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Citric Acid Cycle
citric acid, TCA, krebs
26
Biochemistry
Undergraduate 2
11/26/2013

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Term
How does cytosolic NADH enter the mitochondria? Why is this necessary?
Definition
NADH doesn't enter the mitochondria. Two different shuttle systems bring the reducing equivalents from NADH across the inner mitochondrial membrane, which is impermeable to NADH.
Term
What are the two shuttle systems for transporting reducing equivalents?
Definition
Malate-Aspartate Shuttle & Glycerol-3-phosphate Shuttle
Term
How does the Malate-Aspartate shuttle function?
Definition
1. oxaloacetate is converted to malate by cytosolic malate dehydrogenase, NADH is oxidized to NAD+
2. Malate crosses the inner innermitochondrial membrane.
3. Malate is converted back to oxaloacetate by mitochondrial malate dehydrogenase, NAD+ reduced to NADH
4. Oxaloacetate is converted to aspartate which can cross the inner mitochondrial membrane and is converted back to oxaloacetate on the other side.
Term
What are reducing equivalents?
Definition
Electrons. May be accompanied by protons.
Possibilities: electron on its own, H atoms (e + H) or hydride ions (H-)
Term
How does the Glycerol-3-Phosphate Shuttle function?
Definition
1. Dihydroxyacetone is reduced to glycerol-3-phosphate, NADH2 is oxidized to NAD+.
2. Reducing equivalents are passed on to FAD which is reduced to FADH2, when glycerol-3-phosphate is oxidized to dihydroxyacetone phosphate.
3. FADH2 passes reducing equivalents directly to Coenzyme Q (Complex I is bypassed)
Term
True or False
Molecules freely cross the inner mitochondrial membrane with the glycerol-3-phosphate shuttle
Definition
False
The mitochondrial glycerol-3-phosphate dehydrogenase enzyme is located on the exterior surface of the inner mitochondrial membrane, so no molecules need cross the membrane.
Term
What is the pyruvate dehydrogenase complex?
Definition
The link between glycolysis and the TCA cycle
Pyruvate is oxidized into Acetyl-CoA
Term
Where is the pyruvate dehydrogenase complex located and what does it do?
Definition
In the mitochondrial matrix.
It catalyzes the irreversible oxidative decarboxylation of pyruvate
Term
What 5 coenzymes are required by the  pyruvate dehydrogenase complex?
Definition
-NAD+, FAD, CoA, thiamine pyrophosphate and lipate
Term
Describe the substrate-level phosphorylation that occurs in the TCA cycle
Definition
The high energy thioester bond in succinylCoA is hydrolyzed, this reaction is coupled to GTP/ATP synthesis:
-inorganic phosphate binds to succ.CoA & displaces CoA
-a histidine residue accepts the phosphate group, succinate is released
-phosphate group is transferred to a GDP/ADP molecule releasing a free enzyme
Term
What type if intermediate is formed in the hydrolysis of the thioester bond in succinyl CoA?
Definition
phosphoenzyme intermediate
Term
What is produced by the cataboolism of fatty acids, sugars and some amino acids?
Definition
Acetyl CoA
Term
What is the function of the TCA cycle?
Definition
-completely oxidizes acetyl CoA to CO2
-produces a small amount of ATP/GTP (1 per cycle)
-generates FADH2 & 3NADH2 (important for ATP generation in the ETC later!!)
Term
True or False
The condensation of acetyl CoA and oxaloacetate to form citrate involves the removal of water
Definition
False
This isn't actually a dehydration synthesis reaction, the molecules are simply joined together
Term
Oxidative decarboxylation of isocitrate converts an ______ to a ________
Definition
alcohol to a ketone
Term
What mechanism is identical to the pyruvate dehydrogenase complex? Why is it important?
Definition
alpha-ketoglutarate dehydrogenase
Forms a high-energy thioester bond which will be later involved in substrate level phosphorylation
Term
What are the products of the TCA cycle?
Definition
3 NADH & 3H+
1 FADH2
2 CO2
1 ATP or GTP
1 CoA
Term
What enzyme does this?

acetyl-CoA + oxaloacetate form citrate
Definition
citrate synthetase
Term
What enzyme does this?
malate is converted to oxaloacete
Definition
malate synthetase
Term
What enzyme does this?
Fumerate is converted to malate
Definition
Fumerase
Term
What does this enzyme do?

succinyl-CoA synthetase
Definition
converts succinyl-CoA to succinate
Term
What does this enzyme do?
succinate dehydrogenase
Definition
converts succinate to fumerate
Term
What enzyme converts citrate to aconitate? What is aconitate then converted to?
Definition
enzyme: aconitase
converted to: isocitrate
Term
What enzyme converts alpha-ketoglutarate to succinyl-CoA?
Definition
alpha-ketogultarate dehydrogenase
Term
What does this enzyme do?

isocitrate dehydrogenase
Definition
Converts isocitrate to alpha-ketoglutarate
Term
What is the most important role of the TCA cycle?
Definition
To reduce FAD & NAD+ to form FADH2 and 3 NADH, which will later be used to produce lots of ATP in the ETC
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