Amino acid sequence alignment of human ␣1,3/4-fucosyltransferases (FucTs) demonstrates that three highly conserved Lys residues are present in the catalytic domain of FucTs III, IV, V, and VI. Two of these sites are conserved in FucT VII, with the third located within the ␣1,3-FucT motif as a conservative change to Arg at position 223. and the R223K mutant; however, the purified R223K mutant enzyme had a 2-fold increased specific activity compared with purified native FucT VII. No change in GDP-fucose-protectable pyridoxal-P/NaBH 4 inactivation was observed for native or mutant FucT V or VII, further supporting the absence of involvement of this residue in sugar nucleotide binding. The results indicate that a basic residue in this position is required for enzyme activity, with a Lys residue providing higher intrinsic activity. The lack of influence of this site on substrate binding parameters and its location within the ␣1,3-FucT motif suggest that at least some of the residues within this motif are involved in catalysis rather than substrate binding.Five distinct human ␣1,3/4-fucosyltransferases (FucTs) 1 have been cloned (1-7), as well as related enzymes from other species (8 -16). Analysis of their deduced amino acid sequences demonstrates that a variable number of Lys residues are present. FucTs III, V, and VI are highly homologous and contain 13, 12, and 12 Lys residues, respectively. In contrast, FucTs VII and IV contain only two and five Lys residues, respectively. Sequence alignment analysis of these forms demonstrate that nine of the Lys residues present in FucT III, V, and VI are in equivalent positions. Of these, three Lys residues can be aligned with Lys residues found in the FucT III, IV, V, and VI sequences. The FucT VII sequence contains two of these Lys residues, and a conservative substitution (Arg 223