Semiholoenzyme optimizes activity and stability of a hyperthermostable iron-superoxide dismutase

Metal ion coordination is an essential step for the maturation of metalloenzymes. Generally, the metal coordination sites are thought to be fully occupied to achieve the maximum activity and stability. In this research, we compared the structural features, activity and stability of the apo-, semiholo- and holo-forms of a hyperthermostable tetrameric Fe-superoxide dismutase (SOD). Strikingly, the three forms of enzymes had similar compact tetrameric structures. Removal of iron ions destabilized subunit-subunit interactions during guanidine hydrochloride-induced unfolding. The partially metalized semiholoenzyme possessed most of the activity and identical hyperthermostability of the holoenzyme, but weaker propensity to aggregate. Furthermore, both of the iron content and activity of the semiholoenzyme were unaffected by a 200-fold excess iron ions in solutions, suggesting that conformation of the apo-subunits were forced to the close state by the iron-containing subunits. These observations suggest that fully metalized enzyme is probably nonessential for multimeric metalloenzymes and the semiholoenzyme may be a better choice. The unique properties of semiholoenzyme also provide the organisms a compromised solution to survival under metal deficiency conditions.