Abstract
Purpose :
Oculocutaneous albinism type 1 (OCA1) is an autosomal recessive disorder caused by mutations in the tyrosinase gene. OCA1 exists in two forms: OCA1A and OCA1B. OCA1A is caused by a full loss of human tyrosinase (Tyr), leading to an absence of pigment in skin, hair, and eyes, while OCA1B has reduced Tyr activity and pigment. Here, we used computational methods to find molecular and bioenergetic differences of Tyr in OCA1A and OCA1B.
Methods :
Tyr (residues 19-469 of the native protein) was built by homology modeling, glycosylated in silico, minimized using molecular dynamics in water, and subjected for the global computational mutagenesis to ascertain the DDG values of folding. In total, 19 known mutations of Tyr leading to OCA1A or OCA1B were simulated for 100 ns. OCA1A mutant simulations were run with and without dioxygen binding copper. The DDG values of folding were used to create an unfolding parameter that defined the predicted effect of the mutation as weak, moderate, or severe. Global mutagenesis was conducted to identify residues critical to protein folding.
Results :
The simulation results indicated large differences in the size of each OCA1B mutant when compared with the wild type. The DDG values of folding showed a positive correlation with differences in cavity volume at 100 ns between OCA1B mutations and Tyr with a Pearson’s Coefficient of 0.724. The solvent accessible volume has a trend to increase with the severity of the mutation with a slope of 110.59 Å2mol/kcal as described by the unfolding parameter . Simulations of OCA1A mutants with dioxygen did not suggest expanding or unfolding patterns leading to a full loss of activity. OCA1A mutant simulations without dioxygen were unable to form the alpha helix bundle, interpreted as a full loss of Tyr activity due to denaturation.
Conclusions :
OCA1A mutant simulations without dioxygen binding copper indicate that a full loss of Tyr function could stem from a disruption of the metal-ion protein interaction. Folding of Tyr could fail to incorporate dioxygen. The copper atoms are no longer bound tightly, thereby unfolding the substrate binding site, and causing a full loss of Tyr activity. In patients with a known genotype, the unfolding parameter may help identify the severity of OCA1 by characterizing the amount of lost pigment.
This is a 2020 ARVO Annual Meeting abstract.