Link Reaction in Cellular Respiration

Overview of the Link Reaction

• The link reaction, also known as the pyruvate decarboxylation step, connects glycolysis to the citric acid cycle.
• It converts pyruvate, a three-carbon molecule, into acetyl CoA, a two-carbon molecule.
• This process occurs in the mitochondrial matrix.

Steps in the Link Reaction

• Decarboxylation: The first step involves the removal of a carbon atom from pyruvate, releasing carbon dioxide as a byproduct. This creates a two-carbon molecule called an acetyl group.
• Oxidation: The remaining two-carbon acetyl group is oxidized, meaning it loses electrons. This oxidation process involves the transfer of a pair of hydrogen atoms to NAD+ (nicotinamide adenine dinucleotide), reduced to NADH.
• CoA attachment: The acetyl group combines with coenzyme A (CoA) to form acetyl CoA. This molecule is crucial for transporting the acetyl group into the citric acid cycle.

Key Molecules Involved

• Pyruvate: A three-carbon molecule produced from glycolysis.
• Acetyl CoA: A two-carbon molecule that carries the acetyl group to the citric acid cycle.
• NAD+: A coenzyme that accepts electrons and hydrogen ions during oxidation, producing NADH.
• CO2: Carbon dioxide, a waste product of the reaction, is released into the environment.
• CoA: Coenzyme A (CoA) molecule is a carrier.

Energy Production

• The link reaction produces one molecule of NADH per pyruvate molecule, carrying high-energy electrons to the electron transport chain.
• This step generates no ATP directly. The energy stored within NADH will contribute to ATP production at a later stage (electron transport chain).

Location and Regulation

• It occurs in the mitochondrial matrix.
• The enzyme complex responsible for the process includes specific enzymes that catalyze the steps.
• The rate of the link reaction can be regulated by the availability of substrates (pyruvate) and the activity of enzymes that catalyze the reaction steps. Product inhibition of the link reaction, based on the availability/ accumulation of the products, is common.
• Higher energy charges indicate a metabolically sufficient state which can inhibit aspects of the link reaction. High NADH inhibits the link reaction to avoid excess NADH production.