Structure-Based Correlation of Light-Induced Histidine Reactivity in A Model Protein

Light is known to induce covalently linked aggregates in proteins. These aggregates can be immunogenic and are of concern for drug product development in the biotechnology industry. Histidine (His) is proposed to be a key residue in cross-link generation ( Pattison , D. I. Photochem. Photobiol. Sci. 2012 , 11 , 38 - 53 ). However, the factors that influence the reactivity of His in proteins, especially the intrinsic factors are little known. Here, we used rhDNase, which only forms His-His covalent dimers after light treatment to determine the factors that influence the light-induced reactivity of His. This system allowed us to fully characterize the light-induced covalent dimer and rank the reactivities of the His residues in this protein. The reactivities of these His residues were correlated with solvent accessibility-related parameters both by crystal structure-based calculations of solvent-accessible surface area and by hydrogen-deuterium exchange (HDX) experiments. Through this correlation, we demonstrate that the photoreactivity of His is determined by both solvent accessibility and structural flexibility. This new insight can explain the highly complex chemistry of light-induced aggregation and help predict the aggregation propensity of protein under light treatment.