Structural determination of archaeal UDP-N-acetylglucosamine 4-epimerase from Methanobrevibacter ruminantium M1 in complex with the bacterial cell wall intermediate UDP-N-acetylmuramic acid

The crystal structure of UDP‐N‐acetylglucosamine 4‐epimerase (UDP‐GlcNAc 4‐epimerase; WbpP; EC 5.1.3.7), from the archaeal methanogen Methanobrevibacter ruminantium strain M1, was determined to a resolution of 1.65 Å. The structure, with a single monomer in the crystallographic asymmetric unit, contained a conserved N‐terminal Rossmann‐fold for nucleotide binding and an active site positioned in the C‐terminus. UDP‐GlcNAc 4‐epimerase is a member of the short‐chain dehydrogenases/reductases superfamily, sharing sequence motifs and structural elements characteristic of this family of oxidoreductases and bacterial 4‐epimerases. The protein was co‐crystallized with coenzyme NADH and UDP‐N‐acetylmuramic acid, the latter an unintended inclusion and well known product of the bacterial enzyme MurB and a critical intermediate for bacterial cell wall synthesis. This is a non‐native UDP sugar amongst archaea and was most likely incorporated from the E. coli expression host during purification of the recombinant enzyme.