The effect of manganese and other divalent cations on the activity of a soluble recombinant form of human alpha3/4 fucosyltransferase III (SFT3) expressed in Spodoptera frugiperda (Sf9) insect cells was studied. SFT3 was active in the absence of divalent cations with an optimum pH of 4.5. In the absence of Mn2+ increasing the pH from 4.5 to 7.0 caused a decrease in the affinity of SFT3 for the acceptor Galbeta3GlcNAcO(CH2)(3)NHCO(CH2)(5)NH-biotin, as monitored by the 4-fold increase in the apparent K-M value (0.9 to 3.3 mM). At pH 7.0, the addition of Mn2+ activated the enzyme and caused an increase in the affinity of SFT3 for the acceptor, as monitored by the 5-fold decrease of the apparent K-M value (3.3 to 0.7 mM). In solution, a complex between GDP-Fuc donor and the divalent cation Mn2+ was observed by electrospray ionization mass spectrometry, in a 1:1 stoichiometry. These results indicated that Mn2+ bound the enzyme and increased its affinity for the acceptor; one possible functional role of manganese in catalysis could be as an electrophilic catalyst, co-ordinating the negative charges of the phosphate groups of the GDP-Fuc donor and promoting Fuc transfer. At low pH values such role would be played by the proton.