Helicobacter pylori ␣1,3-fucosyltransferase (FucT) is involved in catalysis to produce the Lewis x trisaccharide, the major component of the bacteria's lipopolysaccharides, which has been suggested to mimic the surface sugars in gastric epithelium to escape host immune surveillance. We report here three x-ray crystal structures of FucT, including the FucT⅐GDP-fucose and FucT⅐GDP complexes. The protein structure is typical of the glycosyltransferase-B family despite little sequence homology. We identified a number of catalytically important residues, including Glu-95, which serves as the general base, and Glu-249, which stabilizes the developing oxonium ion during catalysis. The residues Arg-195, Tyr-246, Glu-249, and Lys-250 serve to interact with the donor substrate, GDP-fucose. Variations in the protein and ligand conformations, as well as a possible FucT dimer, were also observed. We propose a catalytic mechanism and a model of polysaccharide binding not only to explain the observed variations in H. pylori lipopolysaccharides, but also to facilitate the development of potent inhibitors.

Hexaprenyl pyrophosphate synthase (HexPPs) from Sulfolobus solfataricus catalyzes the synthesis of trans-C 30 -hexaprenyl pyrophosphate (HexPP) by reacting two isopentenyl pyrophosphate molecules with one geranylgeranyl pyrophosphate. The crystal structure of the homodimeric C 30 -HexPPs resembles those of other trans-prenyltransferases, including farnesyl pyrophosphate synthase (FPPs) and octaprenyl pyrophosphate synthase (OPPs). In both subunits, 10 core helices are arranged about a central active site cavity. Leu164 in the middle of the cavity controls the product chain length. Two protein conformers are observed in the S. solfataricus HexPPs structure, and the major difference between them occurs in the flexible region of residues 84 to 100. Several helices (␣I, ␣J, ␣K, and part of ␣H) and the associated loops have high-temperature factors in one monomer, which may be related to the domain motion that controls the entrance to the active site. Different side chain conformations of Trp136 in two HexPPs subunits result in weaker hydrophobic interactions at the dimer interface, in contrast to the symmetric -stacking interactions of aromatic side chains found in FPPs and OPPs. Finally, the three-conformer switched model may explain the catalytic process for HexPPs.Isoprenoids are among the most diverse and widely distributed natural compounds (17,22). Using the 5-carbon isopentenyl pyrophosphate (IPP) as building blocks, linear isoprenoids are synthesized by a group of prenyltransferases (PTases), which catalyze the multiple-IPP condensation reaction with allylic substrates, i.e., C 5 dimethyl allyl pyrophosphate (DMAPP), C 15 farnesyl pyrophosphate (FPP), or C 20 geranylgeranyl pyrophosphate (GGPP) for chain elongation (13). These PTases have been classified as E-and Z-types, which synthesize products with trans and cis double bonds, respectively. Different enzymes synthesize varying chain length products, which are then utilized as precursors for steroids, carotenoids, quinones, dilichols, prenylated proteins, and archaeal membrane lipids.Trans-PTases are typically classified into three groups based on the chain length of their final products, i.e., short (C 10 to C 25 ), medium (C 30 to C 35 ), and long chain (C 40 to C 50 ) (12, 16). The crystal structures of short-chain farnesyl pyrophosphate synthase (FPPs) and long-chain octaprenyl pyrophosphate synthase (OPPs) have been determined previously (6,10,25). These proteins are homodimers. Each subunit contains several ␣-helices that form a large central active site cavity with two conserved DDXXD motifs, located at the rims of the helices D and H, respectively. On the other hand, the mediumchain trans-PTases, including hexaprenyl pyrophosphate synthase (HexPPs) and heptaprenyl pyrophosphate synthase (HepPPs), are composed of either homodimers or heterodimers. The HexPPs from Sulfolobus solfataricus (23), which catalyzes the condensation of two IPP molecules with GGPP to yield all-trans-C 30 -HexPP, is a homodimer (8). In contrast, HexPPs from Micrococcus luteus...

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