Schematic overview of the two major pathways from pyruvate towards oxaloacetate: (i) direct carboxylation via pyruvate carboxylase or (ii) pyruvate dehydrogenase and the energy forming reactions of the TCA cycle. The conserved carbon transfer of the underlying reactions yields a specific labelling pattern for oxaloacetate formed by each pathway which is presented in this figure. White circles represent 12C whereas black circles indicate labelled 13C. The numbers given reflect the position of the carbon atom within the molecule. AcCoA: acetyl-Coenzyme A; EDP: Entner-Doudoroff pathway; OAA: oxaloacetate; PYR: pyruvate; TCA: tricarboxylic acid.