Conformational aspects of N-glycosylation of proteins. Studies with linear and cyclic peptides as probes.

Conformational aspects of N-glycosylation of glycoproteins have been studied by using a series of peptides which contained, in addition to the ;marker sequence' Asn-Gly-Thr, two cysteine residues in various positions of the peptide chain. The presence of two cysteines permitted a partial fixation of the above triplet sequence in cyclic structures of various size by intramolecular disulphide bond formation. Comparison of the glycosyl acceptor properties of the linear peptides and their corresponding cyclic analogues allows the following statements. The considerably lower acceptor capabilities of the cyclic derivatives indicate that the restriction of rotational degrees of freedom imposed by disulphide bonding results in a conformation which hinders a favourable interaction of the peptide substrate with the N-glycosyltransferase. On the other hand, the glycosylation rate of linear peptides increases with increasing chain length, suggesting that the amino acids on both the N- and C-terminal side of the ;marker sequence' may contribute to a considerable extent to the induction of an ;active' conformation. Realization of a potential sugar attachment site requires a hydrogen bond interaction within the ;marker sequence' between the oxygen of threonine (serine) as the hydrogen bond acceptor and the beta-amide of asparagine as the donor [Bause & Legler (1981) Biochem. J.195, 639-644]. This interaction is obviously facilitated when the peptide chain can adopt a conformation which resembles a beta-turn or other loop structure. The available experimental and statistical data are discussed in terms of possible structural features for N-glycosylation, with the aid of space-filling models.