Quantifying Reductive Amination in Nonenzymatic Amino Acid Synthesis

Amino acid biosynthesis initiates with the reductive amination of α‐ketoglutarate with ammonia to produce glutamate. However, the other α‐keto acids derived from the glyoxylate and Krebs cycles are converted into amino acids by transamination, rather than by reductive amination. Why is only one amino acid synthesized by reductive amination and not the others? To explore this question, we quantified the inherent reactivities of keto acids in nonenzymatic reduction and reductive amination by using BH(3)CN(−) as a model nucleophile. Biological α‐keto acids were found to show pronounced nonenzymatic reactivity differences for the formation of amino acids (α‐ketoglutarate<oxaloacetate≈pyruvate≪glyoxylate). Accordingly, the flow of ammonia passes through the least reactive α‐keto acid of the Krebs cycle. One possible explanation for this choice is the position of the corresponding amino acid, glutamate, at the top of the thermodynamic landscape for subsequent transamination reactions.