16 tca cycle 2014-2015
DESCRIPTION
HKU science lecture notesTRANSCRIPT
![Page 1: 16 TCA cycle 2014-2015](https://reader034.vdocuments.net/reader034/viewer/2022042718/5695d4f31a28ab9b02a36b64/html5/thumbnails/1.jpg)
The Citric Acid Cycle
Citric acid -a natural preservative in citrus fruits -Accumulation not related to citric acid cycle activities
![Page 2: 16 TCA cycle 2014-2015](https://reader034.vdocuments.net/reader034/viewer/2022042718/5695d4f31a28ab9b02a36b64/html5/thumbnails/2.jpg)
(1) Acetyl-CoA production
(2) Acetyl-CoA oxidation - Citric acid cycle - or Tricarboxylic acid (TCA) cycle - or Kreb cycle
Cellular respiration - Aerobic conditions
- Complete oxidation of glucose, amino acids and fatty acids to CO2 and H2O
- Three major stages:
(3) Electron transfer and oxidative phosphorylation
![Page 3: 16 TCA cycle 2014-2015](https://reader034.vdocuments.net/reader034/viewer/2022042718/5695d4f31a28ab9b02a36b64/html5/thumbnails/3.jpg)
Entry of pyruvate to mitochondria
Inner membrane
![Page 4: 16 TCA cycle 2014-2015](https://reader034.vdocuments.net/reader034/viewer/2022042718/5695d4f31a28ab9b02a36b64/html5/thumbnails/4.jpg)
- A link between glycolysis and TCA cycle - 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)
Production of acetyl-CoA from pyruvate
(Co-enzyme A)
![Page 5: 16 TCA cycle 2014-2015](https://reader034.vdocuments.net/reader034/viewer/2022042718/5695d4f31a28ab9b02a36b64/html5/thumbnails/5.jpg)
![Page 6: 16 TCA cycle 2014-2015](https://reader034.vdocuments.net/reader034/viewer/2022042718/5695d4f31a28ab9b02a36b64/html5/thumbnails/6.jpg)
Pyruvate Dehydrogenase (PDH) Complex - Consisting of 3 different enzymes (E1, E2, E3) - Several reaction steps preformed before a product is released - Avoids diffusion of intermediates and allows efficient metabolite channeling
![Page 7: 16 TCA cycle 2014-2015](https://reader034.vdocuments.net/reader034/viewer/2022042718/5695d4f31a28ab9b02a36b64/html5/thumbnails/7.jpg)
E1: Pyruvate dehydrogenase
- Contains TPP (or called TDP) in active site:
1. Decarboxylation of pyruvate
![Page 8: 16 TCA cycle 2014-2015](https://reader034.vdocuments.net/reader034/viewer/2022042718/5695d4f31a28ab9b02a36b64/html5/thumbnails/8.jpg)
E1: Pyruvate dehydrogenase
Lipoyllysine: A lipoic acid linked to a lysine residue in E2
2. Transfer of acetyl group to a lipoyllysine in E2
![Page 9: 16 TCA cycle 2014-2015](https://reader034.vdocuments.net/reader034/viewer/2022042718/5695d4f31a28ab9b02a36b64/html5/thumbnails/9.jpg)
E2: Dihydrolipoamide acetyltransferase
(Fully reduced form)
Arsenite poisoning:
Transfer of acetyl group to co-enzyme A:
![Page 10: 16 TCA cycle 2014-2015](https://reader034.vdocuments.net/reader034/viewer/2022042718/5695d4f31a28ab9b02a36b64/html5/thumbnails/10.jpg)
E3: Dihydrolipoamide dehydrogenase
- Contains FAD as a prosthetic group
1. Regeneration of lipoamide (oxidized form)
2. Regeneration of FAD (oxidized form)
![Page 11: 16 TCA cycle 2014-2015](https://reader034.vdocuments.net/reader034/viewer/2022042718/5695d4f31a28ab9b02a36b64/html5/thumbnails/11.jpg)
- A link between glycolysis and TCA cycle - 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)
Production of acetyl-CoA from pyruvate
(Co-enzyme A)
![Page 12: 16 TCA cycle 2014-2015](https://reader034.vdocuments.net/reader034/viewer/2022042718/5695d4f31a28ab9b02a36b64/html5/thumbnails/12.jpg)
Regulation of the pyruvate dehydrogenase complex
(1)
![Page 13: 16 TCA cycle 2014-2015](https://reader034.vdocuments.net/reader034/viewer/2022042718/5695d4f31a28ab9b02a36b64/html5/thumbnails/13.jpg)
Regulation of the pyruvate dehydrogenase complex
(2)
![Page 14: 16 TCA cycle 2014-2015](https://reader034.vdocuments.net/reader034/viewer/2022042718/5695d4f31a28ab9b02a36b64/html5/thumbnails/14.jpg)
The citric acid cycle
Mitochondria
![Page 15: 16 TCA cycle 2014-2015](https://reader034.vdocuments.net/reader034/viewer/2022042718/5695d4f31a28ab9b02a36b64/html5/thumbnails/15.jpg)
![Page 16: 16 TCA cycle 2014-2015](https://reader034.vdocuments.net/reader034/viewer/2022042718/5695d4f31a28ab9b02a36b64/html5/thumbnails/16.jpg)
Reactions and enzymes of the citric acid cycle
(1) Citrate synthase
(OAA)
Enzyme-bound intermediate
- An acetyl group transferase - No ATP requirement
![Page 17: 16 TCA cycle 2014-2015](https://reader034.vdocuments.net/reader034/viewer/2022042718/5695d4f31a28ab9b02a36b64/html5/thumbnails/17.jpg)
(2) Aconitase
- an isomerase (mutase)
![Page 18: 16 TCA cycle 2014-2015](https://reader034.vdocuments.net/reader034/viewer/2022042718/5695d4f31a28ab9b02a36b64/html5/thumbnails/18.jpg)
(3) Isocitrate dehydrogenase
- Catalyzes an oxidative decarboxylation
![Page 19: 16 TCA cycle 2014-2015](https://reader034.vdocuments.net/reader034/viewer/2022042718/5695d4f31a28ab9b02a36b64/html5/thumbnails/19.jpg)
(4) α-Ketoglutarate dehydrogenase
- An enzyme complex with 3 components (E1, E2, and E3) - Catalyzes an oxidative decarboxylation
- Resembles pyruvate dehydrogenase in both structure and function:
TPP, Lipoate, FAD
![Page 20: 16 TCA cycle 2014-2015](https://reader034.vdocuments.net/reader034/viewer/2022042718/5695d4f31a28ab9b02a36b64/html5/thumbnails/20.jpg)
(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
![Page 21: 16 TCA cycle 2014-2015](https://reader034.vdocuments.net/reader034/viewer/2022042718/5695d4f31a28ab9b02a36b64/html5/thumbnails/21.jpg)
Proposed mechanism of succinyl-CoA synthetase:
![Page 22: 16 TCA cycle 2014-2015](https://reader034.vdocuments.net/reader034/viewer/2022042718/5695d4f31a28ab9b02a36b64/html5/thumbnails/22.jpg)
Malonate - A structural analog of succinate not present in cells - Strong inhibitor of succinate dehydrogenase - Effectively blocks the TCA cycle activities
(6) Succinate dehydrogenase
- an membrane-bound enzyme
Q = ubiquinone QH2 = ubiquinol
![Page 23: 16 TCA cycle 2014-2015](https://reader034.vdocuments.net/reader034/viewer/2022042718/5695d4f31a28ab9b02a36b64/html5/thumbnails/23.jpg)
(7) Fumarase
- Hydration of fumarate
(8) Malate dehydrogenase
![Page 24: 16 TCA cycle 2014-2015](https://reader034.vdocuments.net/reader034/viewer/2022042718/5695d4f31a28ab9b02a36b64/html5/thumbnails/24.jpg)
Fates of carbon atoms in the citric acid cycle:
![Page 25: 16 TCA cycle 2014-2015](https://reader034.vdocuments.net/reader034/viewer/2022042718/5695d4f31a28ab9b02a36b64/html5/thumbnails/25.jpg)
ATP production from reduced co-enzymes
- Summary of citric acid cycle:
- Electron transport chain: oxidation of reduced co-enzymes and ATP production
![Page 26: 16 TCA cycle 2014-2015](https://reader034.vdocuments.net/reader034/viewer/2022042718/5695d4f31a28ab9b02a36b64/html5/thumbnails/26.jpg)
Net profit of aerobic degradation of glucose (32 ATP)
5 ATP (ETC)
2 ATP
5 ATP (ETC)
15 ATP (ETC)
QH2 3 ATP (ETC)
2 ATP
![Page 27: 16 TCA cycle 2014-2015](https://reader034.vdocuments.net/reader034/viewer/2022042718/5695d4f31a28ab9b02a36b64/html5/thumbnails/27.jpg)
Regulation of citric acid cycle
Pyruvate Dehydrogenase
Complex
Isocitrate Dehydrogenase
α-Ketoglutarate Dehydrogenase
NAD+, CoA-SH
Citrate Synthase
ATP, NADH, citrate, succinyl-CoA
ADP +
- The cycle is precisely regulated to meet the cellular needs for ATP
![Page 28: 16 TCA cycle 2014-2015](https://reader034.vdocuments.net/reader034/viewer/2022042718/5695d4f31a28ab9b02a36b64/html5/thumbnails/28.jpg)
Citric acid cycle is not always a “cycle”
The intermediates are used for both catabolism and anabolism
Cataplerotic reactions - Depletion of citric acid
intermediates
Anaplerotic reactions - Filling up of citric acid
intermediates