Analyzing the functions of large glycoconjugates through the dissipative properties of their absorbed layers using the gel-forming mucin MUC5B as an example

Glyconjugates such as mucins, proteoglycans, and polysaccharides form the structural basis of protective cell-surface layers. In particular gel-forming mucins define a zone between the epithelial cell layer and the environment. Such molecules are of extreme molecular weight 5-100 x 10(6) and size (Rg 20-300 nm). On this account their biochemistry is inseparable from their physical biochemistry. Combining laser light scattering and quartz crystal mass balance with dissipation methods (QCM-D) we have investigated the properties of the MUC5B mucin and its cognate fragments when bound to a hydrophobic surface. MUC5B forms the basis of gels responsible for the protection of the oral cavity, lung, and cervical canal surfaces. Here we show, by analyzing dissipative interactions of hydrophobic, gold, and polystyrene surfaces, with the intact MUC5B molecule, its reduced subunits, and glycosylated tryptic fragments (obtained after reduction), the formation of 40- to 100-nm-thick highly structured, hydrated interfaces. These interfaces are dominated in their geometry and dissipative properties by the negatively charged carbohydrate-rich domains of the molecule, the naked protein domains being responsible for attachment. These carbohydrate-rich surfaces have well-defined absorptive properties and permit the entry and entrapment of albumin-coated micro-beads into the absorbed layer at and below a size of 60 nm. However beads larger than 100 nm are completely excluded from the surfaces. These absorptive phenomena correlate with large changes in film dissipation and thus may not only be important in biological functions, e.g. binding viruses, but could also be informative to the surfaces (often ciliated) onto which such mucus films are attached.