CAC Flashcards
Citrate synthase
Condensation between oxaloacetate and acetyl CoA
NO COFACTOR
Low barrier H bonds stabilize transition state
Energy fueled by thioester bond
“Hugs” substrate
4C – >6C
What reuses CoA?
Succinyl CoA synthetase
PDH
Aconitase
enzyme asymmetrical, only one carboxymethyl arm in reaction
Isomerization
Isocitrate Dehydrogenase
Oxidative decarboxylation of isocitrate to alpha-ketoglutarate
Requires NAD+
Caroxylate group beta ketone –> CO2
Stabilized by Mn2+
6C – >C5
Alpha-ketogluarate dehydrogenase
Requires CoEnzyme A
NADH + CO2
Transfer 4C fragment to COA
Free energy in thioester bond
resembles PDH - E3 in both
Releases different carbons than those that entered
Succinyl-CoA synthetase
GTP
Active site HIs transfers phophoryl group to substrate
Requires movement of protein loop with phospho-his side chain to distant ADP 35A away.
Coupling of exergonic to phosphoryl group transfer to NDP = direct phosphorylation
(Thioester offsets attaching phosphate to GDP)
Succinate Dehydrogenase
Located in mitochondrial membrane
Q accepts e- from FADH2
Fumarase
Water added
hydration/reduction
Reverse hydration of double bond
Malate dehydrogenase
NADH
Citrate synthase pulls it forward, positive delta G
Not spontaneous
Mechanism of Citrate synthase
BASE CATALYSIS
1) Oxaloacetate/acetyl-CoA bind to enzyme (Oxaloacetate binds first)
2) Asp 375 removes proton to make enolate, stabilized by His 274. This is the slowest rate limiting step
3) Enolate attacks oxaloacetate to produce a citryl-CoA intermediate
4) Hydrolysis releases CoA and Citrate
Mechanism of Succinyl-CoA synthase
1) Phosphate group displaces CoA in succinyl-CoA. The product is acyl phosphate while releases free energy when hydrolyzed.
2) Succinyl phosphate donates phosphoryl group to His residue, phospho-His intermediate, releases succinate.
3) Phospho group then transferred to GDP from GTP
