Matrix chloroplast

The I vaginalis hydrogenosomal presequences are generally short, ranging from 5 to 14 amino acid residues for those that have been proven experimentally, and up to 17 residues for the predicted presequences (Table 1). The presequences are enriched in the amino acid residues Ser (20%), Leu (14%), Arg (11%), Ala (8%), Phe (7%), Val (6%), Thr (6%) and Asn (5%). The other amino acids are significantly under-represented. Incidentally, or accidentally, the three amino acids most commonly found in these presequences, Ser, Leu and Arg, are the ones that are each encoded by six codons. This may have been relevant in the evolution of these presequences. The mitochondrial matrix N-terminal presequences are enriched in Arg (14%), Leu (12%), Ser (11%) and Ala (14%). On the other hand, chloroplast leader peptides have a different amino acid composition with 19% Ser and 9% Thr (von Heijne et al. 1989). Markedly under-represented in hydrogenosomal presequences are the acidic residues, as in the case of both mitochondrial and plastidic presequences (von Heijne et al. 1989). 

Electron microscopy of sectioned chloroplasts shows ATP synthase complexes as knoblike projections on the outside (stromal or N) surface of thylalcoid membranes these complexes correspond to the ATP synthase complexes seen to project on the inside (matrix or N) surface of the inner mitochondrial membrane. Thus the relationship between the orientation of the ATP synthase and the direction of proton pumping is the same in chloroplasts and mitochondria. In both cases, the Fl portion of ATP synthase is located on the more alkaline (N) side of the membrane through which protons flow down their concentration gradient the direction of proton flow relative to Fi is the same in both cases P to N (Fig. 19-58). 

Eukaryotic cells also have organelles, mitochondria (Fig. 24-6) and chloroplasts, that contain DNA. Mitochondrial DNA (mtDNA) molecules are much smaller than the nuclear chromosomes. In animal cells, mtDNA contains fewer than 20,000 bp (16,569 bp in human mtDNA) and is a circular duplex. Each mitochondrion typically has two to ten copies of this mtDNA molecule, and the number can rise to hundreds in certain cells when an embryo is undergoing cell differentiation. In a few organisms (trypanosomes, for example) each mitochondrion contains thousands of copies of mtDNA, organized into a complex and interlinked matrix known as a kinetoplast. Plant cell mtDNA ranges in size from... 

It is very difficult to measure the flux of protons across the membrane either out of the mitochondria into the cytoplasm or from the cytoplasm through the ATP synthase into the mitochondria. Therefore, estimates of the stoichiometry have often been indirect. One argument is based on thermodynamics. If Ap attains values no more negative than -160 mV and Rp within mitochondria reaches at least 104 M 1, we must couple AGh of -15.4 kj/ mol to AG of formation of ATP of +57.3 kj/ mol. To do this four H+ must be translocated per ATP formed. Recent experimental measurements with chloroplast ATP synthase188 also favor four H+. It is often proposed that one of these protons is used to pump ADP into the mitochondria via the ATP-ADP exchange carrier (Section D). Furthermore, if Rp reaches 106 M 1 in the cytoplasm, it must exceed 104 M 1 in the mitochondrial matrix. 

Like mitochondria, chloroplasts (when illuminated) pump protons across their membranes (Fig. 23-18). However, while mitochondria pump protons to the outside, the protons accumulate on the inside of the thylakoids. The ATP synthase heads of coupling factor CEj are found on the outside of the thylakoids, facing the stromal matrix, while those of F, lie on the insides of mitochondrial membranes. However, the same mechanism of ATP formation is used in both chloroplasts and mitochondria (Chapter 18). 

Chlorophylls are stabilized in vivo within chloroplasts in a complex environment of lipids, proteins, and other compounds. Once extracted from the biological matrix, chlorophylls are easily oxidized upon exposure to air and/or light to form allomers and other degradation products (unitf4.i). Therefore, care should be taken to minimize the exposure of chlorophyll samples to light and air. For example, sample... 

Most mitochondria and chloroplast proteins are made on free cytosolic ribosomes, released into the cytosol and then taken up into the organelle. Uptake into the mitochondrial matrix requires a matrix-targeting sequence and occurs at sites where the outer and inner mitochondrial membranes come into contact. The process is mediated by hsp70 and hsp60 proteins and requires both ATP hydrolysis and an electrochemical gradient across the inner mitochondrial membrane. Targeting of proteins to other compartments of mitochondria or chloroplasts requires two signals. 

Glycine is then transported to the mitochondrial matrix where the conversion of two glycines to one serine occurs with the loss of CO2 and NH3 from the pool of fixed molecules. The serine is transported into the peroxisome, where it is deaminated to glycerate. The glycer-ate is transported back to the chloroplast, where it is phosphorylated to 3-phosphoglycerate for the Calvin-Benson cycle. 

Chloroplasts of higher plants are saucer-shaped, and from 4 to 10 ym in diameter and 1 to 3 ym thick. The chlorophyll is concentrated in bodies within the chloroplasts called grana, which are about 0.4 ym in diameter. Under the electron microscope, the grana appear as highly organized, precisely stacked lamellae, to which the chlorophyll is bound, imbedded in a stroma matrix. The light and associated electron transport reactions take place in the lamellae, whereas enzymes involved in carbon dioxide fixation are located in the stroma. 

Roise, D and Maduke, M. (1994) Import of a Mitochondrial Presequence into P. Denitrificans, FEBS Letters, 337, 9-13 Cavalier-Smith, T. (1987) The Simultaneous Symbiotic Origin of Mitochondria, Chloroplasts and Microbodies, Annals of the New York Academy of Science, 503, 55-71 Cavalier-Smith, T. (1992) The Number of Symbiotic Origins of Organelles, BioSystems, 28, 91-106 Hartl, F Ostermann, J., Guiard, B and Neupert, W. (1987) Successive Translocation into and out of the Mitochondrial Matrix Targeting of Proteins to the Inner Membrane Space by a Bipartite Signal Peptide, Cell, 51,1027-1037. 

FIGURE 1. Thin section of part of an isolated chloroplast showing the internal thylakoid membrane system which consists of appressed grana lamellae (g) and non-appressed stroma lamellae (s) embedded in the stroma protein matrix and surrounded by a double membrane envelope (e). 

P. jahnii Zingone (Fig. 5e) forms colonies very different from all other Phaeocystis colonies (Zingone et al. 1999). These are loose aggregates of non-motile cells embedded in a sticky mucilaginous matrix probably of polysaccharide nature, with no external layer nor a definite shape. In culture material the colonies may form wide sheets with margins at times sticking to the cell tube. Colonial cells range from 6 to 8.5 gm and have 2-4 chloroplasts. 

Colonial cells have 2-4 parietal chloroplasts, are deprived of body scales, haptonema, and flagella and are embedded in a mucilaginous matrix (Scherffel 1899 Kornmann 1955). They possess on their flagellar pole two short appendages, the... 

As for chloroplast membranes, various compounds in mitochondrial membranes accept and donate electrons. These electrons originate from biochemical cycles in the cytosol as well as in the mitochondrial matrix (see Fig. 1-9) —most come from the tricarboxylic acid (Krebs) cycle, which leads to the oxidation of pyruvate and the reduction of NAD+ within mitochondria. Certain principal components for mitochondrial electron transfer and their midpoint redox potentials are indicated in Figure 6-8, in which the spontaneous electron flow to higher redox potentials is toward the bottom of the figure. As for photosynthetic electron flow, only a few types of compounds are involved in electron transfer in mitochondria—namely, pyridine nucleotides, flavoproteins, quinones, cytochromes, and the water-oxygen couple (plus some iron-plus-sulfur-containing centers or clusters). 

Figure 6-9. Schematic representation of certain electron flow and ATP synthesis components in the inner mitochondrial membrane, emphasizing the directional flows of H+, various protein complexes, and the ATP synthase. The stoichiometry of H+ per pair of electrons for the protein complexes is tentative. The H+, which is moved toward higher jU,H accompanying electron flow along the respiratory chain, can move back through a hydrophobic channel (F0) and another protein factor attached to the inner membrane (F ), leading to ATP synthesis in the matrix. The lumen side is here designated the Inside, as for chloroplasts (Fig. 6-5).

Thus the H+ chemical potential is higher in the lumen than in the matrix (Fig. 6-6). For some cases in which mitochondrial ATP formation occurs, EM is 0.14 V and pH° - pH1 is 0.5, in which case 16 kJ (mol H+) 1 is available for ATP formation from the chemical potential difference of H+ across the inner mitochondrial membrane (Fig. 6-6). We indicated previously that at least 13 kJ per mole H+ is required for ATP formation if four H+ s are used per ATP synthesized. We also note that for chloroplasts, most of the Ais due to the pH term, whereas for mitochondria the electrical term is usually more important for ATP formation. 

The be complexes from mitochondria, chloroplasts, and bacteria all contain three catalytic subunits harboring the four redox centers cytochrome b, the high-potential cytochrome C or /, and the Rieske iron sulfur protein. These subunits are required and sufficient to support electron transport since most bacterial bci complexes only consist of these three subunits. However, some bacterial bc complexes contain a fourth subunit with yet unknown function. Mitochondrial bc complexes contain in addition to the three catalytic subunits 7-8 subunits without redox centers two large core proteins which are peripherally located and which are members of the family of matrix proeessing peptidases (MPP), and 5-6 small subunits. In cytochrome complexes, cytochrome b is split into cytochrome b(, and subunit IV containing the C-terminal part of cytochrome b in addition, 3 small hydrophobic subimits are present [18]. 

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