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1 IX. Metabolism and Energy Production A. Introduction Slide 1. R `ism a. Stage I: Breakdown of macromolecules into their building blocks (1) Proteins amino acids (2) Polysaccharides, disaccharides monosaccharides (3) Fats fatty acids + glycerol b. Stage II: Conversion of amino acids, monosaccharides, fatty acids, glycerol into few smaller molecules - main one is acetyl CoA c. Stage III: Final oxidation of smaller molecules (primarily acetyl CoA) to produce ATP 2. Now examine aspects of the last stage in greater detail B. Oxidation of Acetyl CoA 1. First part of the story is the Krebs cycle (citric acid cycle, tricarboxylic acid cycle) a. Proposed by Hans Krebs in late 1930 s b. Completely verified by use of radioactively-labeled substrates c. In eukaryotes, all reactions occur inside mitochondria 2. Overview of the cycle Slide a. Picture overview b. Text overview (1) Aerobic pathway only (2) Acetyl CoA oxidized to (3) Produces NADH and FADH 2 and 1 GTP (equivalent to ATP) Slide 3. Individual reactions of the Krebs cycle - all are normal organic chemistry reactions a. Condensation of acetyl CoA with oxaolacetate to give citrate O 3 C SCoA acetyl CoA C O citrate synthase Condensation reaction: very favorable HO C oxaloacetate citrate (2) Entry to the cycle and regulated by availability of the two substrates Slide b. Isomerization of citrate (tertiary alcohol) to isocitrate (secondary alcohol) HO C citrate aconitase Conversion of 3 o alcohol into 2 o alcohol: Now able to be oxidized 64 H H C C OH isocitrate

2 Slide c. Oxidation decarboxylation of isocitrate (secondary alcohol) to give α-ketoglutarate H C OH isocitrate C O H C dehydrogenase NAD + NADH + H + isocitrate Oxidative decarboxylation α-ketoglutarate (2) Reaction also gives NADH (reduced NAD + ) - will be reoxidized using the electron transport system with resultant energy used to synthesize ATP (3) Note that the carbon lost in is not one of the ones from the added acetyl CoA (4) Rate of reaction controlled by availability of NAD + (NAD + /NADH ratio) Slide d. Second oxidative decarboxylation (α-ketoglutarate to succinyl CoA) SCoA C O CoASH -ketoglutarate C O dehydrogenase α-ketoglutarate NAD + NADH + H + Oxidative decarboxylation coupled to formation of an energized molecule succinyl CoA (2) Second reaction that gives NADH (reduced NAD + ) - will be reoxidized using the electron transport system with resultant energy used to synthesize ATP (3) Again note that the carbon lost in is not one of the ones from the added acetyl CoA (4) Rate of reaction controlled by availability of NAD + (NAD + /NADH ratio) (5) Product (succinyl CoA) is a high-energy molecule (like ATP) 65

3 Slide e. Use of high-energy succinyl CoA to provide energy for phosphorylation of GDP SCoA C O CoASH succinyl CoA synthetase 2 2 GTP GDP + P i succinate succinyl CoA Substrate-level phosphorylation. The energized succinate is used to drive the phosphorylation of GDP. (2) Called substrate-level phosphorylation - contrasted with oxidative phosphorylation where ATP synthesis is coupled to oxidation of NADH and FADH 2 (3) GTP made can transfer its terminal phosphate to ADP to give ATP GTP + ADP GDP + ATP Slide f. Oxidation of succinate succinate dehydrogenase FAD FADH 2 succinate HC Oxidation fumarate Insertion of double bond as first step towards regeneration of oxaloacetate (2) Uses FAD as the electron acceptor and produces FADH 2 - will be reoxidized via the electron transport system with resultant energy used to synthesize ATP (3) Succinate dehydrogenase is located in the inner mitochondrial membrane Slide g. Hydration of fumarate H 2 O HC fumarase HO C H fumarate Addition of water to create a 2 o alcohol. L- malate (2) Fumarase is stereospecific for the trans isomer and gives only the L isomer of malate 66

4 Slide h. Oxidation of L-malate - completes the cycle HO C H malate dehydrogenase C O NAD + NADH + H + oxaloacetate (2) Enzyme shows stereospecificity - only accepts L-isomer (3) Third reaction that gives NADH (reduced NAD + ) - will be reoxidized using the electron transport system with resultant energy used to synthesize ATP (4) Rate of reaction controlled by availability of NAD + (NAD + /NADH ratio) Slide 4. Summary of the Krebs cycle Slide acetyl CoA + 3 NAD + + FAD + GDP + P i NADH + H + + FADH 2 + GTP + CoA Slide 5. Regulation of the citric acid cycle a. Regulation by energy level - tied to energy needs of the cell (1) Positive modulator - ADP - signal that cell s energy level is low (2) Negative modulator - ATP - signal that cell s energy level is high b. Regulatory enzymes (1) Isocitrate dehydrogenase - in the cycle (2) α Ketodehydrogenase - in the cycle (3) Pyruvate dehydrogenase - entry point from pyruvate c. Regulation by NAD + /NADH ratio (1) Several reactions need NAD + as an electron acceptor (2) Positive modulator - NAD + - acceptor available (3) Negative modulator - NADH - acceptor unavailable C. Reoxidation of NADH, FADH 2 - the Electron Transport System (ETS) 1. Reoxidation occurs inside the mitochondria Slide a. Mitochondrial structure (1) Outer permeable membrane (2) Inner highly-convoluted membrane - (a) (b) Impermeable to most molecules and ions Contains proteins of the electron transport sysyem and succinate dedydrogenase (3) Intermembrane space - compartment between the two membranes (4) Matrix - compartment inside inner membrane - contains Krebs cycle enzymes (except succinate dehydrogenase) b. Structural integrity very important to function 2. Electron transport system a. Function is to take electrons from NADH and FADH 2 and transport them to O 2 b. System is series of electron carriers arranged in four complexes (I, II, III, IV) c. Reduction potential is measure of species tendency to gain electrons and electron carriers are in order of increasing reduction potential 67

5 d. Types of electron carriers in the electron transport system Slide (1) Flavin - flavin mononucleotide (FMN) (a) Similar to FAD (b) Contains riboflavin (c) Transfer two electrons at a time Slide (2) Coenzyme Q (quinone) (a) Transfers two electrons at a time (b) CoQ + 2 e + 2 H + CoQH 2 Slide (3) Cytochromes - contain heme group (a) Iron ion in heme carries the electron (b) Transfer one electron at a time (c) cyt (Fe 3+ ) + e cyt (Fe 2+ ) (4) Iron sulfur clusters (a) Embedded in proteins (b) Transfer one electron at a time Slide e. Electron entry from NADH (1) Electrons transferred to NADH dehydrogenase (contains flavin mononucleotide, FMN) in Complex I (2) Electrons transferred to proteins containing iron-sulfur centers (Fe 3+ reduced to Fe 2+ ) (3) Electrons transferred to Coenzyme Q (ubiquinone) - mobile carrier to take electrons to next complex (Complex III) f. Electron entry from FADH 2 (1) Succinate dehydrogenase is component of Complex II (2) Electrons transferred to proteins containing iron-sulfur centers (3) Electrons transferred to Coenzyme Q (uniquinone) g. Complex III (1) Contains 2 cytochromes (b, c 1 ) and iron-sulfur centers (2) Cytochrome - hemeprotein that transfer electrons using Fe 3+ Fe 2+ (3) Transfer sequence: cyt b Fe. S cyt c1 (4) Final transfer is to cyt c - mobile carrier to take electrons to Complex IV h. Complex IV (1) Contains cytochromes aa 3 (cytochrome oxidase) - has ability to transfer electrons to O 2 - gives H 2 O (2) Cytochrome oxidase can be inhibited by cyanide ion (CN ) - basis of cyanide poisoning 1. Summary reaction from NADH 2 NADH + 2 H + + O 2 2 NAD H 2 O j. This downhill flow of electrons releases energy (53 kcal for electrons from one mol of NADH) - significant portion is used to make ATP (from ADP and P i ) - called oxidative phosphorylation 68

6 3. Oxidative phosphorylation a. Provides over 95% of energy for the organism Slide b. Current picture of mechanism (1) During flow of electrons from NADH and FADH 2 to O 2 protons (H + ions) are pumped from the matrix into the intermembrane space (2) Establishes a ph (H + ) gradient and charge gradient - more H + ( positive charge) outside (intermembrane space) than inside (matrix) Slide (3) Flow of H + down this gradient (outside to inside) through ATP synthase results in phosphorylation of ADP: ADP + P i ATP + H 2 O Slide c. ATP yield (1) Energy potentially available when electrons go from one mole of NADH to O 2 is over 50 kcal (2) This large amount of energy is given up in stages and about 70% is captured as ATP - rest given off as heat (3) ATP yield from reoxidation (a) Reoxidation of 1 NADH yields about 3 ATP (b) Reoxidation of 1 FADH 2 yields about 2 ATP (4) Energy yield from complete oxidation of 1 acetyl CoA (a) Krebs cycle yields 3 NADH, 1 FADH 2 and 1 GTP (b) Reoxidation of 3 NADH, 1 FADH 2 gives 11 ATP (c) Phosphate transfer from GTP give ATP (d) Total yield is 12 ATP 4. Regulation of electron transport and oxidative phosphorylation a. The two processes are tightly coupled - electron transport will occur to an appreciable extent only when ADP is available for phosphorylation Slide b. Regulation by cellular energy need (1) Important here is the ADP/ATP ratio (a) High ADP, low ATP means high energy need (b) Low ADP, high ATP means low energy need (2) Rate of electron transport and oxidative phosphorylation (a) ADP - positive modulator (b) ATP - negative modulator c. Regulation by electron availability (1) Important here is NADH/NAD + ratio (2) High NADH means electrons are available and NADH needs to be converted back to NAD + to be electron acceptor in reactions in oxidative pathways (3) Low NADH (high NAD + ) means low need to recycle the NADH d. Overall picture - the two processes will operate at the fastest rate when (1) ADP is high indicating ATP is needed for cellular processes (2) Oxidative pathways such as the citric acid cycle are producing NADH to provide the electrons 69

7 D. Summary of Energy Production from Glucose Slide 1. Glucose 2 pyruvate gave a net of 2 ATP and 2 NADH 2. 2 NADH produced from glycolysis-electrons transferred to carrier and taken into mitochondrion a. In liver, heart and kidney transport yields 2 NADH inside and finally 6 ATP b. In brain and skeletal muscle electrons transferred to flavoprotein giving 2 FADH 2 and therefore 4 ATP Slide 3. The 2 pyruvates enter the mitochondrion and are converted to 2 acetyl CoA plus 2 NADH a 2 NADH = 6 ATP Slide b. 2 acetyl CoA = 24 ATP Slides 4. Overall a. Glycolysis - 2 ATP b. 2 NADH from glycolysis = 4 6 ATP c. Pyruvate oxidation = 32 ATP d. Total = ATP 5. Percentage of available energy conserved a. Energy potentially avialable from combustion of glucose = 686 kcal/mol b. Energy captured as ATP (standard conditions) = (38 mol ATP)(7.3 kcal/mol) = 280 kcal c. Percent =[ (280 kcal)/(686 kcal)] (100) = 40% d. Under conditions in the cell the engry needed to make ATP is actually about 12 kcal/mol. (1) Under these conditions 38 mol ATP = 460 kcal (2) Percent = [ (460 kcal)/(686 kcal)] (100) = 67% e. Rest of energy lost as heat and used to ensure that overall reaction wants to happen 70

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