The structure of L-ribulose-5-phosphate 4-epimerase: An aldolase-like platform for epimerization

The structure of L-ribulose-5 -phosphate 4-epimerase from E. coli has been solved to 2.4 Angstrom resolution using X-ray diffraction data. The structu re is homo-tetrameric and displays C-4 symmetry. Each subunit has a single domain comprised of a central beta -sheet flanked on either side by layers of alpha -helices. The active site is identified by the position of the cat alytic zinc residue and is located at the interface between two adjacent su bunits. A remarkable feature of the structure is that it shows a very close resemblance to that of L-fuculose-1-phosphate aldolase. This is consistent with the notion that both enzymes belong to a superfamily of epimerases/al dolases that catalyze carbon-carbon bond cleavage reactions via a metal-sta bilized enolate intermediate. Detailed inspection of the epimerase structur e, however, indicates that despite the close overall structural similarity to class II aldolases, the enzyme has evolved distinct active site features that promote its particular chemistry.