BCHM-3050 Lecture Notes - Lecture 11: Dehydrogenation, Ion, Coenzyme Q10
26 Jun 2018
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Fate of Carbon in TCA
Oxaloacetate: reactant for first rxn
Product of 8th
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One turn = 1 acetyl-CoA
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Per turn makes:
2 CO2
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3 NADH
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1 FADH
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Step 1-5: C atoms transfer from acetyl-Coa
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Branching pathway
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Rxn 1: Citrate Synthase (irreversible rxn)
Acetyl-CoA + Oxaloacetate + H2O = Citrate + CoA-SH + H+
The two carbon Acetyl- CoA is introduced
Product = 6C
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Aldol condensation catalyzed by citrate synthase
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Hydrolysis of thioester: highly exergonic
TCA will proceed even at low levels of oxaloacetate
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Rxn 2: Aconitase
Citrate ( 3˚ OH) => [Cis-aconitrate] => D-isocitrate (2˚ OH)
2A 2B
Product = 6C
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Isomerization catalyzed by aconitase
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Step 2A: dehydration
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Step 2B: hydration
Prochiral citrate to chiral D-isocitrate
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Citrate = achiral
Prochiral due to enzymatic mechanism
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Unsymmetrical = make one product
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Rxn slightly unfavorable
BUT it is pulled to the right by rxn 3 by consuming the product
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Aconitase: iron sensor protein during times of low intracellular iron
Due to presence of iron sulfur clusters that’s lost
Iron sulfur clusters are required for catalytic activity
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Rxn 3: isocitrate Dehydrogenase(irreversible rxn)
Isocitrate + NAD+ => Oxalosuccinate + NADH => Alpha-ketoglutarate
Product = 5 C
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NAD+ oxidizes 2˚ alcohol
Form beta keto group
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Beta keto group of oxalosuccinate = electron sink to stabilize transition
state anion
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E releases = store as NADH
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Rxn 4: alpha ketoglutarate dehydrogenase
Alpha ketoglutarate + NAD + CoASH => Succinyl-CoA + NADH + CO2
Whole process is the same as PDH w pyruvate
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Product = 4 C
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Alpha ketoglutarate dehydrogenase = act analogous to pyruvate
dehydrogenase
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TPP = required for decarboxylation of alpha keto acid
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E conserved in NAD and thioester bond
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Succinyl-CoA = energy rich compound
Like acetyl-CoA
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Rxn 5: succinyl-CoA synthetase
Succinyl-CoA + Pi+ ADP/GDP => Succinate + ATP/GTP + CoA-SH
Product = 4 C
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Comparable to substrate level phosphorylation in glycolysis
Phosphate transferred from GTP to ADP
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Energy in the thioester bond: use to form nucleoside triphosphate
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Rxn use phosphohistidine as intermediate
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Rxn 6: succinate dehydrogenase
Succinate => fumarate
Product = 4 C
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dehydrogenation
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Catalyze dehydrogenation of two saturated C to dbl bond
Use enzyme bound FAD
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Succinate dehydrogenase: membrane bound
Deliver e- directly into mitochondria e- transport chain
Via coenzyme Q
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Mechanism: produce only transgeo isomer of fumarate
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Dehydrogenase = inhibited by malonate
Structural analog of succinate
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Rxn 7: fumarase
Fumarate => L-Malate
Hydration
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Rxn 8: malate dehydrogenase
L-Malate + NAD+ => oxaloacetate + NADH + H+
Highly exergonic step 1 pulls this rxn to the right
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Overview of TCA
Energy is released as electron carriers are reoxidized
Energy is used to drive ATP synth
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Regulation of pyruvate decarboxylation and TCA
Control of TCA:
Entry into cycle
Step 1 and PDH
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Key irreversible rxns
Step 3 and 4
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Allosteric activation of isocitrate dehydrogenase by ADP
High ADP = E starved
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NAD/NADH ratio
High NADH = shut down TCA
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Inhibition of relevant enzymes by acetyl-CoA and Succinyl-CoA
Availability of energy rich compound
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TCA= source of biological intermediates
Other paths use the TCA intermediates
Ex. Succinyl-CoA and Oxaloacetate
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Anapleurotic processes = required to restore intermediates
Replenishment of Oxaloacetate = from gluconeogenesis
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Transamination rxns can regenerate intermediates
Reversible rxns that can yield citric acid cycle intermediates
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Ex. Glutamate and aspartate
Undergo transamination to generate alpha-ketoglutarate and
oxaloacetate
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Chapter 13: TCA cycle
Monday, June 18, 2018
9:17 PM
Document Summary
Acetyl-coa + oxaloacetate + h2o = citrate + coa-sh + h+ Tca will proceed even at low levels of oxaloacetate. Citrate ( 3 oh) => [cis-aconitrate] => d-isocitrate (2 oh) But it is pulled to the right by rxn 3 by consuming the product. Aconitase: iron sensor protein during times of low intracellular iron. Due to presence of iron sulfur clusters that"s lost. Iron sulfur clusters are required for catalytic activity. Isocitrate + nad+ => oxalosuccinate + nadh => alpha-ketoglutarate. Beta keto group of oxalosuccinate = electron sink to stabilize transition state anion. Alpha ketoglutarate + nad + coash => succinyl-coa + nadh + co2. Whole process is the same as pdh w pyruvate. Alpha ketoglutarate dehydrogenase = act analogous to pyruvate dehydrogenase. Tpp = required for decarboxylation of alpha keto acid. Succinyl-coa + pi+ adp/gdp => succinate + atp/gtp + coa-sh. Energy in the thioester bond: use to form nucleoside triphosphate.
