Biology Department, University of Massachusetts, Amherst, MA 01003, U.S.A.
Superoxide dismutases (SODs) have been conventionally classified into 2 highly conserved families: a Fe- (or Mn-) type characteristic of prokaryotic cytoplasm, and a Cu,Zn-type characteristic of eukaryotic cytoplasm, but which originated as an extra-cytoplasmic enzyme in alpha-Proteobacteria [1]. Previously, no sequence or structural similarity has been detected between the Fe- and Cu,Zn-types of SOD, which are presumed to have originated independently. Nonetheless, Thermoplasma acidophilum has a Fe,Zn-SOD that shares some features with both types [2]. The gene has been cloned and sequenced by Gupta et al. (McMaster University), and in sequence it is clearly of the Fe- SOD family. However, a distant but statistically significant alignment can be made with Cu,Zn-SOD sequences. Thus, the Thermoplasma enzyme can relate both families of SOD, suggesting that the enzyme originated only once, and before the Archaeal- Eubacterial divergence. Conjecturally, it may have entered eukaryotes with the mitochondrion, and was transformed from being an extracytoplasmic bacterial protein into an ubiquitous eukaryotic cytoplasmic protein.
[1] Imlay, K.R.C. and J. A. Imlay (1996) Cloning and analysis of
sodC, encoding the copper-zinc superoxide dismutase of
Escherichia coli. J. Bact. 178: 2564-2571.
[2] Searcy, K.B. and D.G. Searcy (1981) Superoxide dismutase from
the archaebacterium Thermoplasma acidophilum. Biochim.
Biophys. Acta 670: 39-46.
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