Enzymes involved in the oxidative stress in Sulfolobus solfataricus

The present article summarises our recent work carried out on enzymes involved in the oxidative stress in the hyperthermophilic archaeon Sulfolobus solfataricus. The functional role of three key enzymes, superoxide dismutase (SOD), thioredoxin reductase (TrxR), and NADH oxidase (NOX) has been investigated. The study included a detailed structure–function relationship on these ubiquitous proteins through the characterisation of their molecular and functional properties, the comparison with the features of the corresponding enzymes isolated from taxonomically different sources, and a mutagenic analysis on some specific amino acid residues. S. solfataricus SOD belongs to the family of Fe- and Mn-SOD and possesses a very compact homotetrameric structure, responsible for its great heat resistance. The mutagenic analysis regarded two interacting residues of the active site, namely a conserved tyrosine and a semi–invariant histidine. The data on the tyrosine residue point to its relevance in the catalytic mechanism of superoxide dismutation, and indicate its high reactivity towards modifying agents; vice versa, the histidine residue is important for the structural architecture of the active site. S. solfataricus TrxR, an enzyme previously isolated for its ability to oxidise NADH, belongs to the family of class II pyridine nucleotide–disulphide oxidoreductases. It possesses two cysteine residues in the active site, forming a disulphide bridge essential for both reductase and NADH oxidase activities, a finding supported also by mutagenic analysis. Furthermore, inhibition studies indicate that S. solfataricus TrxR, structurally similar to eubacterial counterparts, is functionally closer to eukaryal TrxR. S. solfataricus NOX is a homodimeric flavo-enzyme able to oxidise both NADH and NADPH, and belongs to the NOX family forming H2O2 as end product of the reaction. The lack of cysteines in its primary structure indicates that the electron transfer from NAD(P)H to molecular oxygen does not involve a disulphide bridge. The results of this investigation indicate that Sulfolobus solfataricus possesses efficient enzyme systems for the protection against oxidative stress. In particular, SOD and TrxR from this archaeal source share similar functional and molecular properties with their mitochondrial counterparts, thus supporting the hypothesis that the Sulfolobus genus is the putative ancestor of animal mitochondria. Vice versa, the properties of S. solfataricus NOX confirm that this enzyme belongs to a protein family scarcely conserved during evolution; the data also suggest a possible involvement of this enzyme in the protection against oxygen toxicity.