AP BIOLOGY

Detailed explanation-1: -To start the cycle, an enzyme fuses acetyl CoA and oxaloacetate together so that citric acid is formed (a 2-carbon molecule + a 4-carbon molecule = a 6-carbon molecule!). This is the first molecule that is made in the cycle and is where the cycle gets its name.

Detailed explanation-2: -Before the Krebs cycle begins, pyruvic acid, which has three carbon atoms, is split apart and combined with an enzyme known as CoA, which stands for coenzyme A. The product of this reaction is a two-carbon molecule called acetyl-CoA.

Detailed explanation-3: -Krebs cycle steps The TCA cycle begins with an enzymatic aldol addition reaction of acetyl CoA to oxaloacetate, forming citrate. The citrate is isomerized by a dehydration-hydration sequence to yield (2R, 3S)-isocitrate.

Detailed explanation-4: -The three-carbon pyruvic acid molecules are converted to a two-carbon molecule attached to Coenzyme A, called acetyl CoA, via the process of pyruvate oxidation. It is the product, acetyl CoA, which enters the Krebs cycle.

Detailed explanation-5: -Moreover, the Krebs cycle also produces intermediates which are important in gluconeogenesis, lipolysis, neurotransmitter synthesis, etc. Acetyl-CoA from pyruvate of glycolysis, beta oxidation of fatty acids, ketogenic amino acids, and ketones enter this pathway for energy production.