CHEMISTRY AND BIOCHEMISTRY OF GLYCOCONJUGATES

Johns Hopkins University, Baltimore MD

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Abstract

We plan to study three health-related galactose-binding systems: i) urinary tract invasive Escherichia coli (UTI-E. coli), containing Gal-binding adhesin at the tip of fimbriae, ii) verotoxin (VT) secreted by some strains of E. coli (e.g., 0-157), and iii) human C-reactive protein (CRP), an important acute phase protein. The first two systems bind Galalpha (1-4)Gal segment (galabiose) of glycoconjugates, but binding of Gal-containing materials by CRP has not been carefully studied. The carbohydrate-binding subunits of both VT and CRP form a cyclic pentamer. Pathogenicity of the first two systems resides in their initial adhesion to the cell surface glycoconjugates, and devising powerful inhibitors for these systems may lead to useful drugs. Carbohydrate-mediated binding is often greatly enhanced by suitable clustering of glycosides. To attain maximum affinity to pentameric proteins, the ligand glycosides should ideally be in a pentameric arrangement to correspond to the combining sites. The crystallographic structures for VT and CRP are known, and we modeled the ideal arrangement for clustered glycoside ligands to be synthesized. Clustered glycosides can be linear or looped. The latter, though more difficult to construct, is expected to give greater binding affinity. The galabiose synthons that contain suitable functional groups for conjugation will be prepared chemically or enzymatically. For this purpose, pigeon egg proteins will play a prominent role, because we have found that major pigeon egg white and yolk glycoproteins are rich in alpha-Gal residue and are shown to be potent inhibitors for UTI- E. coli adhesion. In parallel, we will study the structures of alpha-Gal-containing glycans in pigeon eggs and embark on characterization of the alpha-Gal-transferase responsible for such glycans. These studies are significant from the evolutionary standpoint, since the occurrence of alpha-Gal in glycoproteins is rare in birds, and the responsible transferase seems to be different from that of mammals.