title: Structural properties of porcine submaxillary gland apomucin.
abstract: Porcine submaxillary gland mucin was deglycosylated with a mixture of pure glycosidases to give apomucin containing less than 1% carbohydrate. The resulting apomucin freed of glycosidases was found to contain nine amino acids: threonine, serine, glutamic acid, proline, glycine, alanine, valine, isoleucine, and arginine. Serine, threonine, glycine, and alanine comprise 77% of the composition. The molecular weight of apomucin was 96,500 as determined by gel filtration in guanidine hydrochloride. Its Stokes radius was greater than 68.6 A, a far larger value than expected for a globular protein with Mr = 96,500. Circular dichroism spectroscopy of apomucin suggests that it contains 42% aperiodic or "other" structure, 40% beta-turns, 10% antiparallel pleated sheet, and 8% helical structures. The predicted secondary structure of a 50-residue peptide from ovine submaxillary gland mucin resembles the circular dichroism predictions, being dominated by turns that would lead to an extended nonglobular structure. Analysis for the secondary structure of a 36-residue tryptic peptide derived from porcine submaxillary gland apomucin predicts a similar structure. It is concluded that apomucin is likely devoid of traditional secondary structure and serves as a scaffold upon which oligosaccharides are added in O-glycosidic linkage. When sufficient sialic acid is present in the oligosaccharides, native highly viscous mucin containing about two-thirds carbohydrate by weight is obtained.
mesh_terms: D000595:Amino Acid Sequence; D000596:Amino Acids / Q000032:analysis; D000818:Animals; D002241:Carbohydrates; D055598:Chemical Phenomena; D002627:Chemistry, Physical; D002942:Circular Dichroism; D005752:Gastric Mucins; D006026:Glycoside Hydrolases / Q000378:metabolism; D008970:Molecular Weight; D010455:Peptides / Q000032:analysis*; D011487:Protein Conformation; D013363:Submandibular Gland / Q000032:analysis*; D013552:Swine
chemical_list: D000596:Amino Acids; D002241:Carbohydrates; D005752:Gastric Mucins; D010455:Peptides; C053366:apomucin; D006026:Glycoside Hydrolases