Oxidation of Pyruvate and TCA

Question 1

where does oxidation of pyruvate occur?

Answer 1

mitochondrial matrix

Question 2

linking reaction

Answer 2

the reaction that links glycolysis to the Krebs cycle. It is the oxidation of pyruvate.

Overall reaction:
Pyruvate + NAD+ + coA –> acetylcoA + CO2 + NADH + H+

Question 3

what enzyme completes the linking reaction?

Answer 3

pyruvate dehydrogenase (PDG)

Question 4

pyruvate dehydrogenase function

Answer 4

found in the mitochondrial matrix. oxidatively decarboxylates pyruvate to form acetyl CoA.

Question 5

pyruvate dehydrogenase structure

Answer 5

made of 3 different enzymes put together in a big complex. Each enzyme has its own prosthetic group. There’s a long lipoamide that can move from one active site to another within the complex.

Question 6

regulation of pyruvate dehydrogenase

Answer 6

PDG is regulated by covalent modification and by allosteric effectors. PDG is a committed step - once you make acetyl CoA, you’ve committed to entering the TCA cycle. PDG is inactivated by addition of a phosphate, and it is regulated by energy charge.

Question 7

what happens if ATP is high?

Answer 7

We don’t want to convert pyruvate to acetyl CoA b/c we have plenty of ATP. So we’ll run gluconeogenesis and convert pyruvate to glucose to store it in the liver as glycogen. High ATP –> ATP adds phosphate to pyruvate dehydrogenase to inactivate it

Question 8

what are the steps in pyruvate dehydrogenase?

Answer 8

1. Decarboxylation of pyruvate to remove CO2
2. Oxidation to form an energy-rich bond (thioester with lipoid acid chain)
3. Transfer onto Coenzyme A (also a thioester). Co A carries acetyl units with a high energy bond (high transfer potential).

Question 9

citric acid cycle/TCA/Krebs purpose

Answer 9

to harvest high energy electrons as acetyl CoA is oxidized to CO2. In the process of oxidation, we transfer electrons to NAD+ and FAD to form NADH and FADH2, which will carry electrons to ETS and maintain proton gradient that fuels chemiosmotic synthesis of ATP.

Question 10

Substrates for TCA

Answer 10

acetyl CoA. All amino acids can be degraded by entering TCA at different points.

Question 11

1st step of TCA

Answer 11

Acetyl CoA fuses with oxaloacetate to form a 6-carbon molecule, citrate.

Question 12

citrate

Answer 12

the 6-carbon molecule that forms after 1st step of TCA. It has three carboxylic acid groups on it.

Question 13

alpha-ketoglutarate

Answer 13

one of the intermediates in the TCA cycle. glutamate can be converted to alpha-ketoglutarate, an example of how the amino acids can all be converted into an intermediate of TCA cycle.

Question 14

what is the waste product of TCA cycle?

Answer 14

CO2

Question 15

what is the energy that powers TCA cycle?

Answer 15

decarboxylation events primarily, as well as the high-energy thiol bond in CoA

Question 16

where does energy come from for the substrate level phosphorylation step in TCA cycle?

Answer 16

The thiol bond in CoA breaks as CoA is released from Succinyl coA to become succinate.

Question 17

net reaction of TCA cycle

Answer 17

acetyl coA + ADP + Pi + 3 NAD+ + FAD –> 2 CO2 + 3 NADH + FADH2 + ATP + 2 H+ + CoA

Question 18

what are the regulated steps of the TCA cycle?

Answer 18

The two decarboxylation steps: isocitrate dehydrogenase and alpha-ketoglutarate dehydrogenase (the formation of alpha keto glutarate from isocitrate and the formation of succinyl coA from alpha-keto glutarate). Also pyruvate dehydrogenase is controlled (pyruvate –> acetyl coA is a committed step and highly regulated).

Question 19

Fates of acetyl CoA

Answer 19

1. Enters the Krebs cycle to eventually produce ATP
2. Used as a carbon skeleton in the synthesis of amino acids and nitrogenous bases, as well as used in synthesis of lipids.

Question 20

anaplerosis

Answer 20

word for “replenishing.”