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Gastrointestinal > Terminal Oxidation Pathways > Flashcards

Terminal Oxidation Pathways Flashcards

1
Q

Describe transport of fatty acids into mitochondria by carnitine

A
  • 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
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2
Q

Describe fatty acid oxidation

A

-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

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3
Q

Compare fatty acid synthesis and degradation

A
  • Synthesis= cytoplasm, reductive, NADPH coenzymes, multienzyme complex, intermediates esterified to enzyme complex
  • Degradation= mitochondria, oxidative, NAD+ and ubiquinone coenzymes, separate enzymes, CoA-esters intermediates
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4
Q

Describe oxidation of propionate

A

-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

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5
Q

What is the pyruvate dehydrogenase complex?

A

-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

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6
Q

Describe pyruvate oxidation

A

-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

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7
Q

What causes thiamine deficiency?

A
  • Dietary deficiency
  • Beri-Beri
  • Common in alcoholics as inhibits uptake of vitamin B1 and its processing into coenzyme thiamine pyrophosphate
  • Symptoms= tremor, paralysis
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8
Q

What does alcoholic metabolism cause?

A

-Thiamine deficiency
-Hypoglycaemic crises because it inhibits gluconeogenesis
=Treated by infusion of glucose
-Vitamin B1 deficiency

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9
Q

How is pyruvate dehydrogenase activity regulated?

A
  • 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
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10
Q

What is the TCA cycle?

A
  • 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
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11
Q

What are the gluconeogenic amino acids converted into in the TCA cycle?

A

=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)

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12
Q

Describe the stoichiometry of the TCA cycle

A

-One turn= acetyl-CoA completely burnt up to 2 CO2, 3 NADH, reduced ubiquinone and a GTP
=10 or 11 ATP (10.6)

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13
Q

Describe ketone bodies

A
  • 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)
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14
Q

What happens to coenzymes in the TCA cycle?

A
  • 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
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15
Q

Describe the mitochondrial ultra-structure

A
  • 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
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16
Q

What is an electron transport chain?

A
  • 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
17
Q

Describe the structure of the prosthetic groups of the electron transport chain

A
  • 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
18
Q

Describe ubiquinone

A
  • 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
19
Q

What are cytochromes?

A
  • 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
20
Q

Examples of electron transport chain inhibitors

A
  • 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
21
Q

What do the complexes do in the electron transport chain?

A
  • 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
22
Q

Describe proton translocation by mitochondrial electron transport chain

A
  • 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)
23
Q

What are uncouplers?

A
  • 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
24
Q

What is the P/O ratio?

A

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

25
Q

Describe the control of oxidation by the demand for ATP (respiratory control)

A
  • 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

Gastrointestinal (48 decks)

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