RNA and DNA (mtRNA and mtDNA) and mitochondrial ribosomes.]... 

Some proteins, especially those destined for the eukaryotic mitochondria and chloroplasts, are transported after their synthesis on free polysomes is complete. Such transport is known as posttranslational transport. In the case of posttranslational transport it is believed that the polypeptide to be transported must be unfolded from its native folded configuration by a system of polypeptide-chain-binding proteins (PCBs) before it can pass through the membrane. Posttranslational transport into the mitochondrion requires both ATP and a proton gradient. Presumably the energy from one or both of these sources is used to unfold the protein or separate it from the PCB system so that it can pass through the membrane. 

The oxidative phosphorylation system contains over 80 polypeptides. Only 13 of them are encoded by mtDNA, which is contained within mitochondria, and all the other proteins that reside in the mitochondrion are nuclear gene products. Mitochondria depend on nuclear genes for the synthesis and assembly of the enzymes for mtDNA replication, transcription, translation, and repair (Tl). The proteins involved in heme synthesis, substrate oxidation by TCA cycle, degradation of fatty acids by /i-oxidalion, part of the urea cycle, and regulation of apoptosis that occurs in mitochondria are all made by the genes in nuclear DNA. 

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