In order to understand how mucous glycoproteins, or mucins, confer such striking rheological properties on mucous secretions, it is first necessary to understand the structures of the individual glycoproteins and their components. Although significant variability of structure among mucins from different sources is evident, the "bottlebrush" structure is a common motif. For purposes of hydrodynamic calculations, we take the typical mucin monomer to have a molecular weight of 5 X lo5, 20% protein and 80% carbohydrate. About 25% of the polypeptide chain is unglycosylated or bare, while the remainder is densely substituted with oligosaccharide chains, averaging about eight sugar residues long. Unless the mucins are treated with S-S reducing agents, proteases, and denaturants, they are often found to have molecular weights around 2 X lo6. Thus, we model the normally occurring form as a tetramer, crosslinked like a star in the bare region.
With these structural ideas as a basis, we have confronted published hydrodynamic data with modern hydrodynamic theories to obtain a picture of mucin molecular structure. T o account for the oligosaccharide side chains, we used a treatment developed for partially porous spheres. T o model the composite structure of glycosylated and bare regions, we used subunit hydrodynamic theory. T o consider the effects of branching, we used published theories for branched random coils and extended the broken-rod results of Wilemski to cover branched rodlike structures.
The major conclusions are (1) the bare region is probably not highly extended, (2) the glycosylated region is more like a random coil than a rigid rod, and (3) comparison of hydrodynamic properties of monomers and branched tetramers suggests that the monomer is more rigid in the tetramer. This may be due either to underestimation of aggregation in the analysis of the experimental data or to a real stiffening effect imposed by polyelectrolyte and steric repulsive interactions with the other chains in the branched mucin.