8. TCA cycle 2019.pdf - Citric Acid Cycle Universal metabolic pathway everyone makes citric acid but not everyone accumulated that much as fruits Used

8. TCA cycle 2019.pdf - Citric Acid Cycle Universal...

This preview shows page 1 out of 28 pages.

Unformatted text preview: Citric Acid Cycle Universal metabolic pathway, everyone makes citric acid, but not everyone accumulated that much as fruits Used as food additive which claims to have lemon flavour Citric acid - 6 carbons a natural preservative and a reservoir of carbon in citrus fruits its accumulation in citrus fruits is not related to the role of citric acid in the citric acid cycle 1 Cellular respiration Lose 1 carbon as carbon dioxide and the remaining 2 are used to make cartel-CoA Oxygen is available - Aerobic conditions - Complete oxidation of glucose, amino acids and fatty acids to CO2 and H2O - Three major stages: Coenzyme (a co-factor) (1) Acetyl-CoA production Electrons are important in terms of releasing energy & ATP formation (2) Acetyl-CoA oxidation - Citric acid cycle - or Tricarboxylic acid (TCA) cycle - or Kreb cycle Occurs in matrix of mitochondria (3) Electron transfer and oxidative phosphorylation 2 Entry of pyruvate to mitochondria gluconeogenesis Inner membrane 3 Oxidative decarboxylation (removal of carboxyl group as carbon dioxide) Production of acetyl-CoA from pyruvate - Inside mitochondria (matrix) - A link between glycolysis and TCA cycle Macro protein complex - Involves an enzyme complex with 3 components (E1, E2, and E3) - Overall an “oxidative decarboxylation” reaction - Five cofactors are involved (NAD+ as the final oxidizing agent) (Co-enzyme A) 4 Covalently linked to active sites 5 Pyruvate Dehydrogenase (PDH) Complex - Consisting of 3 different enzymes (E1, E2, E3) catalyzing five reactions Complex formation avoids diffusion of intermediates and allows efficient metabolite channeling 6 E1: Pyruvate dehydrogenase - Catalyzes two reactions 1. Decarboxylation of pyruvate -TDP is used as a prosthetic group in the active site. Thiamine diphosphate (TDP) Vitamin B1 7 E1: Pyruvate dehydrogenase 2. Transfer of acetyl group to lipoamide (a prosthetic group in E2) (a lysine residue in E2) 8 E2: Dihydrolipoamide acetyltransferase Transfer of acetyl group to co-enzyme A: Has to be regenerated back to lipoamide by E3 (Fully reduced form) First product being released to TCA Arsenite poisoning: Inhibition of E2 Couldn’t be regenerated to lipoamide, E2 reaction will be blocked 9 E3: Dihydrolipoamide dehydrogenase - Contains FAD as a prosthetic group 1. Regeneration of lipoamide (oxidized form) 2. Regeneration of FAD (oxidized form) 10 Whole complex is called pyruvate dehydrogenase, same name as E1 11 Regulation of the pyruvate dehydrogenase complex (1) Product inhibition and substrate activation of E2 and E3 Activators Inhibitors 12 Regulation of the pyruvate dehydrogenase complex Covalent regulation (2) Phosphorylation of E1 (inactivation) Substrates inhibit phosphorylation Products from reaction complex, which activates kinase activity to add phosphorus group, which in turn inactivate E1 13 Citric acid (or TCA) cycle All maintained in the same concentration when the cycle is normally proceeded Due to Oxidation Mitochondria Same as generating ATP 14 15 Reactions and enzymes of the citric acid (TCA) cycle (1) Citrate synthase - Condensation of OAA and acetyl CoA (OAA) Enzyme-bound intermediate Citric acid is a tricarboxylic acid (TCA) 16 (2) Aconitase - an isomerase (mutase) Intermediate 17 (3) Isocitrate dehydrogenase - Catalyzes an oxidative decarboxylation 18 this step, the 2 carbons from acetyl Co-A are (4) α-Ketoglutarate dehydrogenase After decarboxylated to carbon dioxide - An enzyme complex with 3 components (E1, E2, and E3) - Catalyzes an oxidative decarboxylation TPP, Lipoate, FAD - Resembles pyruvate dehydrogenase in both structure and function: Mechanism is the same, just the starting substrate is different 19 (5) Succinyl-CoA synthetase - Hydrolysis of the thioester succinyl-CoA - Named for the reverse reaction (ligation of succinate and CoA-SH) - Substrate level phosphorylation - Nucleoside diphosphate (NDP) kinase catalyzes GTP + ADP GDP + ATP Synthetases vs synthase - Both types of enzymes are involved in joining substrates together - Synthase reactions do not require NTP (nucleotide triphosphates) - Synthetase reactions use NTP (ATP or GTP) as an energy source 20 Proposed mechanism of succinyl-CoA synthetase: Release co enzyme A 21 (6) Succinate dehydrogenase Bound to the inner membrane of mitochondria, other enzymes in the cycle are found in the mitochondria matrix - an membrane-bound enzyme Mobile electron carrier Q = ubiquinone QH2 = ubiquinol Malonate - A structural analog of succinate - Strong inhibitor of succinate dehydrogenase - Effectively blocks the TCA cycle activities 22 (7) Fumarase - Hydration of fumarate Oxidation (8) Malate dehydrogenaseServes to regenerate oxaloacetate to start the cycle again 23 Fates of carbon atoms in the citric acid cycle: Oxaloacetate Succinate 24 ATP production from TCA cycle - Summary of TCA cycle: - Oxidation of reduced co-factors (NADH and QH2) in the electron transport chain (ETC) results in more ATP production 25 Net profit of aerobic degradation of glucose (32 ATP) 5 ATP (ETC = electron transport chain) 2 ATP 5 ATP (ETC) 15 ATP (ETC) QH2 2 ATP 3 ATP (ETC) 26 Regulation of citric acid cycle - The cycle is precisely regulated to meet the cellular needs for ATP Pyruvate dehydrogenase complex ✔ NADH, acetyl CoA NAD+, CoA-SH, pyruvate Citrate synthase ATP and NADH Isocitrate dehydrogenase NADH α-Ketoglutarate dehydrogenase NADH and succinyl CoA 27 TCA cycle is not always a “cycle” The intermediates are involved in both catabolism and anabolism CR Metabolites don’t only participate in the TCA cycle, they are also involved in other reactions (e.g. being made into amino acids, lipids, gluconeogenesis, etc) AR AR CR CR CR AR Use of metabolites for other reactions: CR Cataplerotic reactions (CRs) - Depletion of citric acid cycle intermediates AR Reactions to fill up those metabolites for TCA cycle to continue Anaplerotic reactions (ARs) - Filling up of citric acid cycle intermediates AR AR CR 28 ...
View Full Document

  • Fall '15

What students are saying

  • Left Quote Icon

    As a current student on this bumpy collegiate pathway, I stumbled upon Course Hero, where I can find study resources for nearly all my courses, get online help from tutors 24/7, and even share my old projects, papers, and lecture notes with other students.

    Student Picture

    Kiran Temple University Fox School of Business ‘17, Course Hero Intern

  • Left Quote Icon

    I cannot even describe how much Course Hero helped me this summer. It’s truly become something I can always rely on and help me. In the end, I was not only able to survive summer classes, but I was able to thrive thanks to Course Hero.

    Student Picture

    Dana University of Pennsylvania ‘17, Course Hero Intern

  • Left Quote Icon

    The ability to access any university’s resources through Course Hero proved invaluable in my case. I was behind on Tulane coursework and actually used UCLA’s materials to help me move forward and get everything together on time.

    Student Picture

    Jill Tulane University ‘16, Course Hero Intern

Ask Expert Tutors You can ask 0 bonus questions You can ask 0 questions (0 expire soon) You can ask 0 questions (will expire )
Answers in as fast as 15 minutes