TCA cycle:

T/F: it is a difficult step to convert pyruvate to acetyl-CoA

True! This is complicated because pyruvate is a alpha keto-acid, but CO2 is also produced as a by-product. Acetyl-CoA is fed into the citric acid cycle.

What gets fed into the TCA cycle?

acetyl-CoA

How many steps are in the TCA cycle?

8

How many carbons are being fed into the TCA cycle?

2 from acetyl-CoA, and these are being removed as CO2 (2 carbons are leaving as CO2). Oxaloacetate is regenerated in this process!

In the process of the TCA cycle, what do we mostly get out?

electrons and NADH’s. A little ATP and FADH2 is also formed. Because we breath oxygen, we can get energy from those electrons —> eventually oxidative phosphorylation occurs to allow us to convert those electrons to ATP.

• This occurs because we eventually transfer electrons to oxygen, the terminal electron acceptor.

If you are anaerobic, you stop at pyruvate and do fermentation and do those steps to get NAD+ back (to do another round of glycolysis).

• You are not able to get the energy out of those NADH

The TCA cycle derives energy by ________ pyruvate and releasing CO2

oxidizing

What is step 0 of pyruvate?

This is not part of the TCA Cycle:

Pyruvate + CoA-HS + NAD+ ⇌ acetyl-CoA + CO2 + NADH

• This involves pyruvate dehydrogenase that takes pyruvate and gets electrons out and transfers the rest of the acetyl-CoA-HS part to form acetyl-CoA

• The acetyl-group of acetyl-coA contains two carbons

What is interesting about steps 3 and 4 of the TCA cycle?

Oxidative decarboxylation: you remove electrons by oxidizing the molecule and you reduce the NAD+ to NADH, and release CO2 as a byproduct.

T/F: in the TCA cycle, you feed two carbons in and remove two carbons out?

TRUE! This is because acetyl-CoA is a two carbon molecule and there are two decarboxylation steps where two CO2’s are released (this is the removal of the two carbons).

What occurs in step 3 of TCA

isocitrate is oxidized to alpha-ketoglutarate

Isocitrate + NAD+ ⇌ alpha-ketoglutarate + CO2 + NADH + H+

What occurs in step 4 of the TCA cycle?

Alpha-ketoglutarate is oxidized to succinyl-CoA

Alpha-ketoglutarate + CoA-SH + NAD+ ⇌ succinyl-CoA + CO2 + NADH

In aerobic organisms, pyruvate is transported into the _________ to be further oxidized

mitochondria

• pyruvate diffuses across the outer mitochondrial membrane

• pyruvate is transported across the inner mitochondrial membrane via a transporter. This is how pyruvate can be used to create acetyl-CoA that is needed for the TCA cycle (which is done inside the mitochondiral matrix).

  • The inner mitochondiral membrane is very tight and hostile to polar molecules, purivate needs an active transport system: mitochondiral pyruvate carrier to transport pyruvate along with a proton into the matrix.

    • Once inside the matrix, the pyruvate dehydrogenase complex converts pyruvate into acetyl-coA, which then enters into the TCA cycle

Pyruvate oxidation and the citric acid cycle mostly take place within what?

The mitochondiral matrix

• succinate dehydrogenase is the only enzyme of the citric acid cycle that is embedded in the inner membrane

______________ is the only enzyme of the citric acid cycle that is embedded in the inner mitochondrial matrix

succinate dehydrogenase

What forms acetyl-CoA

pyruvate dehydrogenase does this and it transfers the acetyl-group from the reduced lipoamide to Coenzyme A. This is what creates acetyl-CoA

T/F: in the mitochondria, there are membrane proteins that are embedded in the membrane that will synthesize ATP

true

What is the function of the TCA cycle?

• to produce energy

• to produce key anabolic intermediates

What is the very first thing you need to do if you want to do TCA?

First need to convert pyruvate into Acetyl-CoA

- Accomplished by the Pyruvate dehydrogenase complex

- Acetyl-CoA is considered the entry point of the TCA cycle

What is considered the entry point of the TCA cycle?

Pyruvate dehydrogenase complex

What is the overall TCA cycle reaction:

Acetyl-CoA + 3 NAD+ + Q + GDP (or ADP) + Pi + 2 H2O → HS-CoA + 3 NADH + QH2 + GTP (or ATP) + 2 CO2 + 2 H+

The TCA cycle generally creates mostly reducing equivalents to be converted later into ATP during __________

oxidative phosphorylation

Does the TCA cycle mostly generated reducing equivalents or oxidative equivalents

reducing

Oxidation of pyruvate by the pyruvate dehydrogenase complex

• Dehydrogenase: H-, hydride transfer → used to reduce an electron carrier

Requires breaking a C—C bond.

- This is facilitated by forming a carbanion intermediate which is

stabilized by resonance of the negative charge on the C

- Pyruvate is an alpha-keto acid, which cannot stabilize the carbanion

transition state by itself – uses a cofactor

− Beta-keto acids can stabilize the carbanion transition state

What can stabilize a carbanion transition state by itself?

Beta-keto acids can stabilize the carbanion transition state.

• Pyruvate is an alpha keto-acid, which cannot stabilize the carbanion transition state by itself: uses a cofactor