Cellular Biology I_Mitochondria PDF

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It is allowed the use of the material for private and study use, however not for profit and without commercial purposes. Biomedical Sciences I Cellular Biology I Lorena Di Pietro [email protected] Mitochondrion and Aerobic Respiration  Mitochondrion MITOCHONDRIA ARE DYNAMIC STRUCTURES THEY MOVE INTO THE CELLS, COSTANTLY CHANGING SHAPE, DIVIDING, AND FUSING MITOCHONDRIA ARE ASSOCIATED WITH THE CYTOSKELETON, WHICH DETERMINS THEIR DISTRIBUTION  Mitochondrion DEPENDING ON THE CELL TYPE MITOCHONDRIA CAN HAVE A DIFFERENT MORPHOLOGY - individual bean‐shaped organelles (1-4 μm in length) - highly branched, inter-connected tubular network  Mitochondrion Mitochondria are thought to have evolved from an ancient aerobic bacterium that developed a symbiotic relationship with an anaerobic host cell (endosymbiosis)  Mitochondrion ORGANIZATION AND STRUCTURE - Double-membrane system (inner and outer membranes plus cristae) - Intermembrane space - Matrix  Mitochondrion OUTER MITOCHONDRIAL MEMBRANE - Outer boundary that completely encloses the mitochondrion (similar to the outer bacterial membrane) - Smooth and uniform - Freely permeable to ions and small molecules (100 different proteins) and highly impermeable - Contains 2 domains (different proteins and different functions): - Inner boundary membrane (runs parallel to the outer membrane): contains proteins (translocases) that import mitochondrial proteins, devoid of cholesterol and rich in cardiolipin - Cristae (series of invaginated membranous sheets): with a lipid content of 25%, contain enzymes and structural proteins for aerobic respiration and ATP production - Cristae junctions join the two domains  Mitochondrion INTERMEMBRANE SPACE - Contains proteins involved in the initiation of cell death mechanisms - Has the same pH and ionic composition as the cytoplasm MATRIX - Gel-like consistency - High concentration of soluble proteins (enzymes for the Krebs cycle) - DNA (1% cellular DNA content) - Free ribosomes (smaller than those found in the cytoplasm)  Mitochondrion mtDNA encodes: - 13 mitochondrial polypeptides (hydrophobic proteins) - 2 rRNA - 22 tRNA mtDNA has a very high mutation rate → heteroplasmy - lower fidelity of mitochondrial DNA replication - inefficient DNA repair The comparison of mitochondrial DNA is useful for estimating the dates of relatively recent evolutionary events  Mitochondrion The mitochondrial inheritance in both animals and plant is uniparental The mitochondrial DNA passes from one generation to the next by maternal inheritance - A typical human oocyte contains about 100,000 copies of mitochondrial DNA, whereas a sperm cell contains only a few - As sperm mature, the mitochondrial DNA is degraded - Sperm mitochondria are specifically recognized then eliminated from the fertilized egg cell by autophagy  Mitochondrion Germ-line mutations in mitochondrial DNA are transmitted to the next generation by the mother […] although the central dogma of maternal inheritance of mtDNA remains valid, there are some exceptional cases where paternal mtDNA could be passed to the offspring.  Mitochondrion Most mitochondrial proteins are translated on free cytosolic ribosomes and imported into the organelle by specific targeting signals Mitochondrion - Protein translocator complexes are located both in the inner and outer mitochondrial membranes - The TOM complex is required for the import of nearly all nucleus- encoded mitochondrial proteins - The SAM complex is required for the insertion and folding of b- barrel proteins into the outer membrane - Two different TIM complexes mediate protein transport at the inner membrane Mitochondrion - The OXA complex mediates the insertion of membrane proteins encoded by the mitochondrial genome - Mitochondrial precursors proteins remain unfolded in the cytosol (thanks to the interaction with chaperones of the hsp70 family)  Mitochondrion DYNAMIC ORGANELLES CAPABLE OF DRAMATIC CHANGES IN SHAPE: THEY CAN FUSE (FUSION) WITH ONE ANOTHER, OR SPLIT (FISSION) MITOCHONDRIA ARE OFTEN ASSOCIATED WITH THE MICROTUBULAR CYTOSKELETON WHICH DETERMINES THEIR ORIENTATION AND DISTRIBUTION THE ENDOPLASMIC RETICULUM INTERVENES IN MITOCHONDRIAL FISSION, INITIATING THE CONSTRICTION AND SOLUBLE PROTEINS COMPLETE THE PROCESS Mitochondrion THE VIDEO SHOWS THE DYNAMICS OF MITOCHONDRIAL FISSION AND FUSION: A MITOCHONDRION LABELED WITH GREEN FLUORESCENT PROTEIN DIVIDES (FISSION) AND THEN FUSES WITH A MITOCHONDRION LABELED WITH RED FLUORESCENT PROTEIN TO CREATE A YELLOW FUSED MITOCHONDRION.  Mitochondrion MITOCHONDRIA THAT HAVE BECOME NONFUNCTIONAL GO TROUGH AN ORGANIZED DEGRADATION (MYTOPHAGY) THE MITOCHONDRIAL PROTEIN IMPORT BECOMES INEFFICIENT UPON LOSS OF THE ELECTROCHEMICAL GRADIENT, CAUSING THE RETENTION OF PINK1 PROTEIN KINASE ON THE OUTER MEMBRANE  Mitochondrion PINK1 THEN PHOSPHORYLATES BOTH MITOCHONDRIA-ASSOCIATED UBIQUITIN AND THE REGULATOR PARKIN THAT INITIATE THE FORMATION OF THE AUTOPHAGOSOME MUTATIONS IN PINK1 AND PARKIN PROTEINS ARE ASSOCIATED WITH PARKINSON’S DISEASE, WHICH CAN BE CHARACTERIZED BY AN ACCUMULATION OF NONFUNCTIONAL MITOCHONDRIA  Mitochondrion Converts glucose into chemical energy (ATP) through from O2 and C → oxidative respiration CO2 and H2O Is the location of two critical steps of urea cycle in Abundant in cells with liver cells active metabolism (up to 1000) The biosynthesis of the heme groups is shared between the mitochondrion and the cytoplasm Phosphatidylethanolamine, Has a central role in membrane biosynthesis phosphatidylglycerol, and phosphatidic acid are synthetized in the mitochondrion Involved in caspase-dependant apoptosis Regulates the Ca2+ concentration of the cytosol A critical exchange of lipids with ER occurs at special sites of close contact Aerobic Metabolism  Aerobic Metabolism 1 Beginning with glucose, GLYCOLISIS generate pyruvate and NADH in the cytosol Pyruvate and NADH can be metabolized in two different ways, depending on the type of cells and the presence or absence of oxygen In the presence of O2, aerobic organisms extract large amount of additional energy  Aerobic Metabolism 2a In the presence of O2, the pyruvate moves into the matrix of the mitochondrion (facilitated by a membrane transporter) Pyruvate is decarboxylated and linked to coenzyme A (CoA), a reaction that generates NADH  Aerobic Metabolism 2a The acetyl CoA passes through the TRICARBOXYLIC ACID (TCA) CYCLE, which generates NADH and FADH2  Aerobic Metabolism 2b Mitochondria are not able to import the NADH produced during glycolysis The NADH can donate its high‐energy electrons to FAD to produce FADH2 trough the glycerol phosphate shuttle pathway Alternatively, NADH can enter the mitochondrion by the malate-aspartate shuttle and reduce NAD+ to NADH  Aerobic Metabolism 3 The electrons in NADH and FADH2 molecules are passed along the ELECTRON‐TRANSPORT CHAIN Electrons are transferred, through a series of energy- releasing reactions, by carriers that are embedded in the inner mitochondrial membrane, to molecular oxygen (O2) The energy released during electron transport is used in the formation of ATP  Aerobic Metabolism ELECTRON‐TRANSPORT CHAIN 3 4 complexes of electron carriers with redox centers, plus ubiquinone and cytochrome c - Ubiquinone occurs as a pool of molecules dissolved in the lipid bilayer - Cytochrome c is a soluble protein in the intermembrane space 1. Electrons enter the chain from either NADH (via complex I) or FADH2 (a part of complex II) 2. Electrons are passed from either complex I or II to ubiquinone  Aerobic Metabolism ELECTRON‐TRANSPORT CHAIN 3 3. Electrons are subsequently passed from reduced ubiquinone (ubiquinol) to complex III and then to the peripheral protein cytochrome c 4. Electrons are transferred from cytochrome c to complex IV (cytochrome oxidase) and then to O2 to form H2O.  Aerobic Metabolism ELECTRON‐TRANSPORT CHAIN 3 The free energy released as electrons are passed across the complexes, is conserved by translocation of protons from the matrix across the inner membrane through complexes I, II, and IV (coupling sites)  Aerobic Metabolism 4 The energy derived from oxidation is stored as an electrochemical gradient, that drive H+ back into the matrix  Aerobic Metabolism 4 ATP synthase is a protein The H+ gradient is used to drive the reaction in complex composed of two which ADP is principal components: a phosphorylated to ATP by spherical F1 head and a F0 basal section embedded in ATP SYNTHASE the inner membrane Approximately 36 ATP molecules could be generated from a single molecule of glucose  Mitochondrial Apoptosis THE CASPASES ARE PROTEASES CHARACTERIZED BY A CYSTEINE IN THEIR ACTIVE SITE AND THEY CLEAVE THEIR TARGET PROTEINS AT SPECIFIC ASPARTIC ACIDS THEY ARE SYNTHETIZED IN THE CELL AS INACTIVE PRECURSORS AND ACTIVATED ONLY DURING APOPTOSIS THE TWO BEST-UNDERSTOOD ACTIVATION MECHANISMS IN MAMMALIAN CELLS ARE: The extrinsic pathway The intrinsic (or mitochondrial) pathway Mitochondrial diseases LARGE AND HETEROGENEOUS GROUP OF DISORDERS CHARACTERIZED BY MITOCHONDRIAL DYSFUNCTION CAUSED BY MUTATIONS IN mtDNA GENES (IN 15% CASES – INHERITED IN THE REGULAR MENDELIAN FASHION) OR IN NUCLEAR GENES THAT CODE FOR MITOCHONDRIAL PROTEINS A SUBCLASS OF THESE DISEASES THAT HAVE NEUROMUSCULAR SYMPTOMS ARE KNOWN AS MITOCHONDRIAL MYOPATHIES