3-D renderings of ST3Gal I model highlighting the various functionally and structurally important residues and regions. The location of the ligand CMP-3-fluoro-NeuNAc as shown in these renderings has been derived by superposition of the modeled structure on that of the CstII- CMP-3-fluoro-NeuNAc complex. (A, top left) 3-D rendering showing the L-motif (magenta), S-motif (purple), VS-motif (burgundy), motif III (yellow), linkage-specific motifs TTx(4)YPE (orange) and FKxxDxxW (green) and (stick diagram). (B, top right) Location of some of the amino acid residues (colored green) whose roles have been investigated by site-directed mutagenesis studies (Table 2). The loop proposed in this study as important for nucleotide binding is shown in blue. The nomenclature used for identifying helices (cyan) and strands (magenta) is as that in Figure 1. (C, bottom left) The interactions of the conserved Arg168 with Asn147 and Glu178. All three residues are buried within the protein. The disulphide bridge (colored red) proposed as required for structural stability has also been displayed. The ligand is in pink. (D, bottom right) 3-D rendering showing the cluster of apolar residues proposed as important for maintaining the 3-D structure (Table 2). Note that not all these residues are in direct contact with the ligand.