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

Thermal food processing generates Nε-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 glutamate-dependent transaminases responsible for CML and CM-Cad transamination. Our results reveal a branched metabolic network, not a linear pathway: PatA shows specificity toward 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 following CM-Cad transamination and tentatively identify the previously unknown carboxymethyl-tetrahydropicolinic acid (CM-THPA) as a novel metabolite in CML metabolism. Combining molecular microbiology, biochemistry, and analytical chemistry, we define a branched transaminase network that integrates dietary CML into bacterial nitrogen metabolism, providing a model for microbial AGE processing via underground metabolism.