Statistical analysis was carried out to study the sequential aspects of amino acids around the O-glycosylated Ser/Thr. 992 sequences containing O-glycosylated Ser/Thr were selected from the O-GLYCBASE database of O-glycosylated proteins. The frequency of occurrence of amino acid residues around the glycosylated Ser/Thr revealed that there is an increased number of proline residues around the O-glycosylation sites in comparison with the nonglycosylated serine and threonine residues. The deviation parameter calculated as a measure of preferential and nonpreferential occurrence of amino acid residues around the glycosylation site shows that Pro has the maximum preference around the O-glycosylation site. Pro at +3 and/or -1 positions strongly favors glycosylation irrespective of single and multiple glycosylation sites. In addition, serine and threonine are preferred around the multiple glycosylation sites due to the effect of clusters of closely spaced glycosylated Ser/Thr. The preference of amino acids around the sites of mucin-type glycosylation is found likely to be similar to that of the O-glycosylation sites when taken together, but the acidic amino acids are more preferred around Ser/Thr in mucin-type glycosylation when compared totally. Aromatic amino acids hinder O-glycosylation in contrast to N-glycosylation. Cysteine and amino acids with bulky side chains inhibit O-glycosylation. The preference of certain potential sequence motifs of glycosylation has been discussed.

No abstract

An analysis of the frequency of occurrence of various residues at position X was carried out on the consensus glycosylating sequence Asn-X-Ser/Thr using the PDB three-dimensional database. 488 non-homologous proteins bearing 696 Asn-X-Ser/Thr (X T Pro) sequences were analysed. More than 65% of Asn residues, when they occur as part of the consensus sequence, lie on the surface of the protein, implying a potentiality for glycosylation. A deviation parameter (DP) was calculated as a measure of preferential (positive) or nonpreferential (negative) selection. At the X position in the consensus-sequence segment, the amino acids Gly, Asn and Phe have statistically signi®cant positive DP values. The high value of DP for Asn is a consequence of the preferential occurrence of homodoublets, while for Phe it may be a consequence of the stacking interaction of the aromatic ring with the glycan. Gly at the X position in the consensus glycosylating sequence may be functionally signi®cant owing to its preference and its high percentage of occurrence in proteins. The Ramachandran (È,É) angles around Gly in the consensus sequence show clustering in the region which is disallowed for non-glycyl residues. In this region, a hydrogen bond between the side chain of Asn and the peptide backbone/side chain of Ser/Thr is possible, re¯ecting a positional as well as a conformational role in the consensus glycosylating sequence. For the 44 con®rmed N-glycosylating sequences, an in-depth analysis of the (É N , È X , É X , È S/T ) dihedral angles, which position the side chains of Asn and Ser/Thr, shows that these can be grouped into nine conformational states. In most cases, a direct or watermediated hydrogen bond between OD1 of Asn and OG of Ser/Thr is possible, re¯ecting the possible importance of this hydrogen bonding in the glycosylation process.

No abstract

Molecular modeling studies have been carried out to investigate the interactions between substrate sialyloligosaccharide (SOS) fragments bearing different glycosidic linkages and influenza virus N9 neuraminidase, a surface glycoprotein of influenza virus subtype N9. The studies revealed that the allowed orientation for sialic acid (SA) is less than 1% in the Eulerian space at the active site. The active site of this enzyme has enough space to accommodate various SOS fragments, NeuNAcalpha(2-3)Gal, NeuNAcalpha(2-6)Gal, NeuNAcalpha(2-8)NeuNAc and NeuNAcalpha(2-9)NeuNAc, but on specific conformations. In the bound conformation, among these substrates there exists a conformational similarity leading to a structural similarity, which may be an essential requirement for the cleavage activity of the neuraminidases irrespective of the type of glycosidic linkage.