Steady-State Kinetics of a-Synuclein Ferrireductase Activity Identifies the Catalytically Competent Species

a-Synuclein (alpha-syn) is a cytosolic protein known for its association with neurodegenerative diseases, including Parkinson's disease and other synucleinopathies. The potential cellular function of a-synuclein may be of consequence for understanding the pathogenesis of such diseases. Previous work has suggested that a-synuclein can catalyze the reduction of iron as a ferrireductase. We performed a detailed analysis of the steady-state kinetics of recombinant a-syn ferrireductase activity and for disease associated variants. Our study illustrates that the ferrireductase activity we observed is clearly commensurate with bona fide enzyme activity and suggests a mechanistic rationale for the activity and the relationship to cellular regulation of the pool of Fe(III) and Fe(II). Using cell-based studies, we examined the functionally active conformation and found that the major catalytically active form is a putative membrane-associated tetramer. Using an artificial membrane environment with recombinant protein, we demonstrate that secondary structure folding of a-synuclein is insufficient to allow enzyme activity and the absolute specificity of the tertiary/quaternary structure is the primary requirement. Finally, we explored the steady-state kinetics of a range of disease alpha-synuclein variants and found that variants involved in neurodegenerative disease exhibited major changes in their enzymatic activity. We discuss these data in the context of a potential disease-associated mechanism for aberrant a-synuclein ferrireductase activity.