Primary sequence dependence of conformation in oligomannose oligosaccharides.

The oligomannose series of oligosaccharides from bovine thyroglobulin (BTG) and the variant surface glycoprotein (VSG) of Trypanosoma brucei have been isolated and sequenced by 1H NMR. The structure of Man9GlcNAc2, the parent molecule of the series, is shown below. Structural isomerism occurs within this series through the removal of residues D1, D2, D3, and C. Using spin-spin coupling and chemical shift data the rotamer distributions about the dihedral angle omega for the Man alpha 1-6Man beta and Man alpha 1-6Man alpha linkages were determined for each member of the series. It is shown that the dihedral angle omega of the Man alpha 1-6Man beta linkage exhibits low flexibility with a preference for the omega = 180 degrees conformation when residue D2 is present and high flexibility when this residue is absent. Flexibility of omega for the Man alpha 1-6Man alpha is largely independent of primary sequence and is intermediate between the two Man alpha 1-6Man beta extremes, again with a preference for the omega = 180 degrees conformation. [see text] There are, however, data which indicate that removal of residue D3 may confer additional flexibility upon the dihedral angle omega of the Man alpha 1-6Man alpha linkage. Molecular graphics modelling, together with chemical and enzymatic modification studies, suggest that the origin of the observed primary sequence dependence of the Man alpha 1-6Man beta linkage arises from steric factors. On the basis of these observations taken together with previous work, it is postulated that recognition of individual oligomannose conformations may play a role in the control of N-linked oligosaccharide biosynthesis.