Small-angle neutron scattering from native and irradiated superoxide dismutase in aqueous solution.
The approximate size and shape of holo and apo forms of bovine cupro-zinc erythrocyte superoxide dismutase (EC 1.15.1.1) were determined by small-angle neutron scattering from aqueous solutions at neutral pH. A model assuming a cylindrical shape gave the best fit to the data for both forms of the enzyme. The radius of gyration, Rg, of the apoenzyme was found to be marginally larger than that of the holoenzyme. Scattering from the protein vanished for H2O/2H2O mixtures containing 42(+/- 2)% 2H2O, and the negligible dependence of Rg on the 2H2O fraction indicated uniform scattering density. Irradiation with 60Co gamma-rays resulted in aggregation of superoxide dismutase molecules; scattering at small doses was interpreted in terms of pairwise side-by-side aggregation. For large doses (approx. 3.8 X 10(3) Gy) and at relatively high enzyme concentrations (320 microM), the interpretation of the neutron scattering data is ambiguous. The value of Rg suggests that end-to-end stacking of the cylindrical molecules is one possibility. The equilibrium concentration of separated subunits was found to be unobservable and calculations showed that denaturation did not contribute significantly to our neutron scattering measurements for the radiation doses employed (maximum 3.8 X 10(5) Gy). High-performance liquid chromatography (HPLC) data showed that in mixtures the apo and holo forms of superoxide dismutase interact with one another, and that the side-by-side aggregates, induced by irradiation of the enzyme, are readily dissociated, resulting in a single elution peak that is resolved from the later peak common to both the apo and holo forms.
D000818:Animals; D001051:Apoenzymes / Q000097:blood; D002417:Cattle; D002851:Chromatography, High Pressure Liquid; D004912:Erythrocytes / Q000201:enzymology*; D009502:Neutrons; D011487:Protein Conformation; D012542:Scattering, Radiation; D013482:Superoxide Dismutase / Q000097:blood* / Q000528:radiation effects
D001051:Apoenzymes; D013482:Superoxide Dismutase