Oxaloacetate is a four-carbon dicarboxylic acid that plays a crucial role in the citric acid cycle, acting as a substrate for the enzyme citrate synthase and enabling the condensation reaction with acetyl-CoA to form citrate. It also serves as a critical intermediate in gluconeogenesis, linking carbohydrate and amino acid metabolism by being converted to phosphoenolpyruvate through the action of phosphoenolpyruvate carboxykinase.