Investigation of the surface adsorption and biotribological properties of mucins

Tribology is the study of friction, wear, adhesion and lubrication. Biotribology covers all aspects of tribology that are related to biological systems. Most organisms face tribological challenges where increased friction is often desirable, such as walking, gripping and lifting objects or adhering to a surface.

However, in other instances the inverse properties are desirable. Mucins are found on epithelial surfaces throughout the body and are a key component of the mucus barrier. Here, they facilitate friction reduction, thus lowering the impact of physical abrasions, but they also act as a physical barrier that reduces adhesion to, and penetration of, the epithelial cell layer by bacteria.

The composition of the mucin macromolecules includes hydrophobic globular terminal domains that are separated by heavily glycosylated (hydrophilic) central domains. The central domains carry an overall negative charge due to the oligosaccharides being capped by negatively charged species such as sialic acid or sulphate groups.

Mucins display phenotypic diversion according to their expression site. This is most pronounced in the oligosaccharide composition of the central domains. The amphiphilic nature of mucins and their aqueous lubrication properties have led to them being proposed as possible biocompatible lubricants. In this thesis, we investigate the biotribological properties of two commercially available mucins on the soft, elastomeric and hydrophobic surface of PDMS under different conditions.

Due to the presence of a significant amount of non-mucin biomolecules in the commercial mucins, a mild single column protein purification protocol was established. In the mucin biotribology community, many employ the mucins either “as received” or after dialysis. It was therefore investigated how the established purification process impacts their adsorption- and tribological properties in comparison to either no purification or mildly purifying dialysis treatment.

We show that the properties of the mucins are influenced by the presence of other biomolecules. Bovine serum albumin was determined to be the main protein contaminant in bovine submaxillary mucin (BSM). The interactions between these proteins were also investigated spectroscopically. Although these results are partly inconclusive, they hint that the mode of interaction between these proteins may be dependent on time and when the molecules are put into proximity to each other in solution.

We have also investigated the thermostability of BSM. Most proteins denature at elevated temperatures. In this context, BSM displayed amazing resilience. PH was another environmental factor that was investigated. The results showed that the mucins surface adsorption and tribological properties is highly environment dependent.

Lastly, the importance of the hydrophobic terminal domains in 7 surface adsorption and subsequent aqueous lubrication efficiency was investigated by proteolytic digestion. Our studies show that the hydrophobic terminal domains are integral for adsorption and efficient aqueous lubrication of hydrophobic surfaces.