Protein N-glycosylation requires the presence of asparagine (N) in the consensus
tri-peptide NXS/T (where X is any amino acid, S is serine and T is threonine). Several factors affect
the glycosylation potential of NXS/T sequons and one such factor is the type of amino acid at
position X. While proline was shown to negatively affect N-glycosylation, the nature of other amino
acids at this position is not clear. Using Markov chain analysis of tri-peptide NXS/T from viral,
archaeal and eukaryotic proteins as well as experimentally confirmed N-glycosylated sequons from
eukaryotic proteins, we show here that the occurrence of most sequon types differ significantly from
the expected probability. Sequon types with F, G, I, S, T and V amino acids are consistently preferred
while those with P and charged amino acids are under-represented in all four groups. Further, proteins
contained far fewer number of possible sequon types (maximum 20 types for NXS or NXT taken separately)
for any given number of sequons, which may be explained based on random sampling. Consistent with the
present finding, majority of the over-represented sequons found in two important viral envelope
glycoproteins (hemagglutinin of influenza A H3N2 and glycoprotein120 of HIV-1) are indeed preferred
sequon types, which may provide a selective advantage. Accordingly, although there seems to be some
preference for sequons, this preference may not be unique to N-glycosylation.