TCA Cycle

How many steps is the TCA cycle?

8 steps

How many carbons are added by acetyl coA?

2

2 carbons are removed during the cycle as _____

CO2

T/F: The carbons leaving as CO2 are DIFFERENT than the carbons entering in acetyl-CoA

true

T/F: oxaloacetate is regenerated in each cycle

true

What step is where acetyl-CoA (the acetate group has 2 carbons that are brought in) and it reacts with oxaloacetate (4 carbons) to make citrate (a 6 carbon compound).

first step! Citrate synthase is what does this

How does the TCA cycle form GTP?

The CoA is a high energy bond, so we can get some energy out in the form of GTP/ATP and then regenerate oxaloacetate.

What are the products of the TCA cycle?

4 pairs of electrons out (3 NADH + QH2), GTP, 2 H+, HS-CoA, and 2 CO2

T/F: the first step of the TCA cycle is the condensation step?

TRUE: we start with oxaloacetate (4 carbon compound) and it condenses with acetyl-CoA (the acetate group is the methyl, the carbonyl, and the sulfur) —> after they come together, they form citrate (6 carbons). Citrate is symmetrical (3 carboxyl groups, OH in the middle with CH2 on carbon one and 3). Citrate synthase is what is in charge of this step!

• You want to decarboxylate citrate and get some electrons out, but you cannot oxidize this because you cannot oxidize to a double bond right now. You need to move that middle OH to the 3rd carbon, this is what aconitase does (enzyme 2)

What is the first enzyme

citrate synthase

What does aconitase do?

The isomerization step (the molecular formula is the same, the arrangement is slightly different) —> You want to decarboxylate citrate and get some electrons out, but you cannot oxidize this because you cannot oxidize to a double bond right now. You need to move that middle OH to the 3rd carbon, this is what aconitase does (enzyme 2).

• The hydroxyl group is now on the third carbon —> this new form is called isocitrate (it is like citrate, but not symmetric).

  • With the OH at the third carbon, now you can oxidize it to form a double bond when you do a hydride transfer

What does isocitrate dehydrogenase do?

• dehydrogenases are oxidation reduction reactions → they remove electrons. We are going to decarboxylate isocitrate and loose a CO2.

• Bring an NAD+ and get out an NADH (reduced)

• The carboxyl group is lot

• form a carbonyl on the third carbon

• oxidative decarboxylation

• Formed alpha-ketoglutarate

What does alpha-ketoglutarate dehydrogenase do (brief summary)?

• Decarboxylation: TPP attacks the carbonyl at carbon 2 of alpha-ketoglutarate, causing carbon 1 to leave as CO2.

• Oxidation and Transfer: The remaining succinyl group is transferred to lipoic acid, which reduces lipoic acid's disulfide bond.

• Succinyl-CoA Formation: CoA attacks the succinyl group attached to lipoic acid, taking its place to form succinyl-CoA and leaving lipoic acid fully reduced.

• Cofactor Regeneration: FAD oxidizes lipoic acid back to its disulfide form (becoming FADH2), and then transfers those electrons to NAD+ to produce NADH.

• CO2 comes off!

• This is an oxidative decarboxylation step

Down to four carbons (succinyl-CoA)

What happens to allow us to form ATP/GTP?

• CoA is a high energy bond

• Get energy out of the thioester bond.

• GDP, Pi ———→ GTP

• The phopshate attacks the high energy bond between the carbonyl and the sulfur. The