Krebs (Citric Acid) Cycle & Oxidative Phosphorylation

Krebs (Citric Acid) Cycle & Oxidative Phosphorylation The Citric Acid Cycle: The Citric Acid Cycle captures the energy stored in the chemical bonds of acetyl CoA (processed glucose) in a step-by-step process, trapping it in the form of high-energy intermediate molecules (known as electron carriers). They will then be passed on to the ETC for the final step of cellular respiration (does not need O2 but stops in its absence because NAD+ and FADH run out) NADH will indirectly produce 3 ATP molecules (sometimes 2) FADH2 will indirectly produce 2 ATP molecules (sometimes 1) → makes less because its electrons are at a lower energy state GTP can be easily converted into ATP within the cell CO2 is released when pyruvate loses a carbon and 2 oxygen atoms when it converts to Acetyl-CoA (2 carbon compound), known as Pyruvate Oxidation (occurs following glycolysis in the mitochondrial matrix and produces 1 NADH and 1 CO2) To start the CAC, oxaloacetate and acetyl-CoA will fuse together with the help of an enzyme (oxaloacetate will always be regenerated, hence the “cycle”) From one turn of the CAC, you produce: 1 GTP, 3 NADH, 1 FADH2, 2 CO2, and regenerated oxaloacetate The CO2 released possess carbons that came from the original glucose molecule Krebs/Citric Acid Cycle: The Krebs Cycle occurs in the mitochondrial matrix The Krebs Cycle is also the entry point for a lot of other energy sources, such as from proteins and fats Regulation of Pyruvate Dehydrogenase: Pyruvate Dehydrogenase converts pyruvate into acetyl-CoA through oxidation (also produced NADH from NAD+ and CO2) This reaction is irreversible and must therefore be regulated within the cell (pretty large -ΔG value) Fatty acids do produce acetyl-CoA but since the reaction is irreversible, fatty acids cannot contribute to pyruvate production and therefore cannot contribute to gluconeogenesis Acetyl-CoA can however be used to make fatty acids in the body when energy levels are high Pyruvate Dehydrogenase is allosterically regulated by: Activators are: CoA, NAD+, pyruvate, AMP, and Ca2+ (exercising skeletal muscle involves influx of Ca2+, meaning that you’re going to need energy and calcium levels are dependent on this) Inhibitors are: acetyl-CoA, NADH, ATP Regulation of Krebs-TCA Cycle: The CAC is basically always “on” because you always need some sort of ATP production within the cell in order to stay alive (lol) There is no hormonal control of the CAC since it is always on, even if you are in the fed or starve state Regulated through allosteric regulation, even though it’s always on, it still needs to be regulated to some degree (3 specific reactions have large -ΔG values) 1. Citrate Synthase (Oxaloacetate + Acetyl-CoA → Citrate) a. Allosteric Inhibitors: NADH, ATP, Citrate, Succinyl-CoA b. Allosteric Activators: ADP 2. Isocitrate Dehydrogenase (Isocitrate → ⍺-Ketoglutarate) a. Allosteric Inhibitors: NADH, ATP b. Allosteric Activators: ADP, Ca2+ ⍺-Ketoglutarate Dehydrogenase ( ⍺-Ketoglutarate → Succinyl-CoA) 3. a. Allosteric Inhibitors: NADH, Succinyl-CoA b. Allosteric Activators: ADP, Ca2+ It is also regulated through substrate availability (such as NAD+ or FAD, or when citrate is shuttled into the cytoplasm for fatty acid synthesis under conditions of high ATP) Electron Transport Chain: 10 NADH and 2 FADH2 go into the Electron Transport Chain (ETC) The electron carried by NADH/FADH2 are passed through multiple transition molecules going through lower and lower energy states, releasing energy along the way that is needed in order to pump H+ protons into the intermembrane space order to generate a gradient (lower pH in the intermembrane space than the matrix) ATP Synthase uses this gradient in order to generate ATP using the energy of H+ going down its concentration gradient (does it at 3 different sites at the same time) Oxidative Phosphorylation: The Major Energy Provider of the Cell: Can be broken down into two parts: Oxidation of NADH & FADH2 and Phosphorylation (production of ATP) → Together is known as Oxidative Phosphorylation NADH shuttles its electrons to Protein Complex I while FADH2 shuttles its electrons to Protein Complex II Protein Complexes I, III, and IV all pump hydrogen protons into the intermembrane space (Protein Complex II does not) Oxygen is the final electron acceptor (making oxygen necessary for this process to even occur) In the absence of oxygen, fermentation takes over after glycolysis (NADH is oxidized in order to turn pyruvate into lactic acid) → happens all the time in athletes when they aren’t getting enough oxygen Chemiosmosis is the movement of ions across a semipermeable (only allows H+) membrane bound structure, down their electrochemical gradient Substrate Level Phosphorylation is when the compounds themselves directly make ATP (such as in glycolysis and in the Krebs Cycle) Regulation of Oxidative Phosphorylation: The main regulation occurs by assessing the body’s need to energy in the form of ATP There is no major hormonal or even allosteric regulation in oxidative phosphorylation (regulation is more important in the earlier steps anyway) This is mainly regulated through Le Chatelier's Principle Mitochondria, Apoptosis, & Oxidative Stress: Necrosis is uncontrolled type of cell death that is usually in response to extreme stress/trauma Apoptosis is a controlled type of cell death (such as the development of our hand where the tissue between our digits die off to produce five separate fingers) Several factors to induce cell death: Damaged DNA in order to make sure it doesn’t pass it on to its daughter cells Development such as the formation of our fingers Infection (viruses) since viruses like to hang out inside the cell Environmental stress such as deprivation of O2, nutrients, or even connection to other cells Reactive Oxygen Species (ROS) are oxygen species that have acquired an extra electron (ROS is produced because about 4% of oxygen reduced in the ETC is improperly reduced to an ROS) Anti-apoptotic proteins are in abundance when the cell does not need to undergo apoptosis, but when it does, Pro-apoptotic proteins are then in abundance which increase the permeability of the mitochondrial membrane (these proteins are a part of the BCL-2 Family) When mitochondrial permeability increases, the molecule Cytochrome C (a member of the ETC) activate a family of enzymes known as Caspases (which break down proteins with the aspartate residue using a cysteine residue) which enhances the controlled degradation of the cell (surrounding cells can recycle whatever was in the cell) Calculating ATP Produced in Cellular Respiration: 2 ways: Substrate Level Phosphorylation and Oxidative Phosphorylation Protein Complex I pumps 4 H+ per 2 electrons Protein Complex III pumps 4 H+ per 2 electrons Protein Complex IV pumps 2 H+ per 2 electrons (likely less due to the lower energy state of the electrons by the time they get to this last protein complex) Remember, Protein Complex II doesn't really contribute to proton pumping Each NADH contributes to about 2.5 ATP (10 H+ and 4 needed per ATP) while FADH2 contributes to 1.5 ATP ← Generally accepted count of ATP molecules produced Extra Notes: pH Intermembrane Space < pH Cytosol < pH Mitochondrial Matrix