The human ABO(H) blood group antigens are produced by specific glycosyltransferase enzymes. An N-acetylgalactosaminyltransferase (GTA) uses a UDP-GalNAc donor to convert the H-antigen acceptor to the A antigen, whereas a galactosyltransferase (GTB) uses a UDP-galactose donor to convert the H-antigen acceptor to the B antigen. GTA and GTB differ only in the identity of four critical amino acid residues. Crystal structures at 1.8-1.32 A resolution of the GTA and GTB enzymes both free and in complex with disaccharide H-antigen acceptor and UDP reveal the basis for donor and acceptor specificity and show that only two of the critical amino acid residues are positioned to contact donor or acceptor substrates. Given the need for stringent stereo- and regioselectivity in this biosynthesis, these structures further demonstrate that the ability of the two enzymes to distinguish between the A and B donors is largely determined by a single amino acid residue.

The final step in the enzymatic synthesis of the ABO(H) blood group A and B antigens is catalyzed by two closely related glycosyltransferases, an ␣-(133)-N-acetylgalactosaminyltransferase (GTA) and an ␣-(133)-galactosyltransferase (GTB). Of their 354 amino acid residues, GTA and GTB differ by only four "critical" residues. High resolution structures for GTB and the GTA/GTB chimeric enzymes GTB/G176R and GTB/G176R/ G235S bound to a Glycosyltransferases synthesize carbohydrate moieties of glycoconjugates by catalyzing the sequential addition of monosaccharides from specific donors to specific acceptors. The ubiquitous presence of glycolipids and glycoproteins in all living systems underlines the importance of the glycosyltransferases superfamily, and the DNA of all domains of life encode for a large number of these enzymes (1). To date, crystal structures of glycosyltransferases have displayed a high degree of structural similarity even when there is low sequence homology (2-4). As such, glycosyltransferases provide an excellent example of the preferential conservation of structural phenotype over the conservation of sequence identity (2), which indicates that the mechanism of glycosylation, although not yet fully understood, has been conserved.

High-resolution structures reveal how a germline antibody can recognize a range of clinically relevant carbohydrate epitopes. The germline response to a carbohydrate immunogen can be critical to survivability, with selection for antibody gene segments that both confer protection against common pathogens and retain the flexibility to adapt to new disease organisms. We show here that antibody S25-2 binds several distinct inner-core epitopes of bacterial lipopolysaccharides (LPSs) by linking an inherited monosaccharide residue binding site with a subset of complementarity-determining regions (CDRs) of limited flexibility positioned to recognize the remainder of an array of different epitopes. This strategy allows germline antibodies to adapt to different epitopes while minimizing entropic penalties associated with the immobilization of labile CDRs upon binding of antigen, and provides insight into the link between the genetic origin of individual CDRs and their respective roles in antigen recognition.