Schematic representation of FS proteins from LAB. The L. reuteri 121 inulosucrase (Inu) deduced amino acid sequence was used as the template (AF459437). The four different regions shown are (i) the N-terminal signal sequence; (ii) the N-terminal variable region; (iii) the catalytic core; and (iv) the C-terminal variable region (which in some cases contain an LPXTG cell wall anchor). Alignments (SequenceLogo, http://weblogo.berkeley.edu/) are shown of short regions in LAB FS protein amino acid sequences (Table (Table2)2) with conserved amino acid residues for which mutant information is available in literature. (A) Asp86 in B. subtilis SacB identified as the nucleophile based on crystal structure data for (inactive) mutant Glu342Ala with a bound sucrose (113) and biochemical characterization of the L. reuteri 121 Inu Asp272Asn mutant (140). (B) Mutant Glu117Gln in Z. mobilis SacB, resulting in a higher transglycosylation activity (220); SGSA, sucrose binding box 1, a region conserved between sucrose-utilizing enzymes (167). (C) Mutant Asp312Ser in S. salivarius ATCC 25975 FS (possibly involved in acceptor recognition and/or stabilizing a beta turn in the protein) (185). (D) “Sucrose binding box 2,” a region conserved between sucrose-utilizing enzymes (167). (E) RDP motif, with mutant Asp397Ser in S. salivarius ATCC 25975 FS resulting in complete loss of sucrose hydrolysis and polymerization activities (185), mutant Asp309Asn in G. diazotrophicus LsdA, with a 75-fold reduced catalytic activity (9), mutant Asp194Asn in Z. mobilis SacB, with a 3,400-fold decrease in catalytic activity (220), and Asp247 in B. subtilis SacB and Asp272 in L. reuteri 121 Inu (stabilizer of the oxocarbenium-like transition state) (113, 140). (F) Mutant Glu211Gln in Z. mobilis SacB, with sucrose hydrolysis reduced to 28%, and highly reduced transfructosylation activity. (G) Mutant Glu278Asp (30-fold lower catalytic activity), mutant Glu278His (virtually inactive enzyme) in Z. mobilis levansucrase SacB (220), and Glu342 in B. subtilis SacB and Glu523 in L. reuteri 121 Inu (acid/base catalyst) (113, 140). (H) Arg331His (higher oligosaccharide formation) in levansucrase from B. subtilis SacB (22).