Chapter 9 Mitochondrial Structure and Function

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1 Chapter 9 Mitochondrial Structure and Function

2 1 2 3 Structure and function Oxidative phosphorylation and ATP Synthesis Peroxisome Overview 2

3 Mitochondria have characteristic morphologies despite variable appearance. Typical mitochondria are beanshaped organelles but may be round or threadlike. The size and number of mitochondria reflect the energy requirements of the cell. Structure and Function 3

4 The balance between fusion and fission is likely a major determinant of mitochondrial number, length, and degree of interconnection. Fusion and Fission 4

5 Inner and outer mitochondrial membranes enclose two spaces: the matrix and intermembrane space. The outer mitochondrial membrane serves as its outer boundary. The inner mitochondrial membrane is subdivided into two interconnected domains: Inner boundary membrane Cristae where the machinery for ATP is located Mitochondrial Structure 5

6 The outer membrane is about 50%; the inner membrane is more than 75% protein. The inner membrane contains cardiolipin but not cholesterol, both are true of bacterial membranes. The outer membrane contains a large poreforming protein called porin. The inner membrane is impermeable to even small molecules; the outer membrane is permeable to even some proteins. Mitochondrial Membranes 6

7 Contains a circular DNA molecule, ribosomes, and enzymes. RNA and proteins can be synthesized in the matrix. The mitochondrial matrix 7

8 OXIDATIVE METABOLISM Role of Mitochondria in the Formation of ATP 8

9 Overview of carbohydrate metabolism in eukaryotic cells 9

10 The first steps in oxidative metabolism Glycolysis produces pyruvate, NADH, and two molecules of ATP. Aerobic organisms use O2 to extract more than 30 additional ATPs from pyruvate and NADH. Pyruvate is transported across the inner membrane and decarboxylated to form acetyl CoA, which enters the next stage. Glycolysis 10

11 It is a stepwise cycle where substrate is oxidized and its energy conserved. The two-carbon acetyl group from acetyl CoA is condensed with the four-carbon oxaloacetate to form a six-carbon citrate. During the cycle, two carbons are oxidized to CO2, regenerating the four-carbon oxaloacetate needed to continue the cycle. Tricarboxylic acid (TCA) cycle 11

12 Four reactions in the cycle transfer a pair of electrons to NAD+ to form NADH, or to FAD+ to form FADH2. Reaction intermediates in the TCA cycle are common compounds generated in other catabolic reactions making the TCA cycle the central metabolic pathway of the cell. Tricarboxylic acid (TCA) cycle 12

13 The reduced coenzymes FADH2 and NADH are the primary products of the TCA cycle. NADH formed during glycolysis enters the mitochondria via malate-aspartate or glycerol phosphate shuttles. As electrons move through the electrontransport chain, H + are pumped out across the inner membrane. Reduced Coenzymes 13

14 ATP is formed by the controlled movement of H+ back across the membrane through the ATPsynthesizing enzyme. The coupling of H + translocation to ATP synthesis is called chemiosmosis. Three molecules of ATP are formed from each pair of electrons donated by NADH; two molecules of ATP are formed from each pair of electrons donated by FADH2. Reduced Coenzymes 14

15 Strong oxidizing agents have a high affinity for electrons; strong reducing agents have a weak affinity for electrons Redox reactions are accompanied by a decrease in free energy. The transfer of electrons causes charge separation that can be measured as a redox potential. Oxidation-Reduction (Redox) Potentials 15

16 Flavoproteins are polypeptides bound to either FAD or FMN. Cytochromes contain heme groups bearing Fe or Cu metal ions. Three cooper atoms are located within a single protein complex and alternate between Cu2+/Cu3+ Ubiquinone (coenzyme Q) is a lipid-soluble molecule made of five-carbon isoprenoid units. Iron-sulfur proteins contain Fe in association with inorganic sulfur. Electron Carriers 16

17 Electron-Transport Complexes 17

18 Complex I (NADH dehydrogenase) catalyzes transfer of electrons from NADH to ubiquinone and transports four H+ per pair. Complex II (succinate dehydrogenase) catalyzes transfer of electrons from succinate to FAD to ubiquinone without transport of H+. Complex III (cytochrome bc1) catalyzes the transfer of electrons from ubiquinone to cytochrome c and transports four H+ per pair. Electron-Transport Complexes 18

19 Complex IV (cytochrome c oxidase) catalyzes transfer of electrons to O2 and transports H+ across the inner membrane. Cytochrome oxidase is a large complex that adds four electrons to O2 to form two molecules of H2O. The metabolic poisons CO, N3 (nitride), and CN (cyanide) bind catalytic sites in Complex IV. Electron-Transport Complexes 19

20 Electrons are transferred one at a time. Energy released by O2 reduction is presumably used to drive conformational changes. These changes would promote the movement of H+ ions and through the protein. Cytochrome oxidase 20

21 Oxidative phosphorylation 21

22 The F1 particle is the catalytic subunit, and contains three catalytic sites for ATP synthesis. The F0 particle attaches to the F1 and is embedded in the inner membrane. The F0 base contains a channel through which protons are conducted from the inter-membrane space to the matrix. ATP synthase 22

23 Movement of protons through ATP synthase alters the binding affinity of the active site. Each active site goes through distinct conformations that have different affinities for substrates and product. Binding sites on the catalytic subunit can be tight, loose, or open. ATP is synthesized through rotational catalysis where the stalk of ATP synthase rotates relative to the head. There is structural and experimental evidence to support this mechanism Binding Change Mechanism 23

24 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. 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 develop. Additional findings suggest that mutations in mtdna may cause premature aging but are not sufficient for the normal aging process. Diseases that Result from Abnormal Mitochondrial Function 24

25 PEROXISOMES 25

26 Oxidize very-longchain fatty acids Form by splitting from preexisting organelles, import preformed proteins, and engage in oxidative metabolism. Peroxisomes 26

27 Hydrogen peroxide (H2O2), a reactive and toxic compound, is formed in peroxisomes and is broken down by the enzyme catalase. Plants contain a special peroxisome called glyoxysome, which can convert fatty acids to glucose by germinating seedlings. Peroxisomes 27

28 Patients with Zellweger syndrome lack peroxisomal enzymes due to defects in translocation of proteins from the cytoplasm into the peroxisome. Adrenoleukodydstrophy is caused by lack of a peroxisomal enzyme, leading to fatty acid accumulation in the brain and destruction of the myelin sheath of nerve cells. Peroxisomal Dysfunction 28