Download Chapter 15 The Tricarboxylic Acid Cycle

Chapter 15 The Tricarboxylic Acid Cycle
In Bacteria : This happens in cytosol, which is regulated by the presence of oxygen
In Eukaryotic cells : cytosol and mitochondria
All stages are performed in mitochondria.
The components of respiratory metabolism : 4 events for glucose catabolism :
1) Glycolytic pathway : production of pyruvate from glucose (in cytosol)
2) TCA cycle : oxidation of pyruvate to generate reduced electrons (coenzymes)
(mitochondrial matrix)
3) Electron-transport chain : reoxidation of coenzymes at the expense of molecular
oxygen (mitochondrial inner membrane)
4) Oxidative phosphorylation : generation of ATP from ADP using proton-gradient
generated in the process of electron transport. (mitochondrial inner membrane)
Typical efficiency of glucose : about 30 ATPs per glucose
Steps in the TCA Cycle
The Oxidative Decarboxylation of Pyruvate Leads to Acetyl-CoA
The first event in the TCA cycle : entry of pyruvate to mitochondria. : highly conserved during evolution
Generation of Acetyl-CoA by pyruvate dehydrogenase complex
Pyruvate Dehydrogenase Complex : Three enzymes form a complex :
Each of them contains TPP, lipoic acid or CoA as a cofactor :
Pyruvate decarboxylase :
Dihydrolipoyl transacetylase :
Dihydrolipoyl dehydrogenase : perform oxidation of lipoic acid
In Mammals : its molecular weight is 9  106 : It contains 60 of the transacetylases and 20-30 of the other
two enzymes.
Production of Acetyl-CoA :
a) from pyruvate in mitochondria
b) from oxidation of fatty acid :
Acyl-CoA is transported to the space between outer mitochondrial membrane and inner
mitochondrial membrane(It can not pass through the mitochondrial membrane) and pass
through the inner mitochodrial membrane via formation of acyl-carnitine. The acyl-carnitine
becomes acyl-CoA inside mitochodria, which is utilized for acetyl-CoA by -oxidation. :
Carnitine acyltransferase I, carrior protein and carnitine acyltransferase II
Multienzyme complexes in TCA cycle : pyruvate dehydrogenase, -ketoglutarate dehydrogenase
Unaerobically growing bacteria
in the absence of -ketoglutarate dehydrogenase
 GTP-utilizing steps in glucose metabolism : phosphoenolpyruvate carboxykinase
(glyconeogenesis), succinate thiokinase (succinate synthetase: TCA cycle)
Succinate synthetase : use GDP exclusively in animal tissues, however, some
animals have isozyme using ADP in an exclusively manner with the GDP-specific
enzyme.
Action of phosphoenolpyruvate carboxykinase : generate phosphoenolpyruvate from
oxaloacetate using 1 GTP (It is interesting to point out that one GTP would be
generated from conversion of succinyl-CoA to succinate.
Why GTP not ATP which is generated in mitochondria ? : There is a specific
ATP transporting system out of mitochondria, which will keep the concentration of
ATP inside the mitochondria low. Therefore, it is necessary to use another type of
nucleotide to drive turn of TCA cycle in mitochondria.)
Another interesting point : Formations of oxaloacetate from
phosphoenolpyruvate and succinate from succinyl-CoA result in decrease of GDP
and increase of TCA cycle pool.
Cell Mol Life Sci 2002 Feb;59(2):213-9
Evidence of undiscovered cell regulatory mechanisms: phosphoproteins and protein kinases in
mitochondria.
Thomson M.
School of Biological Sciences, The University of Sydney, New South Wales, Australia.
The finding that mitochondria contain substrates for protein kinases lead to the discovery that protein
kinases are located in the mitochondria of certain tissues and species. These include pyruvate
dyhydrogenase kinase, branched-chain alpha-ketoacid dehydrogenase kinase, protein kinase A,
protein kinase Cdelta, stress-activated kinase and A-Raf as well as unidentified kinases. Recent
evidence suggests that mitochondrial protein kinases may be involved in physiological processes
such as apoptosis and steroidogenesis. Additionally, the novel finding of low-molecular-weight
GTP-binding proteins in mitochondria suggests the possibility that these may interact with
mitochondrial protein kinases to regulate the activity of mitochondrial effector proteins. The fact that
there are components of cellular regulatory systems in mitochondria indicates the exciting possibility
of undiscovered systems regulating mitochondrial physiology.
PMID: 11915939 [PubMed - indexed for MEDLINE]
The Amphibolic Nature of the TCA Cycle
Amphibolic : The TCA cycle is catabolic and anabolic.
The intermediates which are coupled with other metabolisms are :
oxaloacetate, -ketoglutarate, succinyl-CoA and citrate.
(amphibolic) :
oxaloacetate : for aspartate
-ketoglutarate : glutamate
succinyl-CoA : heme biosynthesis in animals.
citrate : acetyl-CoA in cytosol for fatty acid synthesis
Anaplerotic reactions to TCA cycle : pyruvate carboxylase reaction to provide
oxaloacetate.
Anaplerotic : the reactions to replenish the intermediates in a biochemical cycle (TCA
cycle in this case)
Oxidation of Other Substrates by the TCA Cycle
The entry of carbohydrates and fats to the TCA cycle : the final products of these
catabolism is acetyl-CoA
The entry of amino acids to the TCA cycle : -ketoglutamate, succinyl-CoA and
oxaloacetate. Oxidation of these intermediates needs supply of acetyl-CoA (or
pyruvate).
Some amino acids become pyruvate or acetyl-CoA
The TCA Cycle Activity Is Regulated at Metabolic Branchpoints
Two major functions of TCA cycle :
1. furnishing reducing equivalents(NADH, FADH2) to the electron-transport
chain
2. providing substrates for biosynthesis pathways.
Factors : TCA Cycle need two entry components, acetyl-CoA and oxaloacetate. Supply
of these molecules must be regulated. And, the fate of citrate is also important.
1. Energy Charge : decrease the TCA cycle rate
2. NADH/NAD+ : decrease the TCA cycle rate
3.
Acetyl-CoA : increase the pool size of TCA cycle intermediates
The Main Regulatory Points : Both are related the above three factors.
a.
b.
Partition of Pyruvate to Acetyl-CoA and Oxaloacetate :
acetyl-CoA는 TCA cycle의 속도를 가속시키기는 하지만 전체의 농도를
증가시키는 효과(pool size를 증가시키는 효과)는 없다. 이런 관점에서
보면 pyruvate에서 oxaloacetate의 합성은 다른 경로(acetyl-CoA의 합성)
와 역할이 다르다.
Citrate Pool : citrate와 isocitrate는 유사하게 보이지만 실재의 역할은 매
우 다르다. citrate의 경우 fatty acid synthesis에 관여하고 있고 또한
mitochondria membrane을 통과할 수 있지만 isocitrate는 그렇지 않다.
Regulation of Amino Acids catabolism in TCA Cycle
Entering TCA cycle of amino acids via -ketoglutarate and succinyl-CoA has no
apparent regulation site during their conversion to oxaloacetate. : Since oxaloacetate can
not go further without acetyl-CoA, pyruvate dehydrogenase will be responsible for
regulation of amino acids.
Electron Transport, Proton Translocation, and Oxidative Phosphorylation
Electron Transport : Oxidation of NADH and FADH2 to secret proton out of the inner
mitochondrial membrane. (generate proton gradient)
Oxidative Phosphorylation : ATP formation utilizing the membrane potential
generated proton gradient.
Oxidative phosphorylation of glucose
1. Electron transport
2. Proton translocation
3. ATP synthesis
Electron Transport Is a Membrane-Localized Process
Outer membrane : semi-permeable
Inner membrane : impermeable
Cristae
Bacteria : plasma membrane이 이 역할을 담당하고 있음.
A Bucket Bridge of Molecules Carries Electrons from the TCA Cycle to O2
Cytochromes : a, b, c
Cytochrome a, a3
Cytochrome bL, and bH
Cytochrome c : water soluble, c1
Cytochrome oxidase : page 349 equation 3
Electron carrier의 구조 : flavoproteins, iron-sulfur proteins and ubiquinone
1) Flavoproteins : one or two-electron transfer
NADH dehydrogenase(FMN), succinate dehydrogenase(FAD),
Mitochondrial inner membrane : glycerol-3-phosphate dehydrogenase(FAD)
Mitochondrial matrix space : at least eight other flavoproteins
2) Iron-sulfur proteins : one-electron carrier
NADH dehydrogenase, succinate dehydrogenase
3) Ubiquinone : on inner membrane, two- or one-electron carriers
In case of single electron carrier, semiquinones can be formed : anionic or neutral semiquinone
depending on pH and on the nature of binding site when the semiquinone is bound to protein.
Freely migrate on the membrane
Most of the Electron Carriers Exist in Large Complexes
4 complexes, cytochrome c and ubiquinone(UQ)
Complex I : NADH dehydrogenase complex
Complex II : succinate dehydrogenase complex is the only enzyme in TCA cycle, which is embedded in
the inner membrane. Formed FADH2 is used for electron transport directly whereas formed NADH in
TCA cycle is a substrate of NADH dehydrogenase complex(complex I).
Complex III : cytochrome bc1 complex
Complex IV : cytochrome oxidase,
Molecular oxygen radicals can be made as a intermediate., which is
a main cause of aging of mitochondria.
Ubiquinone and cytochrome c : electron transfer molecule between complexes
Ubiquinone and cytochrome c can diffuse on the membrane. Ubiquinone diffuses in the
membrane and cytochrome c diffuses on the membrane surface.
Complex I and II Mediate the Transfer of Electrons From NADH and FADH 2 to
Ubiquinone
Complex I : proton이 membrane 밖으로 이동.
complex II : proton의 translocation이 없음.
Complex III, The Cytochrome bc1 Complex, Transfers Electrons from QH2 to
Cytochrome While Translocating Protons by a Redox Loop
Components : cytochrome bL and bH, an iron-sulfur protein
Q cycle
cytochrome bL and bH : UQH2의 regeneration, a redox loop
The iron-sulfur protein : reduction of cytochrome c, a redox loop
Complex IV, the Cytochrome Oxidase Complex, Transfers Electrons from
Cytochrome c to O2 While Pumping Protons across the Membrane
Cytochrome a, a3, copper ions
Four one electron transfers to consume one molecule oxygen
Electrons from Cytosolic NADH Are Imported by Shuttle Systems
Cytosolic NADH
In Plant : another NADH dehydrogenase
In animal : NADH shuttle systems
1) Glycerol-3-phosphate dehydrogenase의 catalytic site는 mitochondria의 cytosolic site에 있음.
2)
malate formation form oxaloacetate, followed by translocation into mitochondria and re-oxidation to
oxalacetate : this process can be coupled with glutamate/aspartate shuttle
Complete Oxidation of Glucose Yields about 30 Molecules of ATP