The first structure of UDP-glucose dehydrogenase reveals the catalytic residues necessary for the two-fold oxidation

Bacterial UDP-glucose dehydrogenase (UDPGlcDH) is essential for formation o f the antiphagocytic capsule that protects many virulent bacteria such as S treptococcus pyrogenes and Streptococcus pneumoniae type 3 from the host's immune system. We have determined the X-ray structures of both native and C ys260Ser UDPGlcDH from S. pyogenes (74% similarity to S. pneumoniae) in ter nary complexes with UDP-xylose/NAD(+) and UDP-glucuronic acid/NAD(H), respe ctively. The 402 residue homodimeric UDPGlcDH is composed of an N-terminal NAD(+) dinucleotide binding domain and a C-terminal UDP-sugar binding domai n connected by a long (48 Angstrom) central alpha-helix. The first 290 resi dues of UDPGlcDH share structural homology with 6-phosphogluconate dehydrog enase, including conservation of an active site lysine and asparagine that are implicated in the enzyme mechanism. Also proposed to participate in the catalytic mechanism are a threonine and a glutamate that hydrogen bond to a conserved active site water molecule suitably positioned for general acid /base catalysis.