PDH and TCA cycle Flashcards
PDH 3 multi-enzyme complex
E1- pyruvate decarboxylase
E2- dihydrplipoyl transacetylase
E3- dihydrolipoyl dehydrogenase
PDH the 5 coenzymes
1) CoA [E2 substrate]- derived from pantothenic acid
2) FAD+ [E3]- derived from riboflavin
3) NAD+ [E3 substrate]- derived from niacin
4) TPP -enzyme E1- derived from thymine
5) lipoamine [E2]- 10-carbon fatty acid that acts as arm to move substrate/product between enzyme complexes
Beri-beri
- thiamine deficiency
- damage to PNS and weakened muscles, cardiovascular disorders
- common in far east
general vitamin deficiencies
riboflavin and niacin precursors of FAD+ and NAD+.
riboflavin deficiency tends to occur with other B vitamin deficiencies.
naicin deficiency cause pellagra [fatal if untreated]
mercury and arsenite poisoning
bind to lipoyl groups in E2; causes CNS pathologies
pyruvate dehydrogenase deficiency
- genetic mutations in components of PDH complex will lead to problems [incidence unknown; rare]
- causes lactic acid build up and CNS pathologies
citrate synthase reaction [1]
- citrate synthase catalyses the first step in the TCA cycle
- acetyl CoA from glycolysis combines with oxaloacetate to form citrate [releasing CoA]
aconitase reaction [2]
- second step in TCA cycle, aconitase converts citrate to isocitrate
- an isomerase reaction
fumarase reaction [7]
- catalyses the hydration of fumarate to malate
- the enzyme requires no cofactors
alpha-ketogluterate DH reaction [4]
alpha-ketoglutarate DH catalyses the oxidative addition of CoA
succinyl CoA synthase reaction [5]
- removal of the CoA moiety to produce succinate provides the energy to phosphorylate GDP
- this is the only triphosphate nucleotide to be produced directly by the TCA cycle
succinate DH reaction[6]
- succinate is oxidised to fumarate
- FAD+ is used as the electron acceptor, forming NADH2
isocitrate DH [3]
- mediated by isocitrate DH, oxidises isocitrate to alpha-ketoglutarate
- NAD+ is reduced to NADH, and carbon dioxide is released
malate DH reaction [8]
- malate DH catalyses the oxidation of malate to form oxaloacetate
control of TCA cycle
- strongest regulation applies to the 2 steps where carbon dioxide is released
- both ATP and NADH inhibit these reactions, while ADP and calcium ions [signals for muscle contraction] are activators
overview of cellular respiration
- electrons from NADH and FADH2 are transferred to oxygen along the ETC [respiratory chain]
- components located in the inner mitochondrial membrane [IMM]
- energy released is used to pump protons into the intermembrane space to create a proton gradient
- protons flow back through ATP-synthase complex to generate ATP
- whole pathway tightly coupled
overview of the ETC
4 protein complexes [I-IV]
- 3 proton pumps [ complex I, III, IV]
- 1 link to TCA cycle [complex II]
2 smaller components:
Coenzyme Q [CoQ]; also called ubiquinone [UbQ]
- cytochrome C
complex I: NADH dehydrogenase
Complex I: NADH dehydrogenase
- At least 34 polypeptides
- Complex I accepts electrons from NADH [oxidizing it to NAD+]
- Transfers electrons to CoQ [via RMN and Fr-S cofactors]
- 4 H+ pumped from matrix into intermembrane space
complex II: succinate dehydrogenase
Complex II: succinate dehydrogenase
- Part of TCA cycle [succinate to fumarate]
- Succinate to fumarate reaction generates FADH2
- FADH2 oxidised to regenerate FAD+
- Released electrons transferred to CoQ via Fe-S proteins
- As a prosthetic group FAD+/FADH2 doesn’t dissociate from the enzyme.
- The enzyme participates directly in both the TCA cycle and the ETC.
complex III: cytochrome c reductase
- Heme prosthetic group
- Accepts electrons from CoQ
- Electrons transferred to small protein cytochrome c
- 2 protons pumped across IMM
complex IV: cytochrome c oxidase
- 13 protein subunits containing 2 heme groups and 3 copper ions
- Reducing oxygen to 2x water requires:
- 4 reduced cytochrome c molecules
- 8 proteins from matrix
- And produces:
- 4 pumped protons [others from the water]
agents affecting oxidative phosphorylation
- Site-specific inhibitors of the ETC
- ATPase inhibitors e.g. oligomycin
- Uncouplers; neutralize the proton gradient and prevent ATP synthesis
ETC uncouplers
Chemicals:
- E.g. dinitrophenol; forms proton ionophore that dissipated H+ gradinent
Natural:
- Uncoupling proteins
- Mitochondria from brown adipose tissue
- ETC energy used to generate heat; non-shivering thermogenesis
- Newborns and hibernating animals
Coenzyme Q
- Also called CoQ10
- Small lipid soluble compound [a hydrophobic quinone]
- Diffuses rapidly within IMM- mobile carrier
- Accept electrons from Fe-S proteins from Complex I and II
- Transfers electrons to complex III/ cytochrome c [‘Q cycle’]
cytochrome c
- Peripheral membrane protein loosely bound to IMM
- Binds to complex III and transfers electrons to complex IV
- Highly conserved
complex V: ATP synthase
- Embedded in inner mitochondrial membrane
- Composed of 2 subunits:
- F1 ATPase (generates ATP)
- F0 coupling factor; a proton channel
- H+ pumps to cytosolic side of mitochondrial membrane r-enter matrix through F0 proton channel
- the passage of protons through the channel drives the rotation of the C-ring of F0
- this alters the conformation of beta-subunit of F1 domain
- this in turn drives formation of ATP from /ADP and PI
