Chapter 5 - Part 3.pdf

Electron-Transport Complexes Disrupting the inner mitochondrial membrane by detergents reveals that various electron carriers exist as part of four distinct membrane spanning complexes: Complex I (NADH dehydrogenase) catalyzes transfer of electrons from NADH to ubiquinone and transports four H+ per...

Electron-Transport Complexes Disrupting the inner mitochondrial membrane by detergents reveals that various electron carriers exist as part of four distinct membrane spanning complexes: Complex I (NADH dehydrogenase) catalyzes transfer of electrons from NADH to ubiquinone and transports four H+ per pair of electrons NADH Dysfunction in complex 1 has been linked to neurological diseases Complex II (succinate dehydrogenase) catalyzes transfer of electrons from succinate to FAD to ubiquinone without transport of H+ FADUz+CoQ – Only enzyme that is used in both the TCA and ETC! Complex III (cytochrome bc1) catalyzes the transfer of electrons from ubiquinone to cytochrome c and transports four H+ per pair of electrons Complex IV (cytochrome c oxidase) catalyzes transfer of electrons to O2 and transports two H+ per pair of electrons across the inner membrane. The metabolic poisons CO, N3–, and CN– bind catalytic 109 sites in Complex IV. Q) How many complexes are involved in electron transport in the electron transport chain? Can you name them? Q) Which of the complex is not involved in the transport of H+ ions? 110 Establishment of a Proton-Motive Force Two components of the proton gradient: – Concentration gradient of hydrogen ions between matrix and intermembrane space creates a pH gradient ( pH). – Separation of charge across the membrane creates an electric potential ( ). Fig 5.21: Visualizing the proton-motive Energy present in both components of the proton electrochemical force in active mitochondria with the gradients is called the proton-motive force ( p). fluorescent, cationic dye rhodamine Treatment of cells with lipid-soluble agents, 2, 4-dinitrophenol (DNP) uncouples glucose oxidation and ATP formation by increasing the permeability of the inner membrane to H+, thus eliminating the proton gradient. DNP is used to inhibit ATP formation in labs. In the 1920s, Physicians prescribed DNP as a diet pill—people died—why? https://publichealthmatters.blog.gov.uk/2018/08/13/deadly-dnp/ This drug can uncouple oxidation and phosphorylation because it is lipid soluble and the electron transport chain requires an electrochemical gradient. The consumption of drug result to send protons back into the matrix and the gradient generated by the electron transport chain is dissipated and leading to the release of energy as heat - cause excessive heat production and organ failure-death Different people have different metabolism rates: Differences in endogenous uncoupling proteins (UCPs) account for these differences. People have natural (endogenous) uncouplers found in brown adipose tissues 111 of hibernating mammals and serve as a source of heat production. Also infants (when exposed to cold). White adipose tissues Brown adipose tissues Protons moving back to matrix Against the concentration gradient results in energy release in the form of ATP Reference://www.ncbi.nlm.nih.gov/pmc/articles/PMC6659641/ DNP (proton uncoupler) insert into the inner mitochondrial membrane and shuttles protons between the intermembrane space and the matrix. The sudden diffusion of proton to the mitochondrial matrix causes current and generate heat 112 The Structure of ATP Synthase Fig 5.22:Electron micrograph of a mitochondrion, with spherical particles attached by a stalk to the cristae membranes Fig 5.23: ATP formation in membrane vesicles reconstituted with the Na+/K+-ATPase Fernandez-Moran discovered a layer of spheres projecting into the matrix. Isolation of coupling factor 1, or F1, showed that it hydrolyzed ATP, and under experimental conditions, it behaves as an ATPase. – Remember that enzymes can catalyze both forward and reverse reactions (depends on prevailing conditions) Led to conclusion that an ionic gradient establishes a proton-motive force to phosphorylate ADP. 113 The Structure of ATP Synthase Fig 5.24 a, b: Schematic diagram and 3D structure of the bacterial ATP synthase. The enzyme consists of two major portions, called F1 and F0 The F1 particle is the catalytic subunit, and contains three catalytic sites synthesis. 3 3 for ATP (The five subunits of F1 are 3 alpha, 3 beta, 1 delta, 1 epsilon and 1 gamma) The F0 particle attaches to the F1 and is embedded in the inner membrane and contains a channel through which protons are conducted from the intermembrane space to the matrix. (Fo subunits are 1a, 2b, 10-14c subunits)- F0 stands for oligomycin- a toxin that binds to the Fo unit 114 The number of subunits in the c ring is 10–14 because structural studies have revealed that this number can vary depending on the source of the enzyme. Q) How does a proton electrochemical gradient provide the energy required to drive the synthesis of ATP? a Paul Boyer of UCLA published an innovative hypothesis in 1979, called the binding change mechanism 115 Using the Proton Gradient What is the path taken by protons as they move through the Fo complex, and how does this movement lead to the synthesis of ATP? • It had been postulated that: & 1. The c subunits of the Fo base are assembled into a ring that resides within the lipid bilayer. 2. The c ring is physically bound to the subunit of the stalk. 3. The “downhill” movement of protons through the membrane drives the rotation of the ring of c subunits. 4. The rotation of the c ring of Fo provides the twisting force (torque) that drives the rotation of the attached subunit, leading to the synthesis and release of ATP by catalytic subunits of the F1 ring. Fig 5.29: A model of the proton diffusion coupled to rotation of c ring in the F0 complex 117 Peroxisomes The Cell: A Molecular Approach. 2nd ed Peroxisomes are small, membrane-enclosed organelles that contain at least 50 enzymes involved in a variety of metabolic reactions, including several aspects of energy metabolism. Peroxisomes are oxidative organelles and contain digestive enzymes that break downs toxic material in the cell and oxidative enzymes for the catabolic activities. Peroxisomes originally were defined as organelles that carry out oxidation reactions leading to the production of hydrogen peroxide. Hydrogen peroxide is harmful to the cell, peroxisomes also contain the enzyme catalase, which decomposes hydrogen peroxide either by converting it to water or by using it to oxidize another organic compound. Zellweger syndrome (ZS) is a rare inherited disease characterized by a variety of neurologic, visual, and liver abnormalities leading to death during early infancy. (patients lacking peroxisomes –problem they can synthesize peroxisomeal enzymes- enzymes fail to incorporate into the peroxisomes and remain largely in the cytosol and cannot carry out their normal function- How we know- found empty membranous structures called “ghosts” 118 The Human Perspective: Diseases that Result from Abnormal Mitochondrial or Peroxisomal Function • Mitochondria – A variety of disorders are known that result from abnormalities in mitochondria structure & function. – Majority of mutations linked to mitochondrial diseases are traced to mutations in mtDNA. – Mitochondrial disorders are inherited maternally. Degenerating muscle shows red-staining “blotches” due to abnormal proliferation of mitochondria a b Mitochondrial DNA (mtDNA)- circular DNA has many special features such as a high copy number in cell, maternal inheritance, and a high mutation rate which have made it attractive to scientists from many fields. Electron micrograph showing crystalline structures within the mitochondrial matrix 119 The Human Perspective: Diseases that Result from Abnormal Mitochondrial or Peroxisomal Function A premature-aging phenotype caused by increased mutations in mtDNA. The defective nuclear gene encodes for the DNA polymerase responsible for mtDNA replication • • • It is speculated that accumulations of mutations in mtDNA is a major cause of aging. In mice encoding a mutation in their mtDNA, signs of premature aging Additional findings suggest that mutations in mtDNA may cause premature aging but are not sufficient for the normal aging process. 120