A close look at the electrostatic properties of Cu, Zn-Superoxide dismutase.


Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease. Mutations in the gene encoding Cu, Zn-superoxide dismutase (SOD1) are responsible for 1-2% of ALS cases. Numerous studies have proved that SOD1 forms neurotoxic aggregates, which add toxicity to motor neurons through a widely accepted “gain of function” mechanism. The electrostatic potential is an overlooked important biophysical property that can affect the aggregation propensity of SOD1. Protein net charge, a good representative of the electrostatic surface potential, is tightly regulated by a network of solvent accessible ionizable amino acid residues and coordinated small molecules such as water and metal ions. Few tools exist to detailed study the electrostatic potential of proteins, however, in this dissertation, we introduced capillary electrophoresis in conjunction with protein charge ladders to i) experimentally measure the net charge of WT and ALS-variant mutant SOD1 under various conditions, and ii) investigate the effect of electrostatic potential on the interaction between SOD1 and sodium dodecyl sulfate (SDS) molecules. Also, we predict and detect the deamidation of asparagine residues of Cu, Zn-SOD1 (human erythrocytes). With our novel tools, we successfully detect this sub-Dalton post-translational modification (< 1 Da) in aged Cu, Zn-SOD1 purified from human erythrocytes. The deamidation of SOD1 was proved to produce an ALS mutant analog which shares similar biochemical and biophysical properties to ALS mutant N139D. This solvent catalyzed spontaneous deamidation of SOD1 is incentive for understanding the mechanism of sporadic ALS. Finally, we study the effect of an external electric field on the structure of cytosolic proteins, and found that the electric field can induce the monomerization of metal replete A4V SOD1 but not apo form or that of the WT SOD1. We hypothesize that this abnormal monomerization on a relatively thermally stable protein is due to the induced rotational torque generated by the applied external electric field on the preexisted macrodipole of each subunit of SOD1. Thus, in this study, we are the first, to the best of our knowledge, to investigate the effect of a physiologically relevant electric field on the structure of a cytosolic protein.

ALS. SOD1. Electrostatic potential.