W3 Tricarboxylic Acid Cycle Flashcards
What is the pyruvate dehydrogenase multienzyme complex (PDC)
used for conversion of pyruvate to acetyl coA by oxidative decarboxylation
a cluster of 3 enzymes: pyruvate dehydrogenase, dihydrolipoul transacetylase, dihydrolipoyl dehydrogenase
require 5 cofactors: thiamines pyrophosphate (TPP), lipoic acid, coenzyme A, FAD and NAD+
reaction occurs in mitochondrial matrix
properties of acetyl coA
functional role: acetyl group carrier
reactive group: thiol (-SH)
acetyl group covalently linked to thiol > forming high energy thioester (hydrolysis is -31.5)
formation of thioester bond in metabolic intermediate conserves a portion of the free energy released during oxidation of a metabolic fuel
high free energy released upon hydrolysis of thioester bond
Why is acyl group readily transferred to other metabolites
Most esters like oxygen esters gave two resonance forms > partial double bond character
lack of this resonance in thioesters in SR bond of acyl coA makes bond weaker > thiolalkoxide ion is a good leaving group in nucleophilic displacement reactions
properties and function of thiamine pyrophosphate (TPP)
pyrophosphate of thiamine (vitamin B1)
function: transfer of an activated aldehyde unit
functional group: thiazolium ring,which has an acidic proton at C2 > loss of proton produces carbanion (the active species)
reaction catalyzed by pyruvate dehydrogenase
colour of oxidized and reduced forms of FAD
oxidized form (FAD): yellow with wavelength max = 450nm
reduced form (FADH2): colourless
summary of pyruvate dehydrogenase complex reaction
- pyruvate reacts with TPP > decarboxylation > hydroxyethyl-TPP of E1
- aldehyde group is transferred to one lipoamide cofactor of E2 and simultaneously oxidized to acetyl
- acetyl group is transferred to the next lipoamide cofactor > transferred to CoA-SH > form acetyl coA
- E3 oxidizes lipoamide by transferring 2 H atoms to FAD
- FADH2 oxidized by NAD+ > enzyme complex ready for next cycle
results of TPP deficiency
vitamin B1 deficiency > TPP deficiency
pyruvate dehydrogenase (E1) inhibited > pyruvate oxidation inhibited > neurological and cardiovascular disorder
detection: increase in pyruvate in blood
step 1 of TCA cycle
acetyl coA adds its two carbon group to oxaloacetate via citrate synthase > produce citrate
step 2 of TCA cycle
citrate is tertiary alcohol > cannot be oxidised > citrate must be converted to isocitrate (chiral secondary alcohol)
reaction catalysed by aconitase
citrate > dehydration to form cis-aconitase intermediate > rehydration to form isocitrate
step 3 of TCA cycle
NAD+ dependent isocitrate dehydrogenase catalyze oxidation of isocitrate to oxalsuccinate (produce NADH) > decarboxylation of carboxyl group (release CO2) > form alpha ketoglutarate
keto beta group to the carboxyl group facilitates the decarboxylation by acting as a electron sink
oxidation occurs with reduction of NAD+
step 4 of TCA cycle
alpha ketoglutarate dehydrogenase catalyses oxidative decarboxylation of alpha ketoglutarate > release CO2 and NADH (mediated by PDC) > produce succinyl CoA
step 5 of TCA cycle
succinyl coA synthase catalyses formation of succinyl phosphate > formation of phosphoryl-His intermediate > transfer of phosphoryl group to GDP to form GTP > GTP can exchange its terminal phosphoryl group with ADP via nucleoside diphophate kinase > produce ATP
succinate is formed
step 6 of TCA cycle
succinate dehydrogenase oxidises succinate to fumarate > produce FADH2
enzyme has FAD covalently bound > can be reoxidised by coenzyme Q in ETC
enzyme is the only membrane bound TCA cycle enzyme
step 7 of TCA cycle
hydration of fumarate to malate by fumarase
step 8 of TCA cycle
regeneration of oxaloacetate from malate by malate dehydrogenase
1 NADH produced
products of one TCA cycle
3 NADH, 2 CO2, 1 FADH2, 1 ATP
net products from both glycolysis and TCA cycle
6 CO2, 10 NADH2, 2 FADH2, 4 ATP
major regulatory interactions in TCA cycle
oxidation of acetyl coA can only go as fast as electrons from NADH enter ETC which is controlled by ATP and ADP content of cells
Increase NADH concentration > isocitrate dehydrogenase, alpha ketoglutarate dehydrogenase and malate dehydrogenase inhibited
during muscular contraction, increased Ca2+ concentration actibates isocitrate DH, alpha keto glutarate DH and pyruvate DH
anaplerotic pathways to replenish TCA cycle intermediates
pyruvate carboxylase converts pyruvate to replenish oxaloacetate
glutamate reversibly converted to alpha ketoglutarate
some compounds enter TCA cycle at level of succinyl CoA
other amino acids also degraded to fumarate
