Analysis of Immobilized Protein Unfolding and Molecular Dynamics Shows How pH, Glycosylation, and OCA3-Related...

Tyrosinase-related protein 1 (Tyrp1) is a melanosomal glycoprotein required for eumelanin biosynthesis through the oxidation of 5,6-dihydroxyindole-2-carboxylic acid (DHICA). Pathogenic variants in Tyrp1 cause oculocutaneous albinism type 3 (OCA3), but the molecular basis by which individual substitutions impair Tyrp1 stability and activity remains incompletely understood. Here, we examined wild-type Tyrp1 and three missense variants associated with OCA3: R356Q and R326H as OCA3-related variants, and D308N as a benign control; these were under conditions relevant to melanosome maturation. To assess stability, we developed a urea-induced unfolding assay in which His-tagged Tyrp1 variants were immobilized to Ni-NTA magnetic beads before chemical denaturation. R356Q was the most destabilized variant, with a ΔΔG of 0.695 kcal/mol at pH 5.0 (acidic conditions) and 1.998 kcal/mol at pH 7.4 (near-neutral conditions) relative to wild-type. R326H showed intermediate destabilization, whereas D308N behaved similarly to wild-type. DHICA oxidation assays in the presence of MBTH showed about 20% reduced catalytic activity for R356Q, particularly under acidic conditions. Molecular dynamics simulations and ligand docking were consistent with these findings and indicated that R356Q increases conformational flexibility and perturbs structural integrity. In contrast, glycosylation reduced conformational fluctuations and enhanced stability across Tyrp1 and mutant variants examined. Together, these results show that pH, glycosylation, and disease-associated substitutions collectively modulate Tyrp1 folding energetics and catalytic competence and identify R356Q as a strongly destabilizing OCA3 variant. By defining how disease-associated Tyrp1 substitutions affect protein stability and function, this study may provide a framework for interpreting genotype-phenotype relationships and improving molecular diagnosis of OCA3.