The 3-keto-L-gulonate 6-phosphate decarboxylase (KGPDC) catalyses the
decarboxylation of 3-keto-L-gulonate 6-phosphate to L-xylulose in the presence
of magnesium ions. The enzyme is involved in L-ascorbate metabolism and plays an
essential role in the pathway of glucuronate interconversion. Crystal structures
of Streptococcus mutans KGPDC were determined in the absence and presence of the
product analog D-ribulose 5-phosphate. We have observed an 8 A alphaB-helix
movement and other structural rearrangements around the active site between the
apo-structures and product analog bound structure. These drastic conformational
changes upon ligand binding are the first observation of this kind for the KGPDC
family. The flexibilities of both the alpha-helix lid and the side chains of
Arg144 and Arg197 are associated with substrate binding and product releasing.
The open-closed conformational changes of the active site, through the movements
of the alpha-helix lid and the arginine residues are important for substrate
binding and catalysis.
