Rational design of a superoxide dismutase into thioredoxin

The rational design of proteins is a new research frontier in protein science. The ability to construct new catalytically active proteins is one of the ultimate goals of biological research. Towards this end, we have successfully incorporated a catalytically active superoxide dismutase site into E. coli thioredoxin which served as the host protein scaffold (TrxSOD). The designed protein incorporates one Fe ion per active site upon reconstitution and catalyzes the disproportionation of superoxide at a rate of 10 5 M-1s-1 at physiological pH.; The focus of the second half of the design cycle is the study of the active site characteristics and catalytic mechanism of the designed protein using various spectroscopic methods. This information is necessary to accurately evaluate the design and gain information for further refinement of the construct. Studies including electron paramagnetic resonance (EPR), stopped-flow and X-Ray absorption spectroscopy (XAS) indicated the presence of a monomer to dimer equilibrium for reconstituted Fe(III)-TrxSOD, in which the dimer is the dominate form. Equivalent studies performed on reduced Fe(II)-TrxSOD suggest the protein adopts a monomeric form. Since the dimer form is catalytically inactive, the formation of dimer in Fe(III)-TrxSOD might account for the reduced catalytic activity. Potentiometric titration showed the designed FeTrxSOD protein has a depressed redox potential compared with that of the wild type FeSOD. This can be remedied in future design by re-constructing the secondary environment in the host protein.; In addition to Fe, Mn(II) was also successfully incorporated into the same coordination site. Mn-TrxSOD displays similar optical and EPR features as wild type Mn-SOD; it is also catalytically active, although with a lower activity than that of the Fe-TrxSOD. Wild type Fe-SOD and Mn-SOD belong to the same protein family, with almost identical active site geometries and superimposable tertiary structures, yet both proteins are highly metal specific. The comparison of the Fe and Mn reconstituted Trx-SOD suggests that the metal specificity of the wild type SODs might be originated at least partially from the small discrepancies between the secondary environment of their active sites.