Citric Acid Cycle Flashcards
Give the other two names for the citric acid cycle.
Krebs cycle
Tricarboxylic acid cycle
Which compound is the most reduced and which is the most oxidised?
R = Citrate O = Oxaloacetate
Octet rule
atoms share e- to get 8 e- in there outer shell
What reaction does citrate synthase catalyse?
- oxaloacetate + acetyl –> Citryl CoA
- Citryl CoA +nH2O –> Citrate + CoA
~ hydrolysis of energy-rich thioester bond drives the reaction forward
Describe the structure of citrate synthase.
~ two identical subunits
~ binding of oxaloacetate (open form) = causes major structural changes (closed form) = binding site for acetyl CoA
–> prevents wasteful enzyme activity
e.g. hydrolysis of acetyl CoA
What reaction does aconitase catalyse?
- Citrate –> cis-Aconitate + H2O (dehydration)
2. cis-Aconitate + H2O –> Isocitrate (rehydration)
What reaction does isocitrate dehydrogenase catalyse?
- Isocitrate + NAD+ –> Oxalosuccinate + NADH + H+ (oxidation)
- Oxalosuccinate + H+ –> alpha-ketoglutarate + CO2 (decarboxylation)
What reaction does alpha-ketoglutarate dehydrogenase catalyse?
alpha-ketoglutarate + NAD+ + CoA –> Succinyl CoA + CO2 + NADH
(oxidative decarboxylation)
VERY SIMILAR TO PYRUVATE DEHYDROGENASE
Which 5 cofactors affect a-ketoglutarate?
~ NAD ~ Coenzyme A ~ FAD ~ Lipoamide ~ Thiamine pyrophosphate
What is the structure of a-ketoglutarate like?
3 different enzyme proteins:
E1 - alpha-ketoglutarate dehydrogenase (decarboxylation)
E2 - transsuccinylase (makes succinylcholine CoA)
E3 - dihydrolipoyl dehydrogenase (identical to PDH & reduces FAD/NAD)
What reaction does succinylcholine CoA synthetase catalyse?
Succinyl CoA + Pi + GTP –> Succinate + GTP + CoA (energy transfer)
What reaction does succinate dehydrogenase catalyse?
Succinate + FAD –> Fumarate + FADH2 (oxidation)
What makes the succinate dehydrogenase different?
~ it is the only membrane-bound enzyme in the krebs cycle
~ contains 3 types of Fe-S clusters (2Fe-S; 3Fe-S & 4Fe-S)
What reaction does fumarase catalyse?
Fumarate + H2O –> Malate (Hydration)
~ can occur via two intermediates either cation or anion depending on whether OH- or H+ gets added first
What reaction does malate dehydrogenase catalyse?
Malate + NAD+ –> oxaloacetate + NADH + H+ (oxidation)
The equilibrium is towards malate not oxaloacetate, so how does the towards oxaloacetate occur?
~ oxaloacetat is unstable & kept in low conc.s
~ NADH is rapidly oxidised by the e- transport chain
Which molecules globally regulate metabolism demand?
ADP/ATP
NAD/NADH
What is the energy yield of the krebs cycle?
2 GTP = 2 ATP
6 NADH = 18 ATP
2 FADH2 = 4 ATP
(everything doubled as 2 pyruvate produced from 1 glucose)
What is the purpose of the krebs cycle?
~ aid energy release & electron transference/create ATP
~ provide intermediates for biosynthesis