Pyruvate is the primary product of glycolysis. It is more oxidized than glucose, but needs to be oxidized all the way to CO₂ to extract the maximum amount of free energy. As you study more biochemistry, you will find slight differences with the citric acid cycle in animals, microbes, and plants. For now, let's just concentrate on the nuts and bolts of the cycle.

Pyruvate is converted to Acetyl-CoenzymeA by pyruvate dehydrogenase complex. This enzyme is a multi-multi-subunit complex with three separate activities. First focus on the overall reaction.

Pyruvate + CoA + NAD⁺ –> Acetyl-CoA + NADH + CO₂

The thioester bond in acetyl-CoA has a high amount of energy (ΔG^o' hydrolysis = -30 kJ/mol)

Cofactors include thiamine pyrophosphate (PP_i,from vit. B₁), lipoamide (from lipoic acid), NAD (from niacin), FAD, and Coenzyme A is produced from pantothenic acid (eat your spinach!)

The abbreviated details: [local]

1. Pyruvate reacts with thiamine-PP, forming hydroxyethyl-TPP and CO₂

2. Hydroxyethyl-TPP reacts with oxidized lipoic acid, forming the oxidized acetyl dihydrolipoamide

3. The acetyl group is transferred to CoA forming acetyl-CoA and reduced lipoic acid (dihydrolipoamide)

4. Reduced lipoic acid is oxidized and the electrons are transfered to FAD (forming FADH₂) and then to NAD⁺, forming NADH

The net result: One carbon down and two to go.

Pyruvate Dehydrogenase and the TCA Cycle [local]

• TCA Cycle [local]
• Chemical Reactions of the Krebs Cycle [local]
• The TCA Cycle
• The TCA Cycle [local]
• The Citric Acid Cycle [local]
• The Citric Acid Cycle [local]
• Step by Step Krebs cycle
• Animated Krebs cycle [local]

Overall score: 3 NADH + QH₂ + GTP + 2CO₂

POINTS TO PONDER

1. The NADH products are free to enter the electron transport chain for oxidative phosphorylation via complex I.

2. Succinate dehydrogenase is complex II of oxidative phosphorylation, so those electrons are already in oxidative phosphorylation.

3. The entire oxidation of pyruvate is done without oxygen. Oxygen acts at the terminal electron acceptor in the following process of oxidative phosphorylation.

4. The cycle acts as a catalyst; the intermediates are not consumed in the reactions. Carbon in the form of acetyl-CoA is put into the cycle and electrons, protons and CO₂ come out.

5. The cycle is the traffic circle at the center of the cell's metabolism. Almost all catabolism directs products toward the citric acid cycle. Almost all macromolecules are formed from building blocks produced from intermediates in the citric acid cycle.

6. Because intermediates can be removed to make amino acids, sugars, lipids, etc. there must be a way of making more of the intermediates to keep the cycle going. These are called anaplerotic reactions. Example: Pyruvate + CO₂ + ATP + H₂O --> OAA + ADP + Pi (pyruvate carboxylase)

7. The cycle is tightly regulated in a way that makes metabolic sense. The whole of metabolism must be coordinated so that futile cycles are not established. The whole cell needs to agree if it is doing anabolism or catabolism.

If times are rich:

• High [NADH] inhibits pyruvate DH, citrate synthase, isocitrate DH and alpha-KGDH
• High citrate inhibits citrate synthase
• High acetyl-CoA inhibits pyruvate DH

If times are poor:

• High ADP stimulates isocitrate DH
• High CoA stimulates pyruvate DH

The Glyoxylate Cycle: a variation on the Tricarboxylic acid cycle. This pathway is used in plants and microbes in situations where 2-carbon units are plentiful and there is a need to utilize them for both catabolism and anabolism. The TCA cycle is able to convert acetate to CO₂ and extract the energy, but does not facilitate anabolic reactions. The glyoxylate cycle uses two new enzymes to circumvent the reactions producing CO₂ (isocitrate DH and alpha-ketoglutarate DH). Isocitrate lyase cleaves isocitrate into succinate and glyoxylate. Malate synthase then adds Acetyl-CoA to glyoxylate to produce malate. Thus, two CO₂ are used to produce a 4 carbon organic acid. Once the four carbons are in the TCA cycle, they can be used to make other metabolites (e.g. sugar from OAA) by standard anabolic pathways.