N-linked glycosylation is a common post-translational modification of 7TMRs. The A118G SNP changes Asn to Asp at position 40 (N40D) in the N-terminal domain and removes one of the five potential N-linked glycosylation sites of hMOPR (Fig. 1). Although the consensus sequence of N-glycosylation sites is a tripeptide motif of -Asn-X-Thr/Ser-, not all such motifs are glycosylated in a glycoprotein [20]. One possible explanation is that the primary, secondary, and/or tertiary structures of the peptide around the consensus N-glycosylation sites play roles in substrate recognition by the enzymes (e.g., oligosaccharyl transferase) [21]. Thus, it is not clear whether the loss of an - Asn-X-Thr/Ser- motif due to A118G SNP, leads to changes in N-glycosylation of the MOPR, and if so, whether this SNP impacts on stability of the MOPR protein as it is well-documented that glycans augment overall stability of glycoproteins [reviewed in [22]], including several 7TMRs [23-25].
