“Metal selectivity of the single superoxide dismutase of Rhodobacter capsulatus

DI CAPUA, CECILIA BEATRIZ; CORTEZ, NESTOR RICARDO

Buzios (Brasil)

Congreso; VII Iberoamerican Congress of Biophysics; 2009

Sociedad Brasilera de Biofísica y Sociedad Argentina de Biofísica

Superoxide dismutases (SODs) are metalloenzymes involved in the defense against oxidative stress by catalyzing the dismutation of the radical anion superoxide. The Fe/Mn SOD family exhibits high conservation in sequence and spatial structure, being able to bind any metal cofactor Fe or Mn, However,  FeSODs and MnSODs attain significant activity with only one of the two metals, with the exception of a small group termed cambialistic that is able to display activity with either Fe or Mn as cofactor. The phototrophic bacterium R. capsulatus encodes a single cambialistic SOD (RcSOD) that preferentially binds Mn. However, metal discrimination depends on the culture conditions, with Fe fractions increasing from 7% in aerobic cultures up to 40% in photosynthetic cultures. To characterize the metal selectivity and the capture process we performed heterologous expression of this protein in E.coli cultured under different culture conditions, finding a correlation between metal availability and the identity of the cofactor of the recombinant RcSOD.   Studies of metal capture on the apoenzyme revealed that this process is thermally triggered in vitro, suggesting a conformational gating of the metal uptake induced by the observed temperature  transition. Surprisingly, when thermal stability was studied using CD spectroscopy, the Fe-RcSOD was shown to be always the more stable metallated form   Altogether, our results suggest that besides metal availability, kinetic factors rather than thermodynamic ones are involved in selectivity during metal capture in Rhodobacter capsulatus SOD