When oxaloacetate concentration is low, what happens to acetyl CoA in the citric acid cycle?

When the concentration of oxaloacetate is low, acetyl CoA cannot enter the citric acid cycle because oxaloacetate is essential for the first step of the cycle, forming citrate. Acetyl CoA and oxaloacetate condense to produce citrate, which then enters the subsequent steps of the cycle.

When there is insufficient oxaloacetate, the reaction cannot proceed effectively. This situation often occurs during periods of fasting or carbohydrate restriction, where oxaloacetate levels can be depleted due to its use in gluconeogenesis or other pathways. Without adequate oxaloacetate to react with acetyl CoA, the citric acid cycle slows down or halts, leading to a reduced capacity for energy production through this pathway. This is critical to understand, as it highlights the interconnected nature of metabolic pathways and the importance of substrate availability in biochemical processes.