ThyX

ThyX, formally known as flavin-dependent thymidylate synthase (FDTS), is an alternative thymidylate synthase found in a broad set of bacteria and archaea. Unlike the classical thymidylate synthase ThyA, ThyX catalyzes the conversion of deoxyuridylate (dUMP) to deoxythymidylate (dTMP) using methylenetetrahydrofolate as a one-carbon donor and flavin adenine dinucleotide (FAD) as a cofactor. The enzyme’s mechanism and cofactor dependencies distinguish it from ThyA and reflect its distinct evolutionary origin.

Distribution and significance: ThyX is present in organisms that lack ThyA or that rely on the pathway encoded by thyX. It is especially common in several pathogens, and in many bacteria whose genomes do not encode a canonical ThyA. Humans and other animals synthesize thymidylate via ThyA, so ThyX has no direct human homolog, a feature that makes it an attractive target for antimicrobial drug development. Inhibitors targeting ThyX are the focus of ongoing research, with the aim of disrupting dTMP synthesis in pathogenic bacteria.

Structure and mechanism: ThyX proteins typically function as oligomers (often dimers) and form an active site that coordinates FAD and dUMP. The catalytic cycle uses FAD in its reduced state to facilitate a hydride transfer, with methylenetetrahydrofolate supplying the one-carbon unit. The precise details of the mechanism differ from ThyA and are the subject of structural and biochemical studies.

History and nomenclature: The family was identified in the late 1990s to early 2000s and named ThyX to distinguish it from the classical ThyA enzyme. The gene is commonly annotated as thyX in bacterial genomes. Ongoing research seeks to map its distribution across taxa, understand its essentiality in different organisms, and develop selective inhibitors.