SURFACE-STRUCTURE OF HUMAN MUCIN USING X-RAY PHOTOELECTRON-SPECTROSCOPY

X-ray photoelectron spectroscopy (XPS) is a surface sensitive analytic al technique that measures the binding energy of electrons in atoms an d molecules on the surface of a material. XPS was used to determine th e distribution of the oligosaccharide side chains in the glycoprotein, MUC1 mucin. Low-resolution XPS spectra provided elemental composition of MUC1 mucin (fully glycosylated), mucin polypeptide (nonglycosylate d), and carbohydrates found in mucin. The nitrogen content of MUC1 muc in was determined to be intermediate between the mucin polypeptide and the carbohydrates. Assuming a uniform distribution of carbohydrate on MUC1 mucin, the average thickness of the carbohydrate layer was calcu lated to be 4.9 nm using the low-resolution N 1s signals. High-resolut ion XPS spectra give detailed information about the chemical bonding o f the surface molecules. Calculations based on the high-resolution O 1 s spectra showed a carbohydrate thickness of 6.6 nm. These experimenta lly determined values agree reasonably well with an estimated 5 nm of carbohydrate thickness from a simple model which assume that the core protein is a rodlike molecule approximately 5 nm in diameter. Although the carbohydrate coating on the MUC1 mucin appears to be thick enough to cover the core protein entirely, fully glycosylated breast milk MU C1 mucin is susceptible to proteolytic digestion without removal of an y oligosaccharide side chain, suggesting areas of exposed core protein . A possible explanation is that the oligosaccharide side chains may f orm patches of carbohydrate along the core protein with regions of exp osed core protein. (C) 1998 John Wiley & Sons, Inc. Biospectroscopy 4: 257-266, 1998.