Quiz 9 study guide: The Electron Transport Chain
Quiz 9 study guide: The Electron Transport Chain BCMB 3100
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This 8 page Study Guide was uploaded by Skyler Tuholski on Friday October 7, 2016. The Study Guide belongs to BCMB 3100 at University of Georgia taught by Wood & Sabatini in Fall 2016. Since its upload, it has received 108 views. For similar materials see Intro to Biochem and Molecular Bio in Biochemistry at University of Georgia.
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Date Created: 10/07/16
Quiz 9 Study Guide: Electron Transport Chain (Ch 20-21) and Glycogen Metabolism (Ch 24-25) Ch 20 + 21: The Electron Transport Chain OBJECTIVES: 1. Origin of reducing equivalents (from previous lecture on CAC) 2. What are the major components of the electron transport chain? 3. How are they arranged? 4. Why is having the electron transport chain located in a membrane necessary? 5. How is the proton motive force converted into ATP Overview of ETC: Exergonic flow of electrons from NADH and FADHS to oxygen takes place in 4 large protein complexes that are embedded in the mitochondrial membrane 3 of these complexes use energy released by the electron flow to pump proteins from the mitochondrial matrix into the space between the inner and outer mitochondrial membranes The proton gradient is used to power the synthesis of ATP by oxidative phosphorylation Chemiosmotic Theory (now known as oxidative phosphorylation): A proton concentration gradient serves as the energy reservoir for driving ATP formation; proton-motive force 1. Intact inner mitochondrial membrane is required (to maintain a proton gradient). 2. Electron transport through “electron transport complexes” generates a proton gradient (pumps H from the matrix to the intermembrane space). 3. The membrane-spanning enzyme, ATP synthase, catalyzes the phosphorylation of ADP in a reaction driven by movement of H back across the inner membrane into the matrix. o Mitochondria have 2 membranes Outer: porous to most small molecules and ions Inner: impermeable to most all polar ions and molecules. Transporters are needed to shuttle metabolites across it, but O2 and CO2 freely diffuse. Location of ETC. (CAC was located in mitochondrial matrix) He said we should be able to draw this and know the names of each complex: Part 1: Setting up the Gradient Protons are pumped out of the matrix as electrons are transferred to 2 The energy stored in the proton gradient is used to produce ATP. The driving force is the electron-transfer potential of NADH/FADH relative to that of2O o Strong reducing: NADH, ready to donate electrons, negative reduction potential o Strong oxidizing agent: O2, ready to accept electrons, positive reduction potential o Ex: NADH is the reductant, NAD + H is the oxidant *Notice: NADH goes first to complex I FADH2 goes first to complex II Lecture Question: How can iron have different reduction potentials (see graph above) and is used as e- carrier is several places? Iron is in different environments in each complex. Iron can form clusters, usually with sulfur, and interacts with proteins in different ways. The environment therefore regulates the reduction potential. Key features of the ETC NADH is oxidized by passing electrons to Flavin mononucleotide (FMN), an electron carrier similar to FAD o Reduced FMN is then oxidized by the next electron carrier in the chain, and the process repeats as electrons flow down the chain until eventually reducing O2 Electrons always flow to components with more positive reduction potentials (higher affinity for electrons) FADH2 enters the ETC at complex II vs. complex I like NADH b/c the electrons of FADH2 have a lower reduction potential o Electrons from FADh2 pump fewer protons and yield less molecules of ATP Coenzyme Q (aka ubiquinone) is a hydrophobic molecule, so it can diffuse rapidly across the inner mitochondrial membrane, where it shuttles protons and electrons o Shuttles electrons from complexes I and II to complex III ETC is not active unless there is a need for ATP to be synthesized Chemiosmotic phosphorylation: The movement of ions across a membrane is converted into a kinetic force that phosphorylates ADP *know the difference* Complex I: NADH-ubiquinone oxidoreductase (NADH dehydrogenase) NADH transfers two electrons to FMN Electrons, one at a time, are then passed to Q via iron-sulfur proteins to be carried to complex III Leads to the pumping of 4 H+ out of the matrix of the mito o Proton removal contributes to proton-motive force Complex II: Succinate- Q reductase complex *Review: FADH2 is formed in CAC in the oxidation of succinate to fumarate by succinate dehydrogenase Complex II accepts electrons from succinate and catalyzes the reduction of Q to Q2 Electron carriers in this complex: FAD, iron-sulfur proteins, and Q o FADH2 does not leave the complex: its electrons are transferred to the Fe-S proteins and then to Q for entry into the ETC o Flavin Adenine Dinucleotide (FAD) of Complex II is reduced in a 2-electron transfer from succinate Complex II does NOT transfer protons, but it supplies electrons from succinate less ATP is formed than from the oxidation of FADH2 than NADH Electrons from succinate are now carried by QH i2 the membrane to Complex III Complex III: Ubiquinol-cytochrome c oxidoreductase Transfers electrons from QH t2 cytochrome c + Oxidation of QH 2s accompanied by the translocation of 4 H across the inner mitochondrial membrane + + Two H are from the matrix, two from QH , r2sulting in a net transfer of 4 H o Cytochrome c can only accept one electron at a time, but QH2 has 2 electrons… Solution: THE Q CYCLE! Q-cycle within Complex III: First half of cycle: 2 e- bound to QH2 are transferred- one to cytochrome c, one to a bound Q at a second binding site to form the semiquinone radical ion Second half: another QH2 gives up its 2 electrons, one to another cytochrome c, one to the semiquinone radical to reduce it back to QH2 o Results in uptake and removal of 2 H+, contributing to formation of proton gradient Summary: 4 H+ are released into the intermembrane space, and 2 H+ are removed from the mitochondrial matrix Complex IV: Cytochrome c oxidase (pg 375-376) Catalyzes a four-electron transfer from the reduced form of 4 cytochrome c molecules to molecular oxygen (O )2 which is then reduced to 2 molecules of water (H O2 o Source of electrons is cytochrome c (links Complexes III and IV) Iron atoms (hemes of cyt a) and copper atoms are both reduced and oxidized as electrons flow + Leads to overall transfer of 4 H from the matrix (2 for each QH 2o Complex III). Thermodynamically favorable Complex V: ATP Synthase ATP Synthase uses the proton gradient energy for the synthesis of ATP o Passage of protons through the F into the matrix is coupled to ATP formation o Estimated passage of 3 H / ATP synthesized F1(knob) contains the catalytic subunits F0(stalk) has a proton channel which spans the membrane. o F0ring turns the subunit, which changes the shapes of the subunits 3 active sites on the enzyme with 3 different functions. Proton-motive force causes active sites to change functions 1) trapping of ADP and P ???? 2) ATP synthesis 3) ATP release and ADP and P bi????ding Thesubunit can be in any one of 3 forms at any given time, and the interconversion of these forms is driven by the rotation of the subunit o L- loose conformation- binds ADP and P ???? o T- tight conformation- binds ATP with high affinity o O- open form- will bind and release adenine nucleotides o Moves TOL with no 2 subunits ever present in same conformation Proton motive force has a chemical gradient (pH) and a charge gradient Mvmt of H+ through the half-channels from the high proton concentration of the inner membrane space to the low proton concentration of the matrix powers rotation of c-ring Each 360 degree rotation of the gamma subunit leads to the synthesis and release of 3 ATP molecules NADH in the cytoplasm is from glycolysis. Only produces 3 ATP from 2 NADH because it costs energy to get the electrons from NADH into the mitochondria. This image is the result of using the glycerol 3-phosphate shuttle, but you would have 32 ATP produced in the heart and liver using malate-aspartate shuttle (see below) Shuttles allow movement across mitochondrial membranes Electrons from NADH are carried across the mitochondrial membrane through 2 methods: o Glycerol 3-phosphate shuttle: takes electrons from NADH and brings them to FADH, which can then use the Q cycle and get in the mitochondria. But this makes you lose some energy by bypassing complex I 1.5 ATP rather than 2.5 Prominent in muscle so it can sustain high rate of oxidative phosphorylation o Malate-aspartate shuttle: Malate dehydrogenase conserves energy of electrons by going from NADH to oxaloacetate, forming malate, and then going into mitochondria. Produces 2.5 ATP Prominent in heart and liver: so you don’t lose potential energy! Regulated Uncoupling leads to generation of heat Brown Fat Cells- Rich in mitochondria with the natural uncoupling protein-1, or thermogenin. Results in short circuiting ox-phosphorylation and no production of ATP, but rather generation of HEAT! Maintains body temp in hibernating animals and mammals adapted to cold Pigs lack this UCP-1 so they have large litters and build a nest for birth to keep babies warm. Humans have it. Unregulated Uncoupling: DNP The proton-motive force is continually dissipated, leading to increased oxygen consumption and oxidation of NADH Heat is formed but NO ATP This was found in herbicides and fungicides but is still a dangerous dieting pill used today This is what he said we need to know for the upcoming quiz during breakout Thursday: Given structures and names of substrates of the CAC: Know the enzyme and products, if energy is produced or consumed, and if it is a regulated step. Ex: NAD+ is a substrate that produces NADH and CO2 Understand the concept of the electron transport chain, what’s happening, electrons are going from strong reducing agent to strong oxidizing agent: all about the electron transfer potential. O2 is the terminal electron acceptor, has the most positive reduction potential. FADH2 doesn’t have high enough reduction potential to transfer e- to NADH-Q reductase (complex I) like NADH does, which is why NADH leads to production of more protons. Know complex names. What Q is and what biochemical characteristics of Q are important for its function of shuttling electrons from complex I or II to III: nonpolar so it can easily travel through inner membrane. Recognize certain structures and their role: iron-sulfur cluster, Q vs QH2, DNP Iron within Iron-sulfur clusters are at different reduction potentials because they are in different environments within enzyme The 2 electrons from the 2 molecules of cytochrome c lead to the pumping of 2 protons Why are the electrons carried by FADH2 not as energy rich as those carried by NADH? NADH has a higher reduction potential and pumps 4 protons across the membrane in complex I vs 0 from FADH in complex II Compare malate vs succinate: malate produced NADH, succinate produces FADH. Not equivalent in terms of ATP. malate makes NADH and succinate does not b/c malate to oxaloacetate has a HIGHER reduction potential than succinate to fumarate. Cyanide kills because it irreversibly binds to the Fe so you can’t have oxygen react there. Kills that Fe-O-O-Cu complex. So you can’t utilize oxygen to make ATP Know what portions of complex 5 play what role and how ATP is being formed DNP generates heat and does not produce ATP. Used as herbicides and fungicides but then as diet pills. Your body is breaking down energy to do ETC but you can’t make ATP: this is not safe. Same concept as brown adipose tissue in hibernating animals, but that is regulated Glycerol 3-phosphate shuttle vs. malate-aspartate shuttle Molecular oxygen is a great terminal electron acceptor, but oxygen is also poisonous. Can result in superoxide ions- free radicals- which can lead to DNA damage and diseases. Superoxide dismutase is an enzyme that will reverse it- take your superoxide ions and convert back to O2. Exercise makes more superoxide dismutase! There is no artificial way to reverse free radicals. Origin of the mitochondria: endosymbiotic theory by Lynn Margulis
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