The crystal structure of dTDP-D-glucose 4,6-dehydratase (RmlB) from Salmonella enterica serovar typhimurium, the second enzyme in the dTDP-L-rhamnosepathway
Stm. Allard et al., The crystal structure of dTDP-D-glucose 4,6-dehydratase (RmlB) from Salmonella enterica serovar typhimurium, the second enzyme in the dTDP-L-rhamnosepathway, J MOL BIOL, 307(1), 2001, pp. 283-295
(L)-Rhamnose is a 6-deoxyhexose that is found in a variety of different gly
coconjugates in the cell walls of pathogenic bacteria. The precursor of L-r
hamnose is dTDP-(L)-rhamnose, which is synthesised from glucose-1-phosphate
and deoxythymidine triphosphate (dTTP) via a pathway requiring four enzyme
s. Significantly this pathway does not exist in humans and all four enzymes
therefore represent potential therapeutic targets, dTDP-(D)-glucose 4,6-de
hydratase (Rm1B; EC 4.2.1.46) is the second enzyme in the dTDP-(L)-rhamnose
biosynthetic pathway. The structure of Salmonella enterica serovar Typhimu
rium Rm1B had been determined to 2.47 Angstrom resolution with its cofactor
NAD (+) bound. The structure has been refined to a crystallographic X-fact
or of 20.4% and an X-free value of 24.9 % with good stereochemistry.
Rm1B functions as a homodimer with monomer association occurring principall
y through hydrophobic interactions via a four-helix bundle. Each monomer ex
hibits an alpha/beta structure that can be divided into two domains. The la
rger N-terminal domain binds the nucleotide cofactor NAD + and consists of
a seven-stranded P-sheet surrounded by alpha -helices. The smaller C-termin
al domain is responsible for binding the sugar substrate dTDP-(D)-glucose a
nd contains four beta -strands and six alpha -helices. The two domains meet
to form a cavity in the enzyme. The highly conserved active site Tyr(167)X
XXLys(171) catalytic couple and the GlyXGlyXXGly motif at the N terminus ch
aracterise Rm1B as a member of the short-chain dehydrogenase/reductase exte
nded family.
The quaternary structure of Rm1B and its similarity to a number of other cl
osely related short-chain dehydrogenase/reductase enzymes have enabled us t
o propose a mechanism of catalysis for this important enzyme. (C) 2001 Acad
emic Press.