Cell Biology: Principles of Cell Function - BIOL2P03 Winter 2024 Lecture Notes PDF

Cell Biology: Principles of Cell Function BIOL2P03 - Winter 2024 Lecture 8/9 - Cellular Energetics Chapter 12: Cellular Energetics Testable Material: Slides 4-9, 11-12, 15- 47 Cellular Energetics Mitochondria Cellular Energetics ATP Cells require a constant supply of energy to survive, gro...

Cell Biology: Principles of Cell Function BIOL2P03 - Winter 2024 Lecture 8/9 - Cellular Energetics Chapter 12: Cellular Energetics Testable Material: Slides 4-9, 11-12, 15- 47 Cellular Energetics Mitochondria Cellular Energetics ATP Cells require a constant supply of energy to survive, grow and reproduce The main energy currency for eukaryotic cells is adenosine triphosphate (ATP) ATP stores energy in the bonds connecting its phosphate groups ▫ Hydrolysis of the terminal phosphate group releases the greatest amount of energy Cellular Energetics ATP LEO says GER Loss Equals Oxidation Gain Equals Reduction Let’s use Iron Oxidation/reduction as an example: Fe2+ >/< Fe3+ Iron (Reduced) Rust (Oxidized) Cellular Energetics ATP The synthesis of ATP requires an input of energy This energy comes from the catabolism (breakdown) and oxidation (stripping of electrons) of nutrients, including carbohydrates, lipids and amino acids + ATP Cellular Energetics ATP ATP production can occur through two main mechanisms: ▫ Substrate-level phosphorylation ▫ Oxidative phosphorylation During substrate-level phosphorylation, a high-energy molecule containing a phosphate group donates its phosphate to ADP to produce ATP ▫ This process does not require O2 and is therefore said to be anaerobic Cellular Energetics Oxidative Phosphorylation - Remember the Proton Pumps Oxidative phosphorylation involves the transfer of electrons from nutrients to high-energy intermediate molecules ▫ These high energy intermediates are then used to fuel the electron transport system and ATP synthase in the mitochondria ▫ This process requires O2 and is therefore said to be aerobic Chapters 11 & 12: ATP-Generating Pumps: F-class Proton Pumps Typically function in the reverse direction of V-class proton pumps Protons move down their concentration gradient and the energy released in the process is used to synthesize ATP ▫ E.g. ATP synthase BIOL2P03 - Cell Biology: ATP Synthase - Smart Biology The Smart Biology resource will be used to provide you with accurate 3D representations of complex cellular processes Cellular Energetics Mitochondria The outer mitochondrial membrane is permeable to ions and most small molecules ▫ Uses channel proteins, known as porins The inner mitochondrial membrane requires protein transporters to move most molecules ▫ The inner membrane folds inwards and generates structures known as cristae This greatly increases the surface area ▫ The inner membrane contains numerous proteins involved in energy production Transport proteins, proteins of the electron transport system and ATP synthase Cellular Energetics Mitochondria There is a space between the inner and outer mitochondrial membranes, known as the intermembrane space ▫ This region accumulates protons during oxidative phosphorylation The mitochondrial matrix is the innermost region of the mitochondrion which is surrounded by the inner mitochondrial membrane ▫ This is where pyruvate decarboxylation, the citric acid cycle and β- oxidation occur Glycolysis Cellular Energetics Glycolysis Glucose catabolism and oxidation begins with a series of enzymatic reactions known as glycolysis ▫ These reactions occur in the cytosol (cytoplasm) of the cell Glycolysis requires an initial investment of energy from ATP ▫ 2 ATP molecules are used to fuel the glycolysis pathway Through glycolysis, glucose (6 carbons) is split into two molecules of pyruvate (3 carbons) ▫ 4 molecules of ATP are produced through substrate-level phosphorylation ▫ There is therefore a net of 2 ATP that are produced through glycolysis for each molecule of glucose Cellular Energetics Glycolysis Cancer Cells are Dependent Upon Glycolysis: The Warburg Effect Cellular Energetics Glycolysis During glycolysis, 4 electrons are oxidized from glucose and used to reduce 2 molecules of nicotinamide adenine dinucleotide (NAD+) ▫ This generates 2 molecules of the high-energy intermediate NADH If O2 is available to the cell, these 2 molecules of NADH can be used to generate additional ATP through oxidative phosphorylation within the mitochondria However, the inner mitochondrial membrane is impermeable to NADH Cellular Energetics Glycolysis High energy electrons from NADH produced in the cytosol are shuttled into the mitochondrial matrix using electron shuttles ▫ E.g. the malate-aspartate shuttle Cellular Energetics Glycolysis Cellular Energetics Glycolysis It is important that cells are able to modify the rate of energy production through glycolysis to meet their energy demands Negative feedback mechanisms are used by several enzymes in this pathway ▫ The product of the reaction inhibits the enzyme catalyzing that reaction E.g. glucose-6-phosphate inhibits hexokinase ▫ ATP can also inhibit certain enzymes Cellular Energetics Glycolysis In the absence of O2, oxidative phosphorylation is not possible ▫ There is therefore no point in sending pyruvate and NADH to the mitochondria We need a constant supply of NAD+ to keep glycolysis going ▫ NAD+ is regenerated from NADH through the reduction of pyruvate (or a derivative of pyruvate) Energy from pyruvate and NADH are wasted ▫ In animal cells, this process generates lactic acid ▫ In yeast, this process generates ethanol Cellular Energetics Glycolysis Cellular Energetics Overview of Cellular Respiration Pyruvate Decarboxylation Cellular Energetics Pyruvate Decarboxylation In the presence of oxygen, pyruvate is transported into the mitochondrial matrix ▫ Freely permeable to outer membrane ▫ Uses transporter to get across inner membrane Citric Acid Cycle Cellular Energetics Citric Acid Cycle The citric acid cycle represents a series of enzymatic reactions that catabolize acetyl-CoA to CO2 and strip electrons from acetyl- CoA to form NADH and FADH2 ▫ Occurs within the mitochondrial matrix ▫ Each acetyl-CoA molecule is used to produce 2 CO2, 3 NADH, 1 FADH2 and 1 GTP NADH and FADH2 enter the electron transport system to produce ATP through oxidative phosphorylation Cellular Energetics Citric Acid Cycle FADH2 FAD Cellular Energetics Citric Acid Cycle Many of the enzymes involved in the citric acid cycle are thought to form large complexes in the mitochondrial matrix ▫ The product from one enzymatic reaction is directly passed on to the next enzyme in the complex ▫ Increases the speed of the cycle as intermediate molecules do not need to diffuse through the mitochondrial matrix to reach their next target Succinate dehydrogenase is attached to the inner mitochondrial membrane ▫ It generates FADH2 which directly enters the electron transport system Cellular Energetics Citric Acid Cycle The Electron Transport System Cellular Energetics The Electron Transport System The electron transport system consists of ▫ 4 multiprotein complexes, known as complexes I-IV ▫ 2 electron carriers, known as Coenzyme Q and cytochrome C The electron transport proteins assemble into supercomplexes ▫ Facilitates the exchange of electrons Different supercomplexes have been identified ▫ Respirasome is a supercomplex consisting of Complexes I-IV, coenzyme Q and cytochrome C Cellular Energetics The Electron Transport System The electron transport system uses the energy from electrons from NADH and FADH2 to pump protons into the intermembrane space ▫ These electrons are ultimately donated to O2 Intermembrane Space Inner Membrane Matrix Cellular Energetics The Electron Transport System The affinity of a molecule for electrons is described in terms of redox potentials ▫ The higher (more positive) the redox potential, the higher the affinity for electrons In each step of the electron transport system, electrons are transferred to proteins with higher redox potentials Energy is released in the process because the newly reduced complex is more stable than the complex that donated the electrons ▫ Most of the energy that is released is used to pump H+ Cellular Energetics The Electron Transport System Cellular Energetics The Electron Transport System NADH is oxidized by complex I and its 2 electrons are then sent to complexes III and IV Complex IV donates these electrons to O2, which then reacts with H+ to generate H2O ▫ This is why the electron transport system requires O2 Intermembrane Space Inner Membrane Matrix Cellular Energetics Proton Motive Force The proton motive force is an electrochemical gradient established by the pumping of protons into the intermembrane space ▫ Protons are positively charged, so a potential difference develops across the inner mitochondrial membrane Favours the movement of H+ into the matrix ▫ The concentration of protons is higher in the intermembrane space than the matrix Also favours the movement of H+ into the matrix The proton motive force stores energy ATP Synthesis Cellular Energetics ATP Synthesis ATP is synthesized using the energy stored in the proton motive force Protons move through the enzyme ATP synthase down their electrochemical gradient ▫ From intermembrane space to matrix Intermembrane Space Inner Membrane Matrix Cellular Energetics ATP Synthase ATP synthase is an F-class proton pump Consists of two multimeric proteins, known as F0 and F1 F0 forms the transmembrane subunit that spans the inner mitochondrial membrane ▫ Rotates as protons pass through the inner mitochondrial membrane F1 is bound to F0 and extends into the matrix ▫ This F1 binds to ADP and Pi and catalyzes the synthesis of ATP ▫ At least 2 protons must pass through ATP synthase for each molecule of ATP synthesized Cellular Energetics ATP Synthase Cellular Energetics ATP Synthase Cellular Energetics ATP Transport Much of the ATP that has been synthesized in the mitochondrial matrix must be transported to the cytosol ▫ ATP/ADP antiporter exchanges ADP for ATP ▫ Inorganic phosphate (Pi) must also be transported across inner membrane β-oxidation Cellular Energetics Fatty Acid Metabolism Fatty acids can also be used as a source of energy for the cell Fatty acids that have entered the cytosol are first converted to fatty acyl-CoA ▫ This reaction uses energy from ATP Fatty acyl-CoA is transported to the mitochondrial matrix and is used to produce energy through β-oxidation Cellular Energetics β-Oxidation β-oxidation refers to the enzymatic catabolism and oxidation of fatty acyl-CoA Fatty acyl-CoA is broken down into 2 carbon acetyl-CoA molecules ▫ In the process, large amounts of FADH2 and NADH are produced ▫ NADH and FADH2 donate electrons to the electron transport system to produce ATP through oxidative phosphorylation Acetyl-CoA enters the citric acid cycle for further energy production Cellular Energetics β-Oxidation end