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Transcript
Energy conversion in mitochondria and chloroplasts 0.5 x 2 um How can you recognize different Organelles? 1.5-2 x 1 um 5-10 x 2 um Chemiosmotic coupling Stage 1 Stage 2 Oxidative phosphorylation NADH produced from catabolized biomolecules transforms to ATP Use oxygen, release carbon dioxide Where is NADPH generated for biosynthesis in animal cells? Photophosphorylation Solar energy transforms to NADPH and ATP Use water, release oxygen and fix carbon dioxide to (CH2O)n Mitochondria tend to be aligned along microtubules. cell stained with rhodamine 123 (mitochondria specific fluorescent dye) cell stained with fluorescent labelled antibody to microtubules Mitochondria locate near sites of high ATP demand in cardiac muscle and a sperm tail. Biochemical fractionation of purified mitochondria into components Each NADH is transformed to ~1.5 to 2.5 ATPs. (Some protons/energy are used for transporting ions.) 1 glucose gives a net yield of ~ 30 ATPs. Outer membrane has porins, forming channels permeable to molecules <5000 dal. Inner membrane has cardiolipin (diphosphatidyl glycerol) and impermeable to ions. ATPase is at inner membrane of mitochondria and is at thylakoid membrane of chloroplasts. Leaky to MgCl2, So no/low potential difference A Chemiosmotic Process Couples Oxidation Energy to ATP Production A comparison of biological oxidation with combustion Energy Derived from Oxidation Is Stored as an Electrochemical Gradient ~150mV 0.5-0.6 pH units 1 pH unit equivalent ~ 60mV Free energy is a gauge to measure the driving force for reactions. Negative free energy favors the reaction. Figure 14-18 (part 4 of 4) Molecular Biology of the Cell (© Garland Science 2008) Redox potential and free energy Pigments and electron acceptors in mitochondria Heme in cytochrome Iron-sulfur cluster NADH and NADPH has an absorption peak at 340nm Conversion between NAD(P)H and NAD(P)+ can be measured by the absorbance change at 340nm Figure 14-9 Molecular Biology of the Cell (© Garland Science 2008) NADH Transfers Its Electrons to Oxygen Through Three Large Enzyme Complexes Embedded in the Inner Membrane Ubiquinone is freely mobile in membrane FADH2 = complex I Flavin, FeS = complex III Uncoupler collapses the electrochemical proton gradient, eg. dinitrophenol = complex IV Redox potential changes along the mitochondrial electron transport chain 2e- 4 /e- 2 /e- 1 /e- Structure of NADH dehydrogenase Directional release and uptake of protons by a quinone pumps protons across a membrane 2e- 2e- Quinone picks up protons and e- on the matrix side, then QH2 diffuses to the crista side of cytochrome C reductase. Cytochrome C reductase is a dimer (2 X 11 subunits = total 480 kdal in size) Two-step mechanism of the cytochrome c reductase Q-cycle Q cycle : while one of the electrons received from each QH2 molecule is transferred from ubiquinone through the complex to the carrier protein cytochrome c, the other electron is recycled back into the quinone pool. The Cytochrome c Oxidase Complex Pumps Protons and Reduces O2 Using a Catalytic Iron–Copper Center The Respiratory Chain Forms a Supercomplex in the Crista Membrane Protons Can Move Rapidly Through Proteins Along Predefined Pathways The ATP Synthase Produces ATP by Rotary Catalysis Generates 100 ATP / second 3 ATP / turn Mitochondrial Cristae Help to Make ATP Synthesis Efficient dimer Special Transport Proteins Exchange ATP and ADP Through the Inner Membrane Chemiosmotic Mechanisms First Arose in Bacteria Synthesis of ATP Use of ATP Figure 14-36 Molecular Biology of the Cell (© Garland Science 2008) Chloroplasts Capture Energy from Sunlight and Use It to Fix Carbon Carbon Fixation Uses ATP and NADPH to Convert CO2 into Sugars Sugars Generated by Carbon Fixation Can Be Stored as Starch or Consumed to Produce ATP Chlorophyll–Protein Complexes Can Transfer Either Excitation Energy or Electrons Chlorophyll a, R= CH3 Chlorophyll b, R= CHO 450 470 675 640 A Photosystem Consists of an Antenna Complex and a Reaction Center Chlorophyll–Protein Complexes Can Transfer Either Excitation Energy or Electrons Purple sulfur bacteria use H2S photosynthesis The Thylakoid Membrane of Chloroplasts Contains Two Different Photosystems Working in Series Z scheme Photosystem II Uses a Manganese Cluster to Withdraw Electrons From Water The Cytochrome b6- f Complex Connects Photosystem II to Photosystem I Photosystem I Carries Out the Second Charge-Separation Step in the Z Scheme The Chloroplast ATP Synthase Uses the Proton Gradient Generated by the Photosynthetic Light Reactions to Produce ATP plastocyanin (a small copper containing protein) ferredoxin (a small protein containing an iron-sulfur center) Figure 14-48 Molecular Biology of the Cell (© Garland Science 2008) All Photosynthetic Reaction Centers Have Evolved From a Common Ancestor Cyclic photophosphorylation Only PS I is involved: the reaction center is P700. Electrons travels in a cyclic manner: electrons travel back to PS I. Only ATP is produced without NADPH and oxygen. This system is predominant in bacteria. The Proton-Motive Force for ATP Production in Mitochondria and Chloroplasts Is Essentially the Same Chemiosmotic Mechanisms Evolved in Stages By Providing an Inexhaustible Source of Reducing Power, Photosynthetic Bacteria Overcame a Major Evolutionary Obstacle. The Photosynthetic ElectronTransport Chains of Cyanobacteria Produced Atmospheric Oxygen and Permitted New Life-Forms Electron-transport chains in bacteria, chloroplasts, and mitochondria are similar. In reconstituted in vitro systems, the different complexes can substitute for one another, and the structures of their protein components reveal that they are evolutionarily related. Photosynthetic Electron-Transport Chains of Cyanobacteria Produced Atmospheric Oxygen and Permitted New Life-Forms ~ 1.5 billion years ago ~ 2 – 2.7 billion years ago ~ 3 – 3.5 billion years ago An endosymbiotic oxygen-evolving cyanobacterium gave rise to chloroplasts, while mitochondria arose from an α-proteobacterium. Over Time, Mitochondria and Chloroplasts Have Exported Most of Their Genes to the Nucleus by Gene Transfer Why the last five mitochondrial genes are conserved? Perhaps they need to be cotranslationally inserted into the inner membrane. Cytosolic production of hydrophobic proteins and their import into the organelle may present a problem to the Cell. Alterations in the genetic code that made the remaining mitochondrial genes nonfunctional if they were transferred to the nucleus. Mitochondria Have a Relaxed Codon Usage and Can Have a Variant Genetic Code Organelle Genes Are Maternally Inherited in Animals and Plants Mutations in Mitochondrial DNA Can Cause Severe Inherited Diseases Accumulation of Mitochondrial DNA Mutations Is a Contributor to Aging The Fission and Fusion of Mitochondria Are Topologically Complex Processes, which constantly remodel the network. Mitochondrial division Mitochondrial fusion Chloroplasts remain closer to their bacterial origins than mitochondria Sequences of proteins encoded in chloroplasts are very similar to bacterial, and several clusters of genes are organized in the same way. Regulatory sequences (promoters and terminators) of chloroplasts are virtually identical to bacteria. The mechanisms by which chloroplasts and bacteria divide are similar. Both utilize FtsZ proteins, which are self-assembling GTPases to form a dynamic ring of membrane-attached protofilaments and to recruitment of other division proteins and generate a contractile force that results in membrane constriction and eventually in division. The machinery for chloroplast division acts from the inside, as in bacteria, while the dynamin-like GTPases divide mitochondria from the outside