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12/1/17 CHAPTER 14: Metabolism and Bioenergetics SUMMARY OF CATABOLISM • Your body is capable of “burning” many sorts of fuel to produce ATP Ø Carbohydrates Ø Lipids Ø Proteins • All three “fuels” feed into the citric acid cycle to complete catabolism: Ø Begins with acetyl-CoA Ø Requires O2 Ø Occurs in the mitochondria CHAPTER 14: Metabolism and Bioenergetics OUTLINE • 14.1 Acetyl CoA and the Citric Acid Cycle • 14.2 Oxidative Phosphorylation • 14.3 Entropy and Bioenergetics (not covered) CHAPTER 14: Metabolism and Bioenergetics ACETYL-CoA PRODUCTION FROM PYRUVATE • Acetyl CoA is produced between the second & third stages of carbohydrate catabolism: Ø Both proteins & fat can also can be converted into acetyl-CoA to produce ATP • Further catabolism of acetyl-CoA forms NADH & FADH2 during the citric acid cycle. Ø NADH & FADH2 are re-oxidized to make ATP 1 12/1/17 CHAPTER 14: Metabolism and Bioenergetics ACETYL CoA • Acetyl CoA is a complex molecule consisting of pantothenic acid (vitamin B5), a thiol amine, and ADP, and the acetyl group • Coenzyme A can carry other groups besides the acetyl in other metabolic pathways (ie. acyl chains) CHAPTER 14: Metabolism and Bioenergetics THE CHEMISTRY OF ACETYL CoA • The key to Coenzyme-A chemistry is the thiol group located at the very end of the molecule: The thiol can react with a variety of carbonyl-containing molecules to form a high- energy thioester bond CHAPTER 12: Carbohydrates: Structure and Function OXIDATION OF PYRUVATE TO ACETYL-CoA • If oxygen is present, pyruvate is oxidized to acetyl- CoA in preparation for the citric acid cycle: Ø Requires both coenzyme A (SH-CoA) and NAD+ Ø Reaction involves the loss of a CO2 (decarboxylation) Oxidation + CO2 loss Oxidative Decarboxylation • Reaction is catalyzed by the mitochondrial enzyme pyruvate dehydrogenase 2 12/1/17 CHAPTER 14: Metabolism and Bioenergetics THE CITRIC ACID CYCLE • The citric acid cycle is called a “cycle” because it begins and ends with the same molecule: The NET reaction is complete oxidation of carbons to CO2 SH-CoA CHAPTER 14: Metabolism and Bioenergetics CARBON ATOMS IN THE CITRIC ACID CYCLE • All eight steps of the citric acid cycle occur in the mitochondrial matrix: Citric acid cycle enzymes are in the mitochondrial matrix • Same location as beta-oxidation • Reduced products (NADH & FADH2) feed directly into the electron transport chain in the inner membrane CHAPTER 14: Metabolism and Bioenergetics DETAILED VIEW OF THE CITRIC ACID CYCLE Reaction Types: 1. Redox reactions: Ø Oxidation of carbonyl Thioester groups produces NADH Redox A hydrolysis (Redox A) Ø Oxidation of an alkane to Hydration Isomerization alkene produces FADH2 Redox B Redox A (Redox B) Thioester 2. Thioester hydrolysis hydrolysis Redox A 3. Isomerization 4. Hydration 3 12/1/17 CHAPTER 14: Metabolism and Bioenergetics PRODUCT ACCOUNTING FOR GLUCOSE • After going thru Glycolysis and the Citric Acid Cycle, what are the net products of glucose catabolism? Glycolysis Intermediate Step Citric Acid Cycle 2 Pyruvates 1 Acetyl-CoA 1 GTP 2 ATP 1 NADH x 2 1 FADH2 x 2 2 NADH 1 CO2 3 NADH 2 CO2 NET PRODUCTS 4 ATP/GTP 2 FADH2 + 6 CO2 10 NADH CHAPTER 14: Metabolism and Bioenergetics ATP PRODUCTION BY OXPHOS • Most of the ATP derived from glucose catabolism comes from the electrons carried by NADH and FADH : 2 Ø Only 4 ATP or GTP molecules are made directly….. • Oxidative phosphorylation (OXPHOS) uses energy derived from the electrons of NADH and FADH2 in order to generate ATP: Ø Requires the mitochondrial electron transport chain Ø Involves addition of an inorganic phosphate (Pi) to ADP to create ATP CHAPTER 14: Metabolism and Bioenergetics MITOCHONDRIAL ARCHITECTURE • Mitochondria have two separate membranes: Ø Inner mitochondrial membrane Ø Outer mitochondrial membrane • The membranes define two unique “compartments”: Folds in the inner Ø Intermembrane space (IMS), mito membrane between the two membranes (cristae) create more surface area Ø Matrix, the region within the inner membrane 4 12/1/17 CHAPTER 14: Metabolism and Bioenergetics OXPHOS PROTEIN COMPLEXES • Proteins involved in the electron transport chain and oxidative phosphorylation are found in the inner mitochondrial membrane (IMM): Ø These are transmembrane protein complexes that actually span the inner membrane phospholipid bilayer Electrons from NADH and FADH2 are carried through the chain, ending at oxygen as the final acceptor NADH e- FADH2 e- e- + 4H + O2 à 2 H2O CHAPTER 14: Metabolism and Bioenergetics ELECTRON TRANSPORT • Electrons are transferred to the FOUR electron transport complexes by oxidation-reduction reactions in metal centers of these proteins: Ø The metal centers consist of iron or copper ions, alternately oxidizing and reducing between Fe2+/Fe3+ or Cu1+/Cu2+ • Transfer of electrons between the complexes involves two extra “electron shuttles”: Ø Coenzyme Q = a small organic co-factor Ø Cytochrome C = protein with heme-like cofactor (iron ion center) What sort of redox reaction is involved in Coenzyme Q electron transfer? CHAPTER 14: Metabolism and Bioenergetics DIRECTION OF ELECTRON FLOW • Electron flow is determined by relative electron affinities for each of the components in the electron transport chain: Ø Electrons always flow from Low e- affinity low electron affinity to high electron affinity: à § NADH & FADH2 have low affinity for electrons § The final electron acceptor (O2) has the highest affinity Ø The difference between these electron affinities is energy Potential inversely proportional to High e- affinity their potential energy 5 12/1/17 CHAPTER 14: Metabolism and Bioenergetics ELECTRON TRANSPORT DRIVES PROTON PUMPING INTO THE IMS The energy from electron transport is used to “pump” protons across the inner membrane…… …..this creates at pH gradient CHAPTER 14: Metabolism and Bioenergetics ANALOGY TO WATER WHEEL PUMPS The potential energy of flowing water can be harnessed to produce mechanical or electrical energy using a water wheel Now imagine using this energy to run a “pump” Complex I Complex II protons Complex III Complex IV CHAPTER 14: Metabolism and Bioenergetics THE REAL ELECTRON TRANSPORT CHAIN • The electron transport chain complexes are molecular “proton pumps” • Each complex is composed of many protein subunits that work together http://www.nature.com/nrm/journal/v16/n6/images/nrm3997-f1.jpg 6 12/1/17 CHAPTER 14: Metabolism and Bioenergetics INITIATING ELECTRON TRANSPORT • Three of the four complexes in the electron transport chain also function as “proton pumps”: 1. Complex I – receives e- from NADH à pumps H+ - 2. Complex II – receives e from FADH2, but does NOT pump 3. Complex III – receives e- from Coenzyme-Q à pumps H+ 4. Complex IV – receives e- from Cytochrome C à pumps H+ • Complex IV reduces molecular oxygen with the electrons it receives: + O2 + 4 H + 4 e- à 2 H2O Ø The deadly poison cyanide (CN-) blocks the final step of electron transport, stopping the entire transport chain CHAPTER 14: Metabolism and Bioenergetics THE PROTON GRADIENT • Protons (H+) are unable to diffuse through the inner mitochondrial membrane by simple diffusion because they are charged: Ø There are more protons in the intermitochondrial space (lower pH) than in the matrix (higher pH). Ø This proton gradient is maintained through the action of the electron transport chain H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ CHAPTER 14: Metabolism and Bioenergetics THE PROTON-MOTIVE FORCE • The proton gradient between the inner membrane space (IMS) and matrix is another form of potential energy for the cell to use: Ø The energy in this unequal distribution of protons is called the proton-motive force Ø The only way for protons to diffuse back down this gradient is through ATP synthase Ø This flow of protons is harnessed to drive ATP synthesis 7 12/1/17 CHAPTER 14: Metabolism and Bioenergetics PHOSPHORYLATION OF ADP • Phosphorylation of ADP to yield ATP ATP synthase is a requires significant energy input: rotary “machine” Energy + ADP + Pi ® ATP + H2O Ø The energy for this process comes from the proton gradient set up by the electron transport chain across the IMM • ATP synthase is the enzyme responsible for producing ATP in the mitochondria: Ø Complex protein that spans the inner mitochondrial membrane (IMM) Ø Contains a “channel” thru which H+ flow Ø Proton flow drives rotary motion to combine ADP with inorganic phosphate (Pi) CHAPTER 14: Metabolism and Bioenergetics ENERGY FROM GLUCOSE OXIDATION • The electron carrying capacity of redox cofactors can be translated into specific amounts of ATP: Ø Each FADH2 à ~1.5 ATPs Ø Each NADH à ~ 2.5 ATPs 1 glucose Glycolysis 2 ATP 7 ATP 2 NADH = 5 ATP 2 pyruvate Intermediate Step 2x 1 NADH = 2.5 ATP 5 ATP 32 ATP Citric Acid Cycle per glucose 2 acetyl- 1 GTP ~ 1 ATP CoA 2x 1 FADH2 = 1.5 ATP 20 ATP 3 NADH = 7.5 ATP CHAPTER 14: Metabolism and Bioenergetics ENERGY FROM PALMITATE OXIDATION • The electron carrying capacity of redox cofactors can be translated into specific amounts of ATP: Ø Each FADH2 à ~1.5 ATPs Ø Each NADH à ~ 2.5 ATPs FA Activation Step 1 palmitate + SH-CoA -2 ATP β-oxidation (of C16:0) 8 acetyl-CoA 1 palmitoyl- 10.5 ATP CoA 7 FADH2 7 NADH 17.5 ATP 106 ATP Citric Acid Cycle per 8 acetyl- 1 GTP ~ 1 ATP palmitate CoA 8x 1 FADH2 = 1.5 ATP 80 ATP 3 NADH = 7.5 ATP 8 12/1/17 CHAPTER 14: Metabolism and Bioenergetics SELF-REVIEW QUESTIONS 1. What important role does oxygen play in the electron transport chain? 2. Explain how cyanide shuts down the electron transport chain. 3. What important chemical reaction is driven by the flow of protons from the intermembrane space back into the matrix? 4.
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