Fig. 7: A proposed working model of the YcfA-YcfC system.

This illustration depicts the dynamic interplay within the YcfA-YcfC system, emphasizing its dual catalytic roles and regulation by GTP and PLP. YcfA alternates between two conformational states: an active GTP-bound form and an inactive PLP-bound form. Binding of PLP induces oligomerization while simultaneously blocking the GTP binding site, acting as a regulatory switch. In its active state, YcfA facilitates a two-step catalytic process involving guanine nucleotides. First, ATP is used to adenylate the guanine nucleotide, releasing pyrophosphate (PPi). This modification enables the subsequent recruitment of L-Cys or L-SeCys, forming a 6-Cys/SeCys-guanine nucleotide S-adduct. YcfC, functioning as a C–S/C-Se lyase, cleaves the S-adduct to yield the final thioamide product. While in vitro data establishes GTP/PLP-mediated regulation of this system, the physiological relevance of PLP-induced oligomerization and metabolite homeostasis maintenance remains to be verified in vivo. Dashed arrows indicate hypothesized connections requiring experimental validation in cellular contexts.