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BSC 300, Week 7

by: Ashley Bartolomeo

BSC 300, Week 7 BSC 300

Ashley Bartolomeo
GPA 3.9

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notes on aerobic respiration and the mitochondrion
Cell Biology
John yoder
Class Notes
Cell, Biology
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This 6 page Class Notes was uploaded by Ashley Bartolomeo on Tuesday September 27, 2016. The Class Notes belongs to BSC 300 at University of Alabama - Tuscaloosa taught by John yoder in Fall 2016. Since its upload, it has received 3 views. For similar materials see Cell Biology in Biology at University of Alabama - Tuscaloosa.


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Date Created: 09/27/16
Chapter 5 Aerobic Respiration and the Mitochondrion Key Concepts  Clarify the overall mitochondrial structure and function  Describe the inner/outer mitochondrial membranes and matrix  Describe the function of the Tricarboxylic Acid Cycle- input/export  Define the role of -oxidation  Emphasize electron transport system in forming a proton gradient  Describe the chemiosmotic mechanism and cellular energy transduction  Clarify mechanisms for transporting protons to the intermembrane space  The function of ATP synthase in ATP synthesis  The function of peroxisomes Oxidative Metabolism in the Mitochondrion An Overview of Glycolysis 1. What are the irreversible steps? Why are they irreversible? 2. What is the committed step? Why does it have this name? 3. Know the substrate, enzyme and product of these 3 steps 4. How is NAD+ reduction linked to ATP synthesis? 5. What is the net production of glycolysis? Ie. What goes in, what comes out? 6. What happens in the presence of oxygen? In the absence of oxygen? 7. What is the role of isomerases? 8. What is the role of dehydrogenases? Introduction  Early Earth was populated by anaerobes, organisms that capture and utilize energy by oxygen-independent metabolism  Oxygen accumulated in the primitive atmosphere after cyanobacteria appeared. Why?  Aerobes evolved to use oxygen in order to extract more energy from organic molecules  In eukaryotes, aerobic respiration takes place in the mitochondria Mitochondrial Structure and Function The Structure of a Mitochondrion  Outer mitochondrial membrane: similar in lipid composition to eukaryotic plasma membrane  Integral membrane proteins called porins allow metabolites, ions and small proteins to freely diffuse into the intermembrane space  Large proteins require specific translocation signals for active transport into the intermembrane space  Inner mitochondrial membrane: lipid composition more similar to prokaryotic membrane  Subdivided into two interconnected domains: o Inner boundary membrane: regulates traffic into and out of the matrix o Cristae: where proteins of the electron transport chain are localized  Mitochondrial matrix o The space enclosed by the inner membrane. Highly concentrated mix of hundreds of different enzymes (most encoded by the nuclear genome) o Major matrix functions include oxidation of fatty acids, and the citric acid cycle o Possesses several copies of a circular DNA chromosome o Fourteen protein coding genes (ETC components) and genes for their own ribosomal and transfer RNAs o Therefore, RNA and proteins can be synthesized in the matrix. But many mitochondrial proteins are nuclear encoded and most be imported from cytoplasm o Unique nuclear encoded RNA polymerase likely evolved from virus rather than either prokaryote or eukaryote  Mitochondria: diverse cellular roles in addition to energy production o Coordinate programmed cell death (apoptosis) o Cell signaling o Differentiation o Control of cell cycle and cell growth o Synthesis of specific amino acids o Uptake and release of Ca2+  Mitochondria: highly variable and dynamic morphology o Diverse shape o Size/ number reflect the varied requirements of the cell o Respond to cellular needs to growth and fission o Fusion protects cells from damaged mitochondrial DNA, ensuring viable organelle function Oxidative Metabolism in the Mitochondrion  The first steps of oxidative metabolism are carried out by glycolysis o Glycolysis produces 2 pyruvate, 2 NADH and 2 ATP per glucose o Aerobic organisms use O2 to extract more than 30 additional ATPs from pyruvate and NADH4 Pyruvate Dehydrogenase Complex Links Glycolysis to TCA Cycle  Pyruvate dehydrogenase: a huge protein complex that catalyzes this critical 3-step reaction o Several vitamin derived co-enzymes including NAD+ and Coenzyme A  Pyruvate dehydrogenase actively transports pyruvate across inner membrane  In three catalytic steps the enzyme: o Decarboxylates pyruvate releasing CO2 o Reduces NAD+ to NADH o The acetyl group is transferred to Coenzyme A forming acetyl CoA  Acetyl CoA links glycolysis to TCA cycle through the action of the pyruvate dehydrogenase complex The TCA Cycle The Tricarboxylic Acid (TCA) Cycle  A stepwise cycle in which Acetyl CoA is completely oxidized and its energy captures in reduced electron carriers (NADH and FADH2) 1. Citrate synthase: The two carbon acetyl group is condensed with the four carbon oxaloacetate to form a six carbon citrate molecule a. Seven subsequent steps oxidize two carbons to CO2, regenerating the four carbon oxaloacetate needed to continue the cycle b. Similar to step 6 of glycolysis, the high energy thioester bond is very unstable and the acetyl group readily transferred to oxaloacetate c. This very favorable reaction (exergonic; ~-7 kcal/mol) kickstarts the cycle d. Like the committed step of glycolysis, citrate synthase is a major site of TCA cycle regulation 2. Isomerization of citrate: the tertiary alcohol of citrate is not oxidizable, but the secondary alcohol is isocitrate is 3. Generation of CO2 by a NAD+ dependent dehydrogenase: isocitrate is oxidized to form the 5-carbon -ketoglutarate a. This step releases one molecule of CO2 and reduces NAD+ to NADH 4. Generation of second CO2: another dehydrogenase complex catalyzes oxidation of -ketoglutarate to succinyl CoA a. Analogous to pyruvate decarboxylase: large multiprotein complex enzyme that uses Coenzyme A to generate a high energy thioeaster bond b. This step also reduces NAD+ to NADH c. This is also a highly favorable reaction (~-7 kcal/mol) 5. Substrate-level phosphorylation: succinyl CoA hydrolysis drives formation of an energy rich phosphate bond a. The high transfer potential of Succinyl-CoA promotes replacement of CoA by an inorganic phosphate which is then transferred to a diphosphate nucleotide b. In plants and bacteria, ATP is formed directly in this step. In animal, GTP is formed and used to synthesize ATP by the actions of another enzyme 6. Flavin-dependent dehydrogenation: succinate is oxidized to fumarate, converting FAD to FADH2. Flavin adenine dinucleotide a. (FAD) a coenzyme similar to NAD+ accepts two hydrogen atoms and is reduced to FADH2 – another high energy carrier molecule b. The enzyme succinate dehydrogenase is directly involved in the electron transport chain 7. Hydration of a C=C bond: fumarate is hydrolyzed to form malate 8. The TCA cycle is completed with the NAD+ dependent dehydrogenation of malate to oxaloacetate a. This is highly endergonic reaction (>+7 kcal/mol) that moves in the forward direction because the exergonic citrate synthase reaction keeps oxaloacetate concentrations very low What You Need to Know  Importance of CoA and the resulting thioester bonds in driving the cycle forward and producing an ATP/GTP molecule  The functional definition of dehydrogenase enzymes and their importance in reducing NAD+ and FAD  Citrate synthase as the kay regulated enzymes in this cycle  Succinate dehydrogenase as a shared enzyme between TCA cycle and ETC 4 Oxidation-Reduction Reactions in the TCA Cycle  3 reactions: NAD+ reduced to NADH + H+ (steps 3,4 and 8)  1 reaction: FAD reduced to FADH2 (step 6)  2 C released as CO2 (Steps 3 & 4)  The purpose of the TCA cycle is not to yield large quantities of ATP  The TCA cycle produces high energy carrier molecule (NADH and FADH2) that will be used in subsequent steps during oxidative phosphorylation to produce ATP -oxidation: The Fatty Acid Cycle  Reaction intermediates of TCA cycle are also common metabolites generated in other catabolic reactions, making the TCA cycle the central metabolic pathway of the cell  Many pathways can feed metabolites into TCA cycle  Example: fats are oxidized in the mitochondria by -oxidation to generate Acetyl CoA  2 dehydrogenase enzymes consecutively oxidize carbons 2 and 3 of the fatty acyl chain. This reduces one FAD and one NAD+  Another enzyme cleaves acetyl CoA and joins another CoA to the shortened chain  Released acetyl CoA fed into TCA cycle  Therefore, each turn of the fatty acid generates: o 4 NADH o 2 FADH2 o 1 ATP equivalent Fats  Significant source of energy (hydrocarbons are highly reduced carbon)  Stored as triglycerides in fat droplets in adipose cells. Upon glucagon stimulation, are cleaved from glycerol and released to bloodstream  Transported into body cells and in ATP dependent manner are covalently attached to CoA at their carboxyl end  Fatty acyl CoA transported into mitochondrion for energy extraction via the -oxidation pathway Peroxisomes  Membrane bound vesicles that contain >50 oxidative enzymes that breakdown diverse biomolecules  Use O2 to oxidize these substrate, producing hydrogen peroxide (H2O2) as a bi-product  The peroxisomal enzyme catalyase converts this toxic molecule to O2 and H2O  Two critical functions include: o -oxidation of very long chain fatty acids (VLCFAs): FA tails longer than 22 carbons. The shorter FA-CoA molecules are then transported to mitochondria o Production of plasmalogen, a lipid enriched in cardiovascular and myelin cell – functioning to insulate excitable cells Compounds from Other Catabolic Pathways Feed into TCA Cycle  Amino acid metabolites enter TCA Cycle at several points, and may therefore yield varying amount of reduced electron carries and therefore ATP  Catabolism of each amino acids is carried out by varied and distinct enzymatic pathways Glycolysis & TCA Cycle Furnish Molecular Building Blocks as well as Energy  Additionally, metabolic intermediates of glycolysis and TCA cycle are substrates for diverse anabolic pathways synthesizing: o Amino acids o Nucleotides o Lipids The Glycerol Phosphate Shuttle  The reduced coenzymes FADH2 and NADH are the primary products of the TCA cycle  Inner mitochondrial membrane is impermeable to NADH  Therefore, the electrons of glycolytically produced NADH must be actively transported into the mitochondrial matrix if they are to be used in the electron transport system  Two processes, the Malate-aspartate and the Glycerol phosphate shuttle systems oxidize cytoplasmic NADH, and shuttle its electrons into the mitochondrial matrix  Cytoplasmic NADH is used to enzymatically reduce either oxaloacetate to malate of DHAP to Glycerol 3-P  In the Glycerol phosphate shuttle system an inner transmembrane dehydrogenase enzyme uses the electrons to reduce FAD to FADH2  The more utilized Malate-Aspartate shuttle first transports (via an antiporter) malate into the matrix where a dehydrogenase reduced NAD+ to NADH Summary of Oxidative Phosphorylation  Electrons from NADH and FADH2 are passed through the electron transport chain  Provides energy to pump H+ across the inner membrane into inter- membrane space  ATP formed by controlled flow of H+ back into the matrix through the TAP-synthesizing enzyme ATP synthase  This coupling of H+ translocation to ATP synthesis is called chemiosmosis  Three molecules of ATP are formed from each pair of electrons donates by NADH; two molecules of TAP are formed form each pair of electrons donated by FADH2


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