Citric Acid Cycle Flashcards
describe some general features of the citric acid cycle
- also called the Tricarboxylic acid cycle or the Krebs cycle
- occurs in the mitochondria
- its a cycle: starts and finishes with the same molecule
- gets acetyl-CoA from glycolysis (aerobic, PDH reaction) and beta-oxidation in the mitochondria
Carbon enters and leaves:
- 2C enters as acetyl-CoA
- 2C out as 2x CO2 (oxidation complete)
Captured energy:
- 1 ATP
- reduced coenzymes: 3NADH and 1 FADH2
Arranged in to two parts:
1. release of C
2. regeneration of the starting molecule
- energy captured in both parts
desciribe the key reaction: condensation of acetyl-CoA with oxaloacetate
- 2C entering in acetyl-CoA is attached to 4C oxaloacetate to produce 6C citrate
- energy for the reaction comes from the hydrolysis of CoA from acetyl-CoA
- as the CAC is a ‘cycle’ these two carbons need to be removed
reaction:
oxaloacetate + acetyl-CoA + H2O –> citrate + CoA-SH
- done by citrate synthase
- not spending any energy to do this reaction because we get energy from breaking the CoA off
- energetically favourable
describe the isomerisation of citrate
- rearrangement of citrate to isocitrate make the molecule susceptible to decarboxylation
- both steps catalysed by aconitase
- rearrangement to get it in the right form
reaction:
citrate - (dehydration) -> cis-aconitate -(hydration)-> isocitrate
describe how you can target the CAC to kill
fluoroacetate is metabolised to fluorocitrate in the isomeration reaction (which is very similar in structure to citrate)
- fluorocitrate is converted into a substrate that binds tightly to aconite and inactivates the enzyme
- this permanent deactivation, if extensive to most/all of the aconitase enzymes, results in the whole cycle not working and a buildup of acetyl-CoA, decreased reduced coenzymes (starts inhibiting the reaction pathways that make acetyl-CoA), less ATP
describe the key reaction: removal of the first carbon
- an oxidative decarboxylation
- reaction occurs in two steps: oxidation then decarboxylation
- energy captured in NADH
reaction:
1. isocitrate oxidised (NAD+ reduced) to oxalosuccinate (enzyme bound)
2. oxalosuccinate then goes to alpha-ketoglutarate and carbondioxide is given off
- reaction done by isocitrate dehydrogensase
describe the key reaction: removal of the second carbon
- a second oxidative decarboxylation
- very similar to the pyruvate dehydrogenase reaction: complex
- energy captured in NADH
reaction:
alphs-ketoglutarate goes to succinyl-CoA
- reaction done by alpha-ketoglutarate dehydrogenase
- NAD+ reduced to NADH, H+
- CoA-SH goes to CO2
- same as the pyruvate dehydrogenase reaction
describe the key reaction: succinyl-CoA to succinate
- the removal of the CoA releases enough energy to drive the synthesis of GTP
- we use the energy from breaking the bond with CoA to phosphorylate GDP to GTP
- GTP is the energy equivalent of ATP: GTP + ADP <=> GDP = ATP
- ATP will be being used and CAC is making GTP and since they are in equalibrium this means that more ATP will be made from GTP as the equilibrium shifts in the forwards direction
- the third substrate level phosphorylation (in pathways)
define substrate level phosphorylations
the DIRECT use of ENERGY from a substrate molecule to drive the synthesis of ATP (or equivalent)
- when the energy to add the substrate on comes from the substrate (P does not have to come from substrate)
how does succinate go to oxaloacetate?
the same way as steps 1,2,3 in beta oxidation
1. carbon single bond molecule (succinate) is oxidised as FAD is reduced, forming a C double bond
2. C double bond molecule (fumarate) undergoes a hydration reaction where a the C double bond is broken and a hydroxyl group is added to a carbon
3. C single bond molecule with a hydroxyl group (malate) is then oxidised while NAD is reduced, to form a C single bond molecule with a double bonded O on one of them (oxaloacetate)
what is the overall reaction for the CAC?
acetyl-CoA + 3NAD+ + FAD + 2H2O + GDP + Pi –> 2CO2 + CoASH + 3NADH + 3H+ + FADH2 + GTP
CAC overall: biological delta G of -44.3kJ/mol
pathway is energetically favourable
heat is also released
describe the reaction in the CAC with the one enzyme that is on the inner mitochondria membrane
- SDH (succinate dehydrogenase) is located in the inner mitochondrial membrane
- SDH uses FAD as a coenzyme: flavin coenzymes are tightly bound to the proteins with which they interact (flavoproteins)
- SDH is part of CAC where FAD is reduced, o needs to also be in the electron transport chain (ETC) to oxidise FADH2 back to FAD
succinate + E-FAD –> fumarate + E-FADH2
(E = enzyme, SDH)
