NUTR 470 1st Edition Lecture 7 -The TCA cycle, also named as citric acid cycle or Krebs cycle, is the final common pathway for the oxidation of carbohydrate, lipid, and protein.• RQ value can tell you the oxidation importance (listen)• The TCA cycle provides substrates for the respiratory chain.--Overview of the cycle:-TCA cycle starts with the reaction between acetyl moiety of acetyl-CoA and C4-oxaloacetate, forming citrate.• we start the cycle with Acetyl-CoA and then we form citrate. where did this come from? from pyruvate oxidation• under fasting— dont have enough glucose from glycolysis, so you have decreased supply to TCA cycle-tying nutrition condition this to physiological condition• you have signals associated with TCA cycle— glucagon and others— increase fatty acid oxidation-Ask you to know how we start TCA cycle— by forming citrate from acetyl coa• send the citrate out from the mitochondria to the cytosol — then start lipogenesis • burn one carb = 4 cals; fat is double• helps us survive when food was hard to findOverview: • 1: The synthesis of Citrate is catalyzed by citrate synthase• 2: Citrate is isomerized to isocitrate by aconitase• 3: First oxidative decarboxylation.• 4: Second oxidative decarboxylation.• 5: Substrate-level phosphorylation• 6: Regeneration of oxaloacetate-the generation of GTP is important for lipogenesis-Generate CO2 and catalyzed by dehydrogenase-Two molecules of CO2 are subsequently released and oxaloacetate is regeneratedRegulation of the cycle:— nutritional signals-Source of acetyl-CoA• Oxidation of pyruvate• β-oxidation— mitochondria-key step: Key step for this/ way to control this is how we transport long chain fatty acid-from cytosol to mitochondria for oxidation• Alanine transamination (through pyruvate)-if we don't have enough— still run transamination -fasting conditions— protein degradation and store it in the muscle-signals associated with the fasting condition: glucagonStructure of mitochondria:-different enzymes responsible for TCA, Beta Oxidation-Generation of Citrate• Regulatory role of citrate• Inhibition of 6-phosphofucto-1-kinase (the enzyme that catalyzes the rate-limiting step of glycolysis)• Significance of the inhibition?-Metabolic Fate of Citrate• In mitochondria-Generation of isocitrate – TCA cycle• In cytosol-Citrate is converted to acetyl-CoA and oxaloacetate.— condition of feeding-The conversion of oxaloacetate to phosphoenolpyruvate is a rate-determing reaction in gluconeogenesis. (talking about this slide) — condition of fasting-Malate, converted from oxaloacetate, is required for the generation of NADPH, which is akey substrate for the synthesis of fatty acids.-Isocitrate —> a-Ketoglutarate• The reducing equivalents (2H) are collected by the respiratory chain for oxidation and coupled generation of ATP.-Reaction: Succinyl-Coa—> Succinate• Significance of GTP• GTP is used for the decarboxylation of oxaloacetate to phosphoenolpyruvate in gluconeogenesis-Succinate—> Fumarate (c4)• The reducing equivalents (2H) are transferred to respiratory chain, where reoxidation of FAD•2H results in formation of ATP.-Fumarate—> Malate—> OAA• Significance—-generation of reducing equivalents-completion of TCA cycle-generate energy— GTP- Significance of TCA cycle• The TCA cycle takes part in gluconeogenesis-Phosphoenolpyruvate carboxykinase (catalyzes first rate determining step) • catalyzes the decarboxylation of oxaloacetate to phosphoenolpyruvate (GTP acting as phosphate donor).-Fasting condition—“when you don't have enough”— but you still have TCA cycle. this means you don't have enough oxidizing carbohydrates, but you have fat stores. This will drive the reaction to provide acetyl—coa and being cycle • The TCA cycle takes part in fatty acid synthesis-When you are feeding— “have plenty”— convert to citrate-In cytosol, citrate is cleaved by ATP-citrate lyase. This reaction, and subsequent reactions provide acetyl-CoA and NADPH, which are essential for the synthesis of fatty acids. The TCA cycle plays a pivotal role in metabolism.-ask us to know the nutrition condition, not the mechanism• The TCA cycle plays a pivotal role in metabolism.• The TCA cycle takes part in transamination• Transaminase reactions form pyruvate from alanine, oxaloacetate from aspartate, and α-ketoglutarate from glutamine.-dephosphorylation— pyruvate kinase • The TCA cycle serves as a source of carbon skeletons for the synthesis of alanine, aspartate, and glutamine.-Transamination reaction• protein degradation• the transfer of one amino acid to another one-Regulation of TCA cycle• Entry of substrates-The generation of acetyl-CoA from carbohydrates is a major control point of the cycle.-The PDH catalyzes the generation of acetyl-CoA from pyruvate.-The PDH is inhibited by acetyl-CoA and NADH and activated by non-acetylated CoA (CoASH) and NAD+.• Key reactions-Three reactions of the TCA cycle need NAD+.-An increase in the cellular ratio of-NAD+/NADH appears to drive the TCA cycle.• The key enzymes of the TCA cycle are also regulated allosterically by Ca2+, ATP and ADP.-Key Components of Respiratory Chain• Complex I-NADH-Q oxidoreductase• Complex III-Q-cytochrome c oxidoreductase• Complex IV-Cytochrome c oxidase• Complex II-Succinate Q reductase-Sequence of Electron Transfer• Q accepts electrons from Complex I and Complex II• The Q cycle couples electron transfer to proton transport in Complex IV• Molecular oxygen is reduced to water via Complex IV-Synthesis of ATP• Generation of proton gradient-Protons are translocated across the membrane, from the matrix to the intermembrane space, as a result of electron transport.-The continued buildup of these protons creates a proton gradient.-Generation of ATP• The resulting current of protons flowing through inner mitochondrial membrane drives the synthesis of ATP.• ADP captures a significant proportion of free energy, in the form of high-energy phosphate – forming ATP.• Reaction: ADP + Pi —> ATP• ATP synthase is embeded in the inner membrane• Several subunits of the protein form F1, which projects into the matrix.• F1 is attached to F0, which also consists of several subunits. F0 spans the membrane and forms a proton channel.- Respiratory Control-Conditions limiting the rate of respiration• Since oxidation and phosphorylation are tightly