Terminal Oxidation Pathways Flashcards
Describe transport of fatty acids into mitochondria by carnitine
- Outer membrane of mitochondria= porins (protein pores) so acyl-CoA can enter
- Inner membrane very impermeable to acyl-CoA
- Acyl group transferred from coenzyme A onto carnitine= acyl- carnitine, catalysed by carnitine palmitoyl transferase 1 on outer membrane, inhibited by malonyl-CoA
- Acyl-carnitine imported through inner membrane in exchange for free carnitine
- Back to coenzyme A after transport so acyl-CoA
Describe fatty acid oxidation
-Once in mitochondria, acyl-CoA oxidised by beta-oxidation pathway
-Enzymes of beta oxidation pathway within mitochondrial matrix, catalyse the oxidation of long-chain fatty acids
-Fatty acid shortened by 2 carbons with acetyl-CoA split off
-Repetitive rounds= shortened
-If odd number of carbons= produces a propanyl-CoA
=Double bond introduced
=Reduction of electron-transferring flavoprotein which has FAD prosthetic group
=Reduces ubiquinone coenzyme
=Double bond hydrated to form hydroxy-acid attached to CoA, oxidised by NAD to form oxo-acid
=Acetyl-CoA split off
Compare fatty acid synthesis and degradation
- Synthesis= cytoplasm, reductive, NADPH coenzymes, multienzyme complex, intermediates esterified to enzyme complex
- Degradation= mitochondria, oxidative, NAD+ and ubiquinone coenzymes, separate enzymes, CoA-esters intermediates
Describe oxidation of propionate
-Propionyl Co-A from odd number fatty acids or metabolism of isoleucine, valine, methionine and a cholesterol side chain (bile acid synthesis)
=Carboxylated then isomerized (involves B12 as coenzyme
=Succinyl CoA, an intermediate in tricarboxylic acid cycle
What is the pyruvate dehydrogenase complex?
-Pyruvate formed by glycolytic pathway in cytoplasm
-Pyruvate dehydrogenase in mitochondrial matrix= 30 copies of one type of subunit, 60 and 12 of others
=Thiamine pyrophosphate (vitamin B1) prosthetic group= first step catalyst
=Transacetylase has lipoic acid
=Dehydrogenase that reoxides enzyme contains flavin adenine dinucleotide derived from B2
Describe pyruvate oxidation
-Pyruvate crosses inner mitochondrial membrane via pyruvate transporter
=Oxidative decarboxylation of pyruvate
=oxidised to acetyl group which is esterified onto coenzyme A, loss of CO2, NAD reduced to NADH
What causes thiamine deficiency?
- Dietary deficiency
- Beri-Beri
- Common in alcoholics as inhibits uptake of vitamin B1 and its processing into coenzyme thiamine pyrophosphate
- Symptoms= tremor, paralysis
What does alcoholic metabolism cause?
-Thiamine deficiency
-Hypoglycaemic crises because it inhibits gluconeogenesis
=Treated by infusion of glucose
-Vitamin B1 deficiency
How is pyruvate dehydrogenase activity regulated?
- Phosphorylation/ dephosphorylation cycle
- Pyruvate dehydrogenase kinase phosphorylates subunit type 1
- Activated by HADH, ATP and acetyl-CoA (products of pyruvate oxidation that inhibit enzyme)
- Pyruvate dehydrogenase phosphatase activates enzyme by dephosphorylation activated by calcium and insulin
What is the TCA cycle?
- Tricarboxylic acid cycle in mitochondrial matrix
- Glucose to pyruvate to acetyl-CoA as does fatty acids, ketogenic amino acids
- Enters cycle by reaction with oxaloacetate to form citrate (irreversible)
- Three dehydrogenases reduce NAD: isocitrate (CO2 lost), oxoglutarate (CO2 lost), malate
- Succinate dehydrogenase reduces ubiquinone (coenzyme, not soluble in water)
- Conversion of succinyl-CoA to succinate linked to formation of GTP
What are the gluconeogenic amino acids converted into in the TCA cycle?
=glutamic acid, histidine, proline and arginine converted to oxoglutarate
=valine, isoleucine, threonine and methionine converted into succinyl CoA
=phenylalanine and tyrosine to fumarate (inner mitochondrial membrane)
Describe the stoichiometry of the TCA cycle
-One turn= acetyl-CoA completely burnt up to 2 CO2, 3 NADH, reduced ubiquinone and a GTP
=10 or 11 ATP (10.6)
Describe ketone bodies
- Acetoacetate and 3-hydroxy butyrate produced in liver, produced during starvation
- Acetyl CoA formed from beta oxidation builds up, diverted to ketone body formation in mitochondria
- HMG-CoA intermediate broken down to acetoacetate, reduced to 3-hydroxy butyrate
- Circulate in plasma, oxidised as fuel
- Acetoacetate can be spontaneously decarboxylated (not stable) to produce acetone
- High concentration= acid-base disturbance (strong acids so metabolic acidaemia)
What happens to coenzymes in the TCA cycle?
- Reduced
- Re-oxidised by electron transport chain
- Electrons reduce oxygen to
- Oxygen is terminal electron acceptor
- Electron transport release energy, conserved for ATP synthesis by ATP synthase
Describe the mitochondrial ultra-structure
- Outer membrane permeability barrier for proteins, porins
- Infoldings of inner membrane increase surface area= cristae
- Inner membrane more impermeable to small molecules, carriers, electron transport chain are integral membrane proteins in inner
- Oxidative enzymes in mitochondrial matrix
What is an electron transport chain?
- Series of carriers, pass electrons to one another
- Reduced by accepting electrons from donor
- Movement is from carriers of lower affinity for electrons to higher affinity
- Expressed as redox potential
Describe the structure of the prosthetic groups of the electron transport chain
- Attached to proteins covalently or by strong non-covalent forces
- Flavoproteins= flavin prosthetic, derived from vitamin B2 or riboflavin, 2 forms= flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD), accepts 2 H to become reduced (hydrogen carrier)
- Iron-sulphur clusters= +side chains of cysteines in the protein, accepts electrons, single electron carrier, iron from ferric to ferrous form
Describe ubiquinone
- Coenzyme free in the mitochondrial inner membrane
- Synthesised not derived on same pathway as cholesterol
- Very hydrophobic, poly-isoprenoid side chain, dissolved in inner membrane, hydrogen-carrier
What are cytochromes?
- Iron atom coordinated by series of four 5-membered rings called haem
- Different types depending on side-chains on rings, may be covalently or non-covalently attached to proteins
- Cytochrome c= smallest and simplest, haem covalently attached to cysteine side chains
- B-type haem present in haemoglobin, reversibly binds to oxygen, always in ferrous state, no redox function
Examples of electron transport chain inhibitors
- Rotenone= plant derived poison (insecticide)- Complex 1
- Atovaquone= complex 2, component of antimalarial drug Malarone
- Cyanide and CO= complex 4, block electron transport to oxygen
What do the complexes do in the electron transport chain?
- Complex 1 oxidises reduced NAD and reduces ubiquinone
- Complex 2 is succinate dehydrogenase, reduces ubiquinone
- Complex 3 reoxidizes ubiquinone, reducing cytochrome c on outside of inner membrane
- Complex 4 reoxidizes cytochrome c, reduces oxygen to water
- Complex 5 makes ATP (ATP synthase), does not have an redox prosthetic groups
Describe proton translocation by mitochondrial electron transport chain
- Proton circuit through mitochondrial membrane (chemiosmotic theory)
- Electron transport chain translocate protons out of the mitochondrial matrix, setting up electrochemical gradient of protons (proton-motive force)
- Small difference in pH (1.5 units, higher inside mitochondria), membrane potential (150 millivolts) because protons positively charged
- Source of energy for ATP synthase as protons return through ATP synthase (inhibitor= oligomycin)
What are uncouplers?
- Protons short-circuit, pumped out but do not make ATP when they come back in, energy released as heat (damage to mitochondria)
- Chemicals that induce this effect, lipid-soluble weak acids which bind protons on the outside, diffuse through the membrane and release them inside
What is the P/O ratio?
Defined as the number of molecules of ATP produced per atom of oxygen reduced
The molecules of ATP per 2 electrons moving along the chain
-For chemiosmotic theory= number of protons from reducing one oxygen divided by the number of protons you need to make an ATP
-For NADH oxidation, 10 protons translocated by chain, ATP synthase makes 3 ATPs from translocation of 8 protons back into mitochondria so= 2.5 (take into account proton required for phosphate import)
-For succinate oxidation, 6 protons translocated from reoxidation of ubiquinone= 1.5
Describe the control of oxidation by the demand for ATP (respiratory control)
- Contracting muscle- myosin-ATPase hydrolyses ATP to ADP and phosphate
- Change in ratio of ATP to ADP stimulates ATP synthase= increased proton translocation= ATP synthase is working faster= more protons are translocated back =lowers the electrochemical gradient of protons= lowers protonmotive force= stimulates electron transport chain= rate of NADH oxidation increases= increases ratio of oxidised to reduced NAD= activates TCA cycle
