Cellular Respiration - Glycolysis (SIJ1003) PDF

12/10/24 RESPIRATION Living organisms obtain energy from respiration. Respiration External resp. - gaseous exchange at the respiratory surface Cellular respiration – sequence of enzyme-controlled...

12/10/24 RESPIRATION Living organisms obtain energy from respiration. Respiration External resp. - gaseous exchange at the respiratory surface Cellular respiration – sequence of enzyme-controlled biochemical reactions. Involves breakdown of complex molecules with the release of energy i.e.heat and potential energy used to form ATP. Cellular Respiration Metabolic pathways are sequence of reactions Different metabolic pathways have different types of reaction sequences. i) Linear A B C D E 1  12/10/24 Cellular Respiration ii) Cyclic D A B C E F iii) Branched D E F A B C G H Cellular Respiration Respiration can be aerobic (O2 is present) or anaerobic (O2 is absent) Equation of aerobic respiration C6H12O6 + 6O2 6CO2 + 6H2O ATP 2  12/10/24 video* Cellular Respiration 3  12/10/24 How Cells Harvest Energy 1.Cellular Energy Harvest 2.Cellular Respiration – Glycolysis – Oxidation of Pyruvate – Krebs Cycle – Electron Transport Chain 1. Cellular Energy Harvest Cells harvest energy by breaking bonds and shifting electrons from one molecule to another. – aerobic respiration - final electron acceptor is oxygen – anaerobic respiration - final electron acceptor is inorganic molecule other than oxygen – fermentation - final electron acceptor is an organic molecule 4  12/10/24 1. Cellular Energy Harvest Energy Carriers Pi (inorganic phosphate) - ATP Electron carrier – NAD + H to NADH – FAD + 2H to FADH2 Chapter 5 1. Cellular Energy Harvest Electron Carriers Photosynthesis – NADP + H to NADPH Respiration – NAD + H to NADH – FAD + 2H to FADH2 Chapter 5 5  12/10/24 1. Cellular Energy Harvest Final Electron Acceptor Photosynthesis – CO2 + H’s to (CH2O)n – Stores energy Respiration – Aerobic 1/2 O2 + H 2 to H2O Chapter 5 Figure 9.3 Methane combustion as an energy- yielding redox reaction Reactants Products becomes oxidized CH4 + 2 O2 CO2 + Energy + 2 H2O H becomes reduced H H H C O O O O H C O H Methane Oxygen Carbon dioxide Water (reducing (oxidizing agent) agent) 6  12/10/24 Chapter 5 Example of Redox Equations Chapter 5 7  12/10/24 Examples ATP è ADP + Pi – Oxidation, release energy ADP + Pi è ATP – Reduction, stores energy NAD + H è NADH FADH2 è FAD + 2H 2H2 + O2 è 2H2 O Chapter 5 Examples Cellular Respiration C6H12 O6 + 6O2 è6H2O + 6CO2 + 38 ATP Photosynthesis 6H2O + 6CO2 + light è C6H12 O6 + 6O2 Chapter 5 8  12/10/24 ATP Adenosine Triphosphate (ATP) is the energy currency of the cell. – used to drive movement – used to drive endergonic reactions ADENOSINE TRIPHOSPHATE (ATP) 9  12/10/24 ATP Most of the ATP produced in cells is made by the enzyme ATP synthase. – Enzyme is embedded in the membrane and provides a channel through which protons can cross the membrane down their concentration gradient. ATP synthesis is achieved by a rotary motor driven by a gradient of protons. ATP acts as temporary energy store. Hydrolysis of ATP produced ADP and Pi and energy released ATP + H2O ADP + Pi + 30.6 kJ (per mole of ATP) 10  12/10/24 Production of ATP = phosphorylation -phosphorylation – adding of phosphate group - Occurs most commonly by transfer of the phosphate group from ATP to the hydroxyl group of a serine, threonine, or tyrosine residue in the protein. - Enzymes that catalyze the phosphorylation of other enzymes (or other proteins) are called protein kinases. Dephosphorylation - The reversal proses of phosphorylation - Involves the removal of phosphate group from a phosphorylated protein. - Enzymes that catalyze the dephosphorylation are called protein phosphatases. Depending on the particular enzyme, phosphorylation may activate or inhibit the enzyme. 11  12/10/24 Three main types of phosphorylation:- (a) Substrate level phosphorylation A phosphate group transferred directly from a phosphorylated compound (substrate) to ADP. e.g. glycerate – 1,3 – diphosphate ADP ATP Glycerate – 3 - phosphate Figure 9.7 Substrate-level phosphorylation Enzyme Enzyme ADP Substrat P e + ATP Product 12  12/10/24 (b) Oxidative phosphorylation Involves chemiosmosis process. Occurs in the inner membrane of mitochondria. (c) Photophosphorylation In light dependant reaction. Occurs in thylakoid membrane in the chloroplast. NAD+ & NADH Nicotinamide adenine dinucleotide, NAD+, is a coenzyme found in all living cells. The compound is a dinucleotide, since it consists of two nucleotides joined through their phosphate groups: with one nucleotide containing an adenosine ring, and the other containing nicotinamide. In metabolism, NAD+ is involved in redox reactions, carrying electrons from one reaction to another. The coenzyme is therefore found in two forms in cells: NAD+ is an oxidizing agent – it accepts electrons from other molecules and becomes reduced, this reaction forms NADH, which can then be used as a reducing agent to donate electrons. These electron transfer reactions are the main function of NAD+. 13  12/10/24 NAD+ & NADH NADH can pass the hydrogen atom to other compounds like pyruvate in the cytoplasm to form lactate catalysed by lactate dehydrogenase during lactate fermentation. Similarly, NADH can pass the hydrogen atom to ethanol to form alcohol during alcohol fermentation. NADH usually binds to several types of dehydrogenases as it is a coenzyme. NADH receives the hydrogen atom from metabolite during dehydrogenation reaction. (Three ATPs can be formed from one NADH) NAD+ & NADH 14  12/10/24 Figure 9.4 NAD+ as an electron shuttle 2 e– + 2 H+ 2 e– + H+ NAD+ NADH H+ Dehydrogenase H O O H H Reduction of NAD+ C NH2 + 2[H] C NH2 + H+ (from food) Oxidation of NADH N N+ Nicotinamide Nicotinamide O CH2 (oxidized form) (reduced form) O O P O– O H H O P O– HO OH NH2 HO O CH2 N N H N N H O H H HO OH FAD FAD (Flavin Adenine Dinucleotide is derived from the vitamin riboflavin. The dimethylisoalloxazine ring system undergoes oxidation/reduction. FAD is a prosthetic group, permanently part of E3. Reaction: FAD + 2 e- + 2 H+ ßà FADH2 15  12/10/24 FAD FADH2 is a reduced form of FAD consisting of flavin, ribitol, adenine, two phosphates and two hydrogen atoms. FADH2 passes the two hydrogen atoms to coenzyme Q becoming oxidised FAD. Coenzyme Q is the second electron carrier in the electron transport chain in the inner mitochondrial membrane. Only two ATPs can be formed from one FADH2. FADH2 is a prosthetic group bonded to succinate dehydrogenase that found in the mitochondrial matrix. dimethylisoalloxazine O O H H H C N C − H3C C C C NH 2 e + 2 H+ H3C C C C N C C NH H3C C C C C O H3C C C C C O C N N C N N H H H CH2 CH2 HC OH HC OH HC OH HC OH FAD Adenine FADH2 HC OH O O Adenine HC OH O O H2C O P O P O Ribose H2C O P O P O Ribose O- O- O- O- 16  12/10/24 How Cells Harvest Energy √ 1.Cellular Energy Harvest 2.Cellular Respiration – Glycolysis – Oxidation of Pyruvate – Krebs Cycle – Electron Transport Chain Figure 9.6 An overview of cellular respiration Electrons Electrons carried carried via NADH and via NADH FADH2 Oxidative Glycolsis Citric phosphorylation: acid electron transport Glucose Pyruvate cycle and chemiosmosis Mitochondrion ATP ATP ATP Substrate-level Oxidative Substrate-level phosphorylation phosphorylation phosphorylation 17  12/10/24 Three main stages of aerobic respiration Stage I : Glycolysis (occurs in cytosol) Stage II : Link reaction (Oxidation of Pyruvate) Stage III : Krebs cycle (Citric Acid Cycle) Stage IV : Electron Transport Chain (Step II - IV occur in mitochondria) ATP produced: to 34 ATP 18  12/10/24 NADH & FADH2 produced: + 2 NADH Total: 10 NADH 2 FADH2 to 34 ATP + 2 NADH + 6 NADH + 2 FADH2 How Cells Harvest Energy √ 1.Cellular Energy Harvest 2.Cellular Respiration – Glycolysis – Oxidation of Pyruvate – Krebs Cycle – Electron Transport Chain 19  12/10/24 Introduction to carbohydrate metabolism Carbohydrates are the major source of energy in the diet Complex carbohydrates (di- and polysaccharides) in the diet are broken down by enzymes and stomach acid to produce monosaccharides, the most important of which is glucose Glucose also comes from the enzymatic breakdown of glycogen that is stored in the liver and muscles until needed Once monosaccharides are produced, they can either to build new oligo- and polysaccharides or to provide energy. The specific pathway by which energy is extracted from monosaccharides is called glycolysis 39 Major pathways of carbohydrate metabolism Glycogen synthetase Glycogen phosphorylase 40 20  12/10/24 Introduction to glycolysis Localized in the cytosol of cells Basically an anaerobic process, principal steps occur with no requirement for O2 Catabolic process, stepwise degradation of glucose One molecule of glucose (a six-carbon compound) is converted to fructose-1,6- bisphosphate (also a six-carbon compund), which eventually gives rise to two molecules of pyruvate (a three-carbon compound). Each reaction in the pathway is catalyzed by an enzyme specific for that reaction. The glycolytic pathway (also called the Embden-Meyerhof pathway) involves 10 enzyme-catalyzed steps Glycolysis is considered the 1st metabolic pathway to be elucidated. Most of the details of this pathway were worked out in the 1st half century by German scientists. O. Meyerhof G. Embden Otto Warburg (1884-1951) (1874-1933) (1883-1970) 41 Glycolysis The breakdown of glucose to pyruvate can be summarized as follows: Glucose (6C) + 2 Pi + 2 ADP + 2 NAD+ → 2 Pyruvate (3C) + 2 ATP + 2 NADH + 2 H+ + 2 H2O 42 21  12/10/24 Both the beginning and end products of glycolysis – glucose and 2 pyruvic acids: CH2 OH O C3H4O3 H OH H C6H12O6 OH H OH H C3H4O3 H OH D-glucose Pyruvic acid Both have 6 carbon atoms, and 6 oxygen atoms The 2 pyruvic acids have a total of 8 hydrogen atoms, which is 4 less than 1 molecule of glucose Therefore 2 events have occurred during glycolysis: Glucose has been split into 2; There has been a loss of 2 hydrogen molecules / 4 hydrogen atoms Loss of hydrogen molecule is a form of oxidation and therefore during glycolysis, glucose is oxidized by loss of 2 hydrogen 43 molecules to form 2 pyruvates The reactions of glycolysis 2 phases: 1st phase: - Glucose is converted to 2 molecules of glyceraldehyde-3-P 2nd phase: - Involve 5 subsequent reactions, convert these 2 molecules of glyceraldehyde-3-P into 2 molecules of pyruvate. 44 22  12/10/24 Stage I : Glycolysis Oxidation of glucose (6C) to pyruvate (3C) Occurs in the cytoplasm Does not require the presence of oxygen A sequence of ten reactions produced ten intermediate compounds The most ancient known metabolic pathway Embden-Meyerhof-Parnas pathway (Gustav Embden, Otto Meyerhof and Jakub Karol Parnas) Stage I : Glycolysis Involves: ENERGY INVESTMENT PHASE (Reaction 1 – 5) (Preparatory Phase) Ø Phosphorylation of sugar - activates the sugar - use energy (ATP) Ø Lysis - phosphorylated 6C sugar is split into 3C sugar phosphate (isomers) 23  12/10/24 ENERGY PAYOFF PHASE (Reaction 6 – 10) Ø Oxidation (by dehydrogenation) - each 3C sugar phosphate is converted into pyruvate - product:- (i) reduced NAD molecule X2 = 2NADH +2H (ii) 2 ATP molecule X2 = 4 ATP (X2 because there are 2 molecules of 3C sugar phosphate) In Glycolysis, 2 ATP used in the investment phase 4 ATP produced in oxidation phase Net gain of ATP = 4 – 2 = 2 4 H added to NAD+ = 2NADH + 2H+ The breakdown of glucose to pyruvate can be summarized as follows: Glucose (6C) + 2 Pi + 2 ADP + 2 NAD+ → 2 Pyruvate (3C) + 2 ATP + 2 NADH + 2 H+ + 2 H2O 24  12/10/24 1st phase: 2nd phase: 25  12/10/24 Stage I : Glycolysis 1st phase: 2nd phase: Figure 9.9 A closer look at glycolysis: energy investment phase CH2OH 1. Glucose enters the cell and is O phosphorylated by the enzyme H H H hexokinase, which transfers a OH H OH phosphate group from ATP to HO the sugar. The charge of the H OH phosphate group traps the sugar Glucose in the cell because the plasma membrane is impermeable to ATP 1 ions. Phosphorylation also 1 Hexokinase makes glucose more chemically reactive. In this diagram, the ADP transfer of a phosphate group or pair of electrons from one CH2O P reactant to another is indicated O by coupled arrows. H H H HO H OH OH H OH Glucose-6-phosphate 26  12/10/24 Figure 9.9 A closer look at glycolysis: energy investment phase CH2O P O H 2. Glucose-6-phosphate is H rearranged to convert it to its HO OH H OH isomer, fructose-6-phosphate. H OH 2 Glucose-6-phosphate 2 Phosphoglucoisomerase CH2O P O CH2OH H HO H HO HO H Fructose-6-phosphate 3. This enzyme transfers a phosphate group from ATP to the sugar, investing another molecule of ATP in glycolysis. So far, 2 ATP have been used. With phosphate groups on its opposite ends, the sugar is now ready to be split in half. This is a key step for regulation of glycolysis, phosphofructokinase is allosterically regulated by ATP and its products. 27  12/10/24 4. This is the reaction from which glycolysis gets its name. The enzyme cleaves the sugar molecules into two different three- carbon sugars: dihydroxyaceton phosphate and glyceraldehyde-3- phosphate. These two sugars are isomers of each other. 4. Fructose bisphosphate aldolase 5. Isomerase catalyzes the reversible conversion between the two three-carbon sugars. This reaction never reaches equilibrium in the cell because the next enzyme in glycolysis uses only glyceraldehydes-3-phosphate as its substrate (and not dihydroxyaceton phosphate). This pulls the equilibrium in the direction of glyceraldehydes-3-phosphate, which is removed as fast as it forms. Thus, the net result of steps 4 and 5 is cleavage of a six-carbon sugar into two molecules of glyceraldehydes-3-phosphate; each will progress through the remaining steps of glycolysis. Triose phosphate isomerase 28  12/10/24 6. This enzyme catalyzes two sequential reactions while it holds glyceraldehydes-3-phosphate in its active site. First, the sugar is oxidized by the transfer of electrons and H+ to NAD +, forming NADH (a redox reaction). This reaction is very exergonic, and the enzyme uses the released energy to attach a phosphate group to the oxidized substrate, making a product of very high potential energy. The source of the phosphates is the pool of inorganic phosphate ions that are always present in the cytosol. Glyceraldehyde-3-phosphate dehydrogenase 7. Glycolysis produces some ATP by substrate-level phosphorylation. The phosphate group added in the previous step is transferred to ADP in an exergonic reaction. For each glucose molecule that began glycolysis, step 7 produces 2 ATP, since every product after the sugar- splitting step (step 4) is doubled. Recall that 2 ATP were invested to get sugar ready for splitting; this ATP debt has now been repaid. Glucose has been converted to two molecules of 3-phosphoglycerate, which is not a sugar. The carbonyl group that characterizes a sugar has been oxidized to a carboxyl group (-COO-), the hallmark of an organic acid. The sugar was oxidized in step 6, and now the energy made available by that oxidation has been used to make ATP. Phosphoglycerate kinase 29  12/10/24 8. Next this enzyme relocates the remaining phosphate group. This step prepares the substrate for the next reaction. Phosphoglycerate mutase 9. This enzyme causes a double bond to form in the substrate by extracting a water molecule, yielding phosphoenolpyruvate (PEP). The electrons of the substrate are rearranged in such a way that the remaining phosphate bond becomes very unstable, preparing the substrate for the next reaction. 30  12/10/24 10. The last reaction of glycolysis produces more ATP by transferring the phosphate group from PEP to ADP, a second example of substrate-level phosphorylation. Since this step occurs twice for each glucose molecule, 2 ATP are produced. Overall, glycolysis has been used 2 ATP in the energy investment phase (step 1 and 3) and produced 4 ATP in the energy payoff phase (step 7 and 10), for a net gain of 2 ATP. Glycolysis has repaid the ATP investment with 100% interest. Additional energy was stored by step 6 in NADH, which can be used to make ATP by oxidative phosphorylation if oxygen is present. Glucose has been broken down and oxidized to two molecules of pyruvate, the end product of glycolytic pathway. If oxygen is present, the chemical energy in pyruvate can be extracted by the citic acid cycle. Figure 9.8 The energy input and output of glycolysis Glycolysis Citric acid Oxidative cycle phosphorylation ATP ATP ATP Energy investment phase Glucose 2 ATP + 2 P 2 ATP used Energy payoff phase 4 ADP + 4 P 4 ATP formed 2 NAD+ + 4 e- + 4 H + 2 NADH + 2 H+ 2 Pyruvate + 2 H2O Net Glucose 2 Pyruvate + 2 H2O 4 ATP formed – 2 ATP used 2 ATP 2 NAD+ + 4 e– + 4 H + 2 NADH + 2 H+ *Glycolysis explained.mp4 31  12/10/24 The possible fates of pyruvate in glycolysis Aerobic condition (presence of O2) § Pyruvate à Acetyl-CoA § Pyruvate is oxidized with loss of the carboxyl group as CO2. The remaining two-carbon unit becomes the acetyl group of acetyl-CoA § Catalyzed by pyruvate dehydrogenase (PDH) § Metabolized in Tricarboxylic acid cycle (TCA cycle) in mitochondria for complete oxidation to CO2 and H2O 63 The possible fates of pyruvate in glycolysis Anaerobic condition (absence of O2) § Pyruvate à Lactate (in some microorganisms and animals) § Pyruvate can be reduced to lactate through oxidation of NADH to NAD+ § Catalyzed by lactate dehydrogenase (LDH) § Lactic acid fermentation § E.g. anaerobic glycolysis in contracting muscle / muscle exercise In yeast (anaerobic condition) § Pyruvate à Acetaldehyde à Ethanol § Pyruvate can be reduced to ethanol, with oxidation of NADH to NAD+ § Catalyzed by pyruvate decarboxylase and ADH (alcohol dehydrogenase) § Alcoholic fermentation: § Brewing for beers; fermentation of grape sugar in making wine; yogurt; cheese; kim chi 64 32  12/10/24 How Cells Harvest Energy √ 1.Cellular Energy Harvest 2.Cellular Respiration √ – Glycolysis – Oxidation of Pyruvate – Krebs Cycle – Electron Transport Chain Stage II : Oxidation of Pyruvate (Link Reaction) Link reaction (linking glycolysis to Krebs cycle) Pyruvate enter mitochondria – decarboxylated (CO2 removed) and oxidised ( H removed) to become acetate (2C) Acetate combines with coenzyme A to become acetyl CoA Summary, 2 pyruvates + 2 CoA + 2NAD+ 2 acetyl CoA + 2CO2 + 2NADH + 2H+ 33  12/10/24 CYTOSOL MITOCHONDRION NAD+ NADH + H+ O– S CoA 2 C O C O C O CH3 1 3 CH3 Acetyle CoA Pyruvate CO2 Coenzyme A Transport protein Because pyruvate is a charged molecule, it must enter the mitochondrion via active transport, with the help of a transport protein. Upon entering the mitochondrion via active transport, pyruvate is first converted to a compound called acetyl coenzyme A, or acetyl CoA. A complex of several enzymes or multi-enzyme complex (the pyruvate dehydrogenase complex) involved in these reactions. The acetyl group of acetyl CoA will enter the Krebs cycle. The CO2 molecule will diffuse out of the cell. CYTOSOL MITOCHONDRION NAD+ NADH + H+ O– S CoA 2 C O C O C O CH3 1 3 CH3 Acetyle CoA Pyruvate CO2 Coenzyme A Transport protein Because pyruvate is a charged molecule, it must enter the mitochondrion via active transport, with the help of a transport protein. Upon entering the mitochondrion via active transport, pyruvate is first converted to a compound called acetyl coenzyme A, or acetyl CoA. A complex of several enzymes or multi-enzyme complex (the pyruvate dehydrogenase complex) involved in these reactions. The acetyl group of acetyl CoA will enter the Krebs cycle. The CO2 molecule will diffuse out of the cell. 34  12/10/24 CYTOSOL MITOCHONDRION NAD+ NADH + H+ O– S CoA 2 C O C O C O CH3 1 3 CH3 Acetyle CoA Pyruvate CO2 Coenzyme A Transport protein Three reactions: 1. Pyruvate’s carboxyl group (--COO-), which is already fully oxidized and thus has little chemical energy, is removed and given off as a molecule of CO2. (This is the first step in which CO2 is released during respiration). CYTOSOL MITOCHONDRION NAD+ NADH + H+ O– S CoA 2 C O C O C O CH3 1 3 CH3 Acetyle CoA Pyruvate CO2 Coenzyme A Transport protein 2. The remaining two-carbon fragment is oxidized, forming a compound named acetate (the ionized form of acetic acid). An enzyme transfers the extracted electrons to NAD+, storing energy in the form of NADH. 35  12/10/24 CYTOSOL MITOCHONDRION NAD+ NADH + H+ O– S CoA 2 C O C O C O CH3 1 3 CH3 Acetyle CoA Pyruvate CO2 Coenzyme A Transport protein 3. Finally, coenzyme A, a sulfur-containing compound derived from a B vitamin, is attached to the acetate by unstable bond that makes the acetyl group (the attached acetate) very reactive. The product of this chemical grooming, acetyl CoA, is now ready to feed its acetyl group into the Krebs cycle for further oxidation. Stage III : Krebs cycle Hans Krebs, 1930s Citric acid cycle Tricarboxylic acid cycle (TCA) Very important in all living cells A series of 8 enzyme-catalysed chemical reactions Use oxygen All glucose are totally broken apart/ oxidized 36