Citric Acid Cycle Flashcards
1
Q
Citric Acid Cycle
A
- Cyclic pathway in aerobic eukaryotic cells
- Located in the mitochondrial matrix
- Central role: links other biochemical pathways to the mitochondrial electron transport system (ETS)
2
Q
Amphibolic pathway
A
- has catabolic functions (break down of metabolites to generate ATP, NADH and FADH2)
- and anabolic functions (supplies precursors for other pathways)
3
Q
Other complex roles of CAC
A
- macrophages and dendritic cells respond to proinflammatory stimuli they undergo metabolic shifts, which include a ‘rewiring’ of the citric acid cycle
4
Q
Inside the mitochondrion
A
- Outer membrane (permeable to many metabolites)
- Intermembrane space
- Inner membrane (impermeable to many metabolites) – with numerous folds (cristae)
- Matrix (inner compartment)
5
Q
Citric Acid Cycle and Electron Transport System
A
- Acetyl-CoA from other pathways enters the first reaction of the CAC: From (PDH) reaction, Fatty Acid beta-oxidation, Amino Acids reactions
- NADH and FADH2 from the CAC and from other pathways transfer electrons to complexes in the ETS and “fuel” ATP synthesis
6
Q
Strategy to citric acid cycle
A
- 2 carbon atoms enter the cycle as the acetyl group of acetyl-CoA
- transferred to a 4C organic acid, oxaloacetate to yield a 6C tricarboxylic acid, citric acid (citrate ion in solution)
- citrate enters a series of reactions during which two carbons are released as CO2
- then remaining four carbons are regenerated as oxaloacetate through 4 reactions, for another run of the cycle
- overall, irreversible pathway
7
Q
Enzymes involved in citric acid cycle
A
- citrate synthase
- aconitase
- isocitrate dehydrogenase
- a-ketoglutarate dehydrogenase complex
- succinyl CoA synthase
- succinyl dehydrogenase
- fumarase
- malate dehydrogenase
8
Q
1st reaction of CAC
A
- citrate synthase catalyses condensation of oxaloacetate and acetyl CoA
- forms citrate
9
Q
2nd reaction of CAC
A
- aconitase catalyses isomerisation of citrate to isocitrate
- citrate is dehydrated to form intermediate cis-aconitate
- then rehydrated to form isocitrate
10
Q
3rd reaction of CAC
A
- isocitrate oxidised and decarboxylated to a-ketoglutarate
- catalysed by isocitrate dehydrogenase
11
Q
4th reaction of CAC
A
- a-ketoglutarate undergoes oxidative decarboxylation to form succinyl-CoA
- catalysed by a-ketoglutarate dehydrogenase complex
12
Q
5th reaction of CAC
A
- succinyl CoA hydrolysed to form high energy phosphate (i.e GTP)
- catalysed by succinyl CoA synthase
- intermediate formation of succinyl phosphate
13
Q
GTP
A
- May be used to form ATP in a reaction catalysed by nucleoside diphosphokinase
14
Q
6th reaction of CAC
A
- oxidation of succinate to fumarate
- catalysed by succinate dehydrogenase
15
Q
7th reaction of CAC
A
- addition of water to fumarate to form malate
- catalysed by fumarase
16
Q
8th reaction of CAC
A
- oxidation of malate to regenerate oxaloacetate
- catalysed by malate dehydrogenase
17
Q
Irreversible reactions in CAC
A
- low product and slow reverse reaction
- drive cycle in forward direction
18
Q
Reversible reactions in CAC
A
- favour production of malate and citrate which exit cycle and allow citrate to act as inhibitor
19
Q
CAC as a source of biosynthetic precursors
A
- citrate: fatty acids, sterols
- a-ketoglutarate: glutamate, other amino acids, purines
- succinyl CoA: porphyrins, heme, chlorophyll
- oxaloacetate: aspartate, phosphoenol-pyruvate, glucose
- aspartate: other amino acids, purines, pyrimidines
20
Q
Anaplerotic reactions
A
- replenishing pathways/reactions
- removal of intermediates from the pathway implies the need to replenish
21
Q
Fluoroacetate as a CAC inhibitor
A
- isolated from plants
- ‘suicide’ substrate as parent compound is non-toxic, but metabolism produces toxic product
- binds to aconitase where fluorine binds to iron ion in active site, causing inhibition and death
22
Q
Malonate as a CAC inhibitor
A
- 3 carbon homologue of succinate
- similar in structure to succinate, allows it to bind to succinate dehydrogenase
- competitive inhibitor
- causes block - succinate, citrate and a-ketoglutarate build up
23
Q
Reduced cofactors generated in CAC
A
- NADH
- FADH2
24
Q
Two phases in CAC
A
- intro and oxidation if 2 carbons to carbon dioxide
- regeneration of oxaloacetate
25
Q
Redox coenzymes
A
- 3 dehydrogenation reactions used NAD+ as redox coenzyme
- step 6 uses FAD as redox coenzyme
- FAD is more powerful oxidant than NAD+ and bound covalently to enzyme complex
26
Q
Overall equation for CAC
A
- acetyl CoA + 2H2O + 3NAD+ + FAD + GDP + Pi —> 2CO2 + FADH2 + CoA-SH + GTP
- then GTP + ADP —> ATP + GDP
27
Q
ATP yield - cell respiration
A
- most of ATP generated through oxidative phosphorylation
- NADH and FADH2 electrons will be transferred toward molecular O2
- coenzymes will be oxidised again
- proton gradient generated across inner mitochondrial membrane
- ATP synthesis will take place through chemiosmotic coupling
