5.19.06 OVERVIEW OF RESPIRATION AND LOOSE ENDS What agents? What war? 1
Ubiquinone or Coenzyme Q: small hydrophobic molecule that can pick up or donate electrons The respiratory chain contains 3 large enzyme complexes: each complex acts as an electron-transport-driven H + pump NADH dehydrogenase complex (22 polypeptide chains!) accepts electrons from NADH electrons are passed via a number of molecules including ubiquinone (coenzyme Q or just Q) to the cytochrome b-c 1 complex 2
Cytochromes: family of proteins that are related by the presence of a bound heme group whose iron atom changes from Fe +3 to Fe +2 whenever it accepts and electron Cytochrome b-c 1 complex: accepts electrons from ubiquinone passes electrons to cytochrome c, which carries electrons to the cytochrome oxidase complex Cytochrome oxidase complex (cytochrome aa 3 ): accepts electrons from cytochrome c and passes them to oxygen also picks up a pair of protons to from water: 1/2 O 2 + 2 H + + 2e - ---> H 2 O The two components that carry electrons between the three major enzyme complexes (ubiquinone and cytochrome c) diffuse rapidly in the plane of the membrane The three enzyme complexes appear to function as independent entities in the plane of the membrane Order of transfer of electrons is due entirely to the specificity of the functional interactions among the components of the chain and the tendancy of a specific component to be oxidized or reduced 3
Electron transfer in the cytochromes involves metal ions such as Cu and Fe (A) The reaction of O 2 with electrons in cytochrome oxidase The iron atoms are linked to a heme residue (B) 4
Back to cyanide Cyanide affects virtually all body tissues, attaching itself to ubiquitous metalloenzymes and rendering them inactive Principle toxicity probably results from inactivation of cytochrome oxidase and thus oxidative phosphorylation Oxygen dependent tissues (highest rate of respiration?) -- brain, heart, liver -- are the most profoundly affected by acute cyanide poisoning Heme Found in what other protein? Inhibitors of Oxidative Phosphorylation: Inhibit cytochrome c oxidase by binding to its heme group cyanide azide carbon monoxide lethal in small doses! [SeeMVH pg. 535 poisons] 5
CORRECTION TO comment in Wednesday's lecture Cytochrome C oxidase (last member of the electron transport chain) : 1. reduces O 2 to water 2. conserves the considerable free energy available from this highly favorable reaction by pumping protons via three independent proton pumping pathways (aka proton wires!) H-bonding is always with us: net translocation of protons can occur over a long distance through a protein by hopping between pairs of hydrogen bonded donor and acceptor residues; a string of such residues connected by hydrogen bonds can be thought of as a proton wire... 6
What can t bacteria do? Nature 441:274 May 18,2006 7
In the case of aerobic respiration, used by eukaryotes and many bacteria, the terminal electron acceptor is molecular oxygen But in principle it could be any atom or molecule that has a hunger for electrons or at least more hunger than the atom or molecule upstream in the electron transfer chain! See next page 8
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Back to the regular process.. The complete oxidation of 1 mole of glucose generates about 38 moles of ATP (synthesized from ADP) ATP yield from complete oxidation of glucose Process Direct product Final ATP Glycolysis Pyruvate oxidation (2 per glucose) Acetyl CoA oxidation (Citric Acid cycle) two per glucose Total ATP yield per glucose molecule 2 NADH (cytosolic) 2 ATP 2 NADH (mitochondrial matrix) 6 NADH (mitochondrial matrix) 2 FADH 2 2ATP or 2 GTP 3 or 5* 2 5** 15 3 2 30-32*** * depends on which "shuttle system" transfers reducing equivalents into the mitochondria ** check reference for # protons per ATP *** This number varies from reference to reference.. Proton and ATP accounting 10
Why is ATP high energy ATP has stored potential energy: ATP ADP + P i + energy ΔG o = - 7.3 kcal/mole exergonic reaction (corresponds to an Keq of >10 5 Under cellular conditions, the hydrolysis of ATP creates two molecules of much lower energy and releases a great deal of usable energy 1. The phosphates in ATP can be considered to exist in an activated state: the presence of four negative charges in close proximity destabilizes the molecule -- electrostatic repulsion between negative charges favors hydrolysis 2. Increased hydration ADP and P -- energetically favored 3. Release of a phosphate increases entropy because the PO 4 molecule released is capable of resonance forms (delocalized proton and oxygen binding) not possible when phosphate is bound to another molecule 11
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What do cells do with ATP? 13
Drives anabolic (endergonic) chemical reactions Used to do work (move stuff around) used for active transport to make/maintain ion and solute gradients Lots of other roles 14
Cells drive active transport in three main ways 15
Remember lysosomes? Lysosomes contain hydrolytic enzymes that are active under acidic conditions. The interior of this organelle is maintained at an acidic ph (high proton concentration) by a H+ ATPase in the membrane that pumps protons against the concentration gradient 16
WHAT HAPPENS DURING GLUCOSE OXIDATION IF NO OXYGEN IS PRESENT? AEROBIC AND ANAEROBIC ORGANISMS One basic metabolic distinction among organisms is whether or not they can use O 2 as an electron acceptor in energy producing pathways AEROBES: CAN USE O 2 Obligate Aerobe: O 2 is obligatory for life ANAEROBES: CAN SUBSIST WITHOUT O 2 Facultative Anaerobe: can adapt to anaerobic conditions by substituting other electron acceptors for O 2 (yeast, bacteria) Obligate Anaerobe: cannot use O 2 and are poisoned by it 17
BEAKER CONTAINS YEAST AND SUGAR What is the gas being produced in the beaker? WHAT HAPPENS DURING GLUCOSE OXIDATION IF NO OXYGEN IS PRESENT? Cell has a limited amount of NADH which must be recycled if glycolysis is to continue under anaerobic conditions FERMENTATION: an anaerobic biological reaction process 18
FERMENTATION: THE ANAEROBIC FATE OF PYRUVATE Commercially Valuable fermentation reactions: Alcoholic fermentation by yeast used in brewing and winemaking Bacteria also can carry out alcoholic fermentation under anaerobic conditions 19
This type of fermentation occurs in some fungi and bacteria (used to make yogurt and cheese) and in human muscle cells when oxygen is limiting 20
The three common metabolic fates of pyruvate generated by glycolysis: 1. Under aerobic conditions, the pyruvate is completely oxidized via the citric acid cycle to CO 2 and H 2 O [NADH acts as a high energy compound] 2. Under anaerobic conditions, pyruvate must be converted to a reduced end product in order to reoxidize the NADH produced by the GAPDH reaction alcoholic fermentation: in yeast, pyruvate is converted to ethanol + CO 2 [free energy of NADH oxidation is dissipated as heat] in muscle cells, under anaerobic conditions, pyruvate is reduced to lactate [free energy of NADH oxidation is dissipated as heat] 21
FERMENTATION: an anaerobic biological reaction process in which a reduced organic compound (like glucose) acts as an electron donor and another organic compound acts as an electron acceptor An even more formal definition of fermentation fermentation: catabolic reactions producing ATP in which organic compounds serve as both the primary electron donor and ultimate electron acceptor and ATP is produced by substrate level phosophorylation Note: fermentation is extremely inefficient compared to aerobic respiration. From first principles: Why? 22
The potential energy drop between glucose and an electron acceptor like pyruvate is a fraction of the potential energy drop that occurs during cellular respiration where O 2 is the electron acceptor 23