Biochemistry Chapter: TCA Cycle and Link Reaction

Pyruvate dehydrogenase is an enzyme that catalyzes the conversion of pyruvate to acetyl-CoA in the link reaction.

The link reaction, also known as the pyruvate dehydrogenase complex reaction, takes place in the mitochondrial matrix, the innermost compartment of mitochondria.

One molecule of pyruvate generates one molecule of NADH during the link reaction. This NADH will later be used in oxidative phosphorylation to generate ATP.

During the conversion of pyruvate to acetyl-CoA, one carbon atom is removed from pyruvate as carbon dioxide (CO₂). This is a significant part of cellular respiration, where carbon is oxidized.

The TCA cycle begins with the reaction between acetyl-CoA and oxaloacetate, forming citrate. This is the starting point of the cycle.

Citrate synthase catalyzes the first reaction of the TCA cycle, combining acetyl-CoA and oxaloacetate to form citrate.

For each acetyl-CoA molecule entering the TCA cycle, three NADH molecules are produced. These NADH molecules are crucial for ATP production in oxidative phosphorylation.

Citrate, the first intermediate in the TCA cycle, is a direct precursor for the synthesis of fatty acids. This connection highlights the interplay between different metabolic pathways.

Succinate dehydrogenase is the only enzyme in the TCA cycle that is embedded in the inner mitochondrial membrane. This allows it to participate in the electron transport chain.

One molecule of GTP (or ATP) is generated per turn of the TCA cycle. This ATP is produced by substrate-level phosphorylation.

Isocitrate dehydrogenase is an important regulatory enzyme in the TCA cycle. It is inhibited by high levels of ATP and NADH, signifying sufficient energy and reducing power.

Complex IV, also known as cytochrome c oxidase, is the final complex of the electron transport chain. It pumps protons across the membrane and produces water.

Oxygen acts as the final electron acceptor in oxidative phosphorylation. This is crucial for the continuous flow of electrons and ATP production.

The proton gradient, a difference in proton concentration across the inner mitochondrial membrane, directly drives ATP synthesis by ATP synthase.

Complex II, also known as succinate dehydrogenase, is the only complex of the electron transport chain that does not pump protons across the membrane. This is because Complex II is also part of the TCA cycle.

One molecule of NADH generates 2.5 molecules of ATP in oxidative phosphorylation. This is not a direct 1:1 conversion, as proton movement is involved.

The liver is the primary site of gluconeogenesis in the body. It is responsible for maintaining blood glucose levels during fasting or starvation.

Pyruvate is first converted to oxaloacetate before it can enter the gluconeogenic pathway. This is a key step in reversing glycolysis.

Phosphoenolpyruvate carboxykinase (PEPCK) catalyzes the conversion of oxaloacetate to phosphoenolpyruvate in gluconeogenesis. This is an important step in reversing glycolysis.

Glucagon is a hormone that promotes gluconeogenesis. It is released when blood glucose levels are low, stimulating the production of glucose from non-carbohydrate sources.

Gluconeogenesis consumes ATP and NADH to synthesize glucose. This process requires energy to reverse glycolysis.

The link reaction connects glycolysis to the TCA cycle by converting pyruvate, the end product of glycolysis, into acetyl-CoA, which can enter the TCA cycle.

The TCA cycle is a central metabolic pathway that generates energy in the form of ATP, NADH, and FADH2. It also provides intermediates for other metabolic pathways.

Oxidative phosphorylation is the final stage of cellular respiration, where electrons are passed through a series of carriers to generate a proton gradient that drives ATP synthesis.

Gluconeogenesis is the metabolic pathway for the synthesis of glucose from non-carbohydrate sources, such as amino acids and glycerol. This is essential for maintaining blood glucose levels during fasting or starvation.

The electron transport chain is a series of membrane-bound protein complexes that transport electrons from NADH and FADH2, using the energy released to pump protons and generate a proton gradient.