Oxaloacetate reacts with acetyl-CoA to produce citrate

Condensation reaction

Enolization of Acteyl-CoA allows it to condense with oxaloacetate.

Inside citrate synthase, the thioester bond of acetyl-CoA is brought to and Asp residue, catalyzing it's hydrolysis.

**Hydrolysis of the thioester bond makes the overall reaction favorable**

Citrate synthase participates in a far from equilibrium reaction

Citrate (3^o alcohol) is rearranged to isocitrate (2^o alcohol) by aconitase

Citrate is dehydrated and bound to aconitase, forming cis-aconitase

Cis-aconitase is then hydrated to isocitrate

Fe₄S₄ coordinates the substrate and water correctly so there isn't a premature C loss in the forst step of the cycle.

Isocitrate undergoes decarboxylation to α-ketoglutarate using isocitrate dehydrogenase.

CO₂ and NADH are also produced

**Decarboxylation makes the overall reaction favorable**

Isocitrate + NAD⁺ --> Oxalosuccinate + NADH

Oxalosuccinate --> α-ketoglutarate + CO₂

Isocitrate dehygrogenase is NAD⁺ dependent and participates in a far from equilibrium reaction

α-ketoglutarate undergoes decarboxylation to form succinyl-CoA using α-ketoglutarate dehydrogenase

CO₂ and NADH are also produced

α-ketoglutarate dehydrogenase undergoes a far from equilibrium reaction

Succinyl-CoA to Succinate via Succinyl-CoA synthetase GTP is also produced.

The CoA bond in succinyl-CoA is very high energy. This energy is conserved throughout the reaction, even in the intermediates. It is stored in:

1. Succinyl-phosphate

2. 3-phospho-His

3. GTP

This is an example of substrate level phosphorylation

Succinate dehydrates to form fumarate via succinate dehygrogenase.

FADH₂ is also produced.

Enzyme is inhibited by Malonate

1. Substrate availability

2. Product inhibition

3. Competitive feedback

The control is spread out through many enzymes, instead of relying on just one

Major regulators include the substrates, acetyl-CoA, oxaloacetate, and NADH

When respiration and muscle workload increase, so does production in TCA cycle

Reactions that utilize intermediates from TCA. This helps drive TCA to create more products, but it also prevents the build-up of intermediates in the mitochondria.

ex: Gluconeogenesis uses oxaloacetate. It is converted to malate or aspartate before moving into the cytosol

ex:Fatty acid biosynthesis uses Acetyl-CoA. It comes from TCA as citrate and is brokendown using ATP-citrate lyase

ex: AA biosynthesis uses α-ketoglutarate and oxaloacetate. This is in the cytosol, so both intermediates are converted to other AA's first

Reactions that replenish intermediate in TCA cycle

Pyruvate decarboxylase senses when there is a lack of intermediates becaue a lack of intermediates leads to a lack of oxaloactetate and an increase in Acetyl-CoA. The increase in acetyl-CoA activate the reaction of pyruvate dehydrogenase, causing the production of oxaloacetate. Oxaloacetate can then react with the excess acetyl-CoA to continue TCA

Groups of noncovalently associated enzymes that catalyze two or more sequential steps in a metabolic pathway. Increase effiency of reactions.

Advantages:

1. Reaction rates are increased because there is less space the substrates must travel before reaching their next active site to bind with.

2. "wasting" of metabolites (intermediates) is diminshed because there is less chance of side reactions occuring

3. Multienzyme complexes are coordinately controlled