The Mitochondrion

About 99% of the mitochondrial proteins are nuclear-encoded and are synthesized in the cytosol, from where they are imported into mitochondria 

This a-helix is amphipathic, containing patches of positively charged and hydrophobic amino acids, respectively, on opposite surfaces of the theoretical cylinder. The presequence is usually processed by the mitochondrial processing peptidase (MPP) and the mature protein is sorted to either the matrix, or to the inner membrane if it bears a hydrophobic stop-transfer sequence. Some mitochondrial proteins, mostly destined to the membranes, do not have cleavable N-terminal presequences but have internal targeting signals that are not well characterized (Pfanner and Geissler 2001). 

The insertion and assembly of -barrel outer membrane proteins, including Tom40, are assisted by the sorting and assembly machinery (SAM) complex (Fig. 1). The SAM complex consists of four subunits, the core translocase Sam50, which is itself a putative -barrel protein (Kozjak et al. 2003 Paschen et al. 2003 Gentle et al. 2004), and the additional proteins Sam35, 

Fig-i Mitochondrial protein import machinery as defined in S. cerevisiae. TOM translo-case of the outer mitochondrial membrane SAM sorting and assembly machinery TIM translocase of the inner mitochondrial membrane MIA mitochondrial IMS import and assembly machine PAM presequence translocase associated motor IMP inner membrane protease MPP mitochondrial processing peptidase. The numbers on the individual Tom, Sam, Tim or Pam components represent their approximate molecular masses in kDa. See text for mechanistic details. Adopted from Dolezal et al. 2006 

Tim23 forms a protein-conducting channel that is regulated by the action of Tim50, and Timl7 (Martinez-Caballero et al. 2006 Meinecke et al. 

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It may seem surprising that isocitrate dehydrogenase is strongly regulated, because it is not an apparent branch point within the TCA cycle. However, the citrate/isocitrate ratio controls the rate of production of cytosolic acetyl-CoA, because acetyl-CoA in the cytosol is derived from citrate exported from the mitochondrion. (Breakdown of cytosolic citrate produces oxaloacetate and acetyl-CoA, which can be used in a variety of biosynthetic processes.) Thus, isocitrate dehydrogenase activity in the mitochondrion favors catabolic TCA cycle activity over anabolic utilization of acetyl-CoA in the cytosol. 

An electrophoresis gel of the bovine heart complex is shown in Figure 21.14. The total mass of the protein in the complex, composed of 13 subunits, is 204 kD. Subunits I through III, the largest ones, are encoded by mitochondrial DNA, synthesized in the mitochondrion, and inserted into the inner membrane from the matrix side. The smaller subunits are coded by nuclear DNA and synthesized in the cytosol. 

The link with the final electron acceptor, O2, is the enzyme cytochrome c oxidase which spans the inner membrane of the mitochondrion. It consists of cytochromes a and a3 along with two, or possibly three, Cu atoms. The details of its action are not fully established but the overall reaction catalysed by the enzyme is ... 

The mitochondrion has an outer and an inner membrane (Figure 1). The outer membrane contains pores formed from a protein, porin, which allow exchange of molecules with molecular weights up to about 2,000 between the cytosol and the intermembrane space. The inner membrane is extensively invaginated to increase its surface area. It has a different lipid composition from the outer membrane and is rich in the acidic phospholipid cardiolipin (diphosphatidyl-glycerol) which is only found in animal cells in mitochondria. Cardiolipin confers good electrical insulating properties on the inner membrane which is impermeable... 

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). 

Rotenone is a complex flavonoid found in the plant Derris ellyptica. It acts by inhibiting electron transport in the mitochondrion. Derris powder is an insecticidal preparation made from the plant, which is highly toxic to hsh. 

Rotenone A complex flavonoid produced by the plant Denis ellyptica. It has insecticidal activity due to its ability to inhibit electron transport in the mitochondrion. 

Atractyloside inhibits oxidative phosphorylation by inhibiting the transporter of ADP into and ATP out of the mitochondrion (Figure 12-10). 

Figure 12-11. Combination of phosphate transporter ( ) with the adenine nucleotide transporter ((2)) in ATP synthesis. The H+ZP, symport shown is equivalent to the P /OH antiport shown in Figure 12-10. Four protons are taken into the mitochondrion for each ATP exported. However, one less proton would be taken in when ATP is used inside the mitochondrion.

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. 

Figure 12-13. Malate shuttle for transfer of reducing equivalents from the cytosol into the mitochondrion. Ketoglutarate transporter , glutamate/aspartate transporter (note the proton symport with glutamate).

The central role of the mitochondrion is immediately apparent, since it acts as the focus of carbohydrate, hpid, and amino acid metabohsm. It contains the enzymes of the citric acid cycle, P-oxidation of fatty acids, and ketogenesis, as well as the respiratory chain and ATP synthase. 

Glycolysis, the pentose phosphate pathway, and fatty acid synthesis are all found in the cytosol. In gluconeo-genesis, substrates such as lactate and pyruvate, which are formed in the cytosol, enter the mitochondrion to yield oxaloacetate before formation of glucose. 

Pymvate dehydrogenase is a mitochondrial enzyme, and fatty acid synthesis is a cytosohc pathway, but the mitochondrial membrane is impermeable to acetyl-CoA. Acetyl-CoA is made available in the cytosol from citrate synthesized in the mitochondrion, transported into the cytosol and cleaved in a reaction catalyzed by ATP-citrate lyase. 

The main source of GTP fot phosphoenolpytuvate catboxykinase inside the mitochondrion is the teaction of succinyl-CoA synthetase (Chaptet 16). This provides a hnk and hmit between cittic acid cycle activity and the extent of withdtawal of oxaloacetate fot gluconeogenesis. 

A summary of the steps in the biosynthesis of the porphyrin derivatives from PBG is given in Figure 32-8. The last three enzymes in the pathway and ALA synthase are located in the mitochondrion, whereas the other enzymes are cytosolic. Both erythroid and non-erythroid ( housekeeping ) forms of the first four enzymes are found. Heme biosynthesis occurs in most mammalian cells with the exception of mature erythrocytes, which do not contain mitochondria. However,... 

ALASl. This repression-derepression mechanism is depicted diagrammatically in Figure 32-9. Thus, the rate of synthesis of ALASl increases greatly in the absence of heme and is diminished in its ptesence. The turnover rate of ALASl in rat liver is normally rapid (half-life about 1 hour), a common feature of an enzyme catalyzing a rate-limiting reaction. Heme also affects translation of the enzyme and its transfer from the cytosol to the mitochondrion. 

With the exception of the acetyl-CoA thiolase, all these enzymes are located exclusively in the peroxisomes, whereas the enzymes that are involved in lipid synthesis are found in the microsomes and the mitochondrion. 

DNA, mitochondrial (mt-DNA) A double-stranded molecule of DNA that controls the development and functioning of the mitochondrion containing it mt-DNA is passed along female lines. 

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. 

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