Divergent specificity of PatA, GabT, and IlvE defines the branched transamination of N _ε -carboxymethyllysine and its metabolite N _ε -carboxymethylcadaverine in Escherichia coli

Thermal food processing generates N _ε -carboxymethyllysine (CML), a key advanced glycation end product (AGE) and marker of the Maillard reaction in food. Escherichia coli utilizes CML as a nitrogen source. While SpeC initiates degradation by decarboxylating CML to N -carboxymethylcadaverine (CM-Cad), the enzymes liberating the nitrogen remained unknown. Here, we identify PatA, GabT, and IlvE as the glutamate-dependent transaminases responsible for CML and CM-Cad transamination. Our results reveal a branched metabolic network rather than a linear pathway: PatA shows specificity towards both substrates, while GabT and IlvE selectively process CM-Cad and CML, respectively. We further demonstrate that the carboxymethyl piperideinium ion (CM-Pip) is formed spontaneously following CM-Cad transamination and reveal the previously unknown carboxymethyl-tetrahydropicolinic acid (CM-THPA) as novel metabolite in CML metabolism. Combining molecular microbiology, biochemistry, and analytical chemistry, we demonstrate that these transaminases are essential for integrating dietary CML into bacterial nitrogen metabolism, providing a model for microbial AGE processing via underground metabolism.