The effect of point mutations on the biophysical properties of an anti-microbial peptide: development of a screening protocol for peptide stability screening.

Therapeutic peptides and proteins show an enormous potential on the pharmaceutical market, but high costs in discovery and development are limiting factors so far. Single or multiple point mutations are commonly introduced in protein drugs to increase their binding affinity or selectivity. They can also induce adverse properties, which might be overlooked in a functional screen, such as a decreased colloidal or thermal stability, leading to problems in later stages of the development. In this study, we address the effect of point mutations on the stability of the 4.4 kDa anti-microbial peptide plectasin, as a case study. We combined a systematic high throughput biophysical screen of the peptide thermal and colloidal stability using dynamic light scattering and differential scanning calorimetry in combination with structure-based methods including small angle X-ray scattering, analytical ultracentrifugation and nuclear magnetic resonance spectroscopy. Additionally, we applied molecular dynamics simulations to link obtained protein stability parameters to the protein's molecular structure. Despite their predicted structural similarities, all four plectasin variants showed a substantially different behavior in solution. We observed an increasing propensity of plectasin to aggregate at higher pH, and the introduced mutations influenced the type of aggregation. Our strategy for systematically assessing the stability and aggregation of protein drugs is generally applicable and is of particular relevance given the increasing number of protein drugs in development.