2rj0: Difference between revisions

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 a-(1-3)-N-acetylgalactosaminyltransferase (GTA) and an a-(1-3)-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 panel of donor and acceptor analog substrates reveal 'open', 'semi-closed' and 'closed' conformations as the enzymes go from the unliganded to the liganded states. In the 'open' form the internal polypeptide loop (amino acid residues 177-195) adjacent to the active site in the unliganded or H-antigen-bound enzymes is composed of two a-helices spanning Arg180-Met186 and Arg188-Asp194 respectively. The 'semi-closed' and closed forms of the enzymes are generated by binding of UDP or of UDP and H-antigen analogs respectively, and show that these helices merge to form a single distorted helical structure with alternating a-310-a character that partially occludes the active site. The 'closed' form is distinguished from the 'semi-closed' form by the ordering of the final nine C-terminal residues through the formation of hydrogen bonds to both UDP and H-antigen analogs. The 'semi-closed' forms for various mutants generally show significantly more disorder than the 'open' forms, while the 'closed' forms display little or no disorder depending strongly on the identity of residue 176. Finally, the use of synthetic analogs reveals how H-antigen acceptor binding can be critical in stabilizing the 'closed' conformation. These structures demonstrate a delicately-balanced substrate recognition mechanism and give insight on critical aspects of donor and acceptor specificity, on the order of substrate binding, and on the requirements for catalysis.

[[Category: Alfaro, J A.]]

[[Category: Evans, S V.]]

''Page seeded by [http://oca.weizmann.ac.il/oca OCA ] on Thu Feb 21 18:47:44 2008''