This review gives an overview about the structural organisation of different evolutionary lines of all enzymes capable of efficient dismutation of hydrogen peroxide. Major potential applications in biotechnology and clinical medicine justify further investigations. According to structural and functional similarities catalases can be divided in three subgroups. Typical catalases are homotetrameric haem proteins. The three-dimensional structure of six representatives has been resolved to atomic resolution. The central core of each subunit reveals a characteristic "catalase fold", extremely well conserved among this group. In the native tetramer structure pairs of subunits tightly interact via exchange of their N-terminal arms. This pseudo-knot structures implies a highly ordered assembly pathway. A minor subgroup ("large catalases") possesses an extra flavodoxin-like C-terminal domain. A > or = 25 A long channel leads from the enzyme surface to the deeply buried active site. It enables rapid and selective diffusion of the substrates to the active center. In several catalases NADPH is tightly bound close to the surface. This cofactor may prevent and reverse the formation of compound II, an inactive reaction intermediate. Bifunctional catalase-peroxidase are haem proteins which probably arose via gene duplication of an ancestral peroxidase gene. No detailed structural information is currently available. Even less is know about manganese catalases. Their di-manganese reaction centers may be evolutionary.

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Five single replacement mutants of catalase A fromSaccharomyces cerevisiae were prepared (F148V, F149V, F156V, FI59V, and VIllA). The exchanges were expected to relieve steric constraints in the lowest part of the major substrate channel. The overall stability of the isolated enzymes is unaffected by the respective amino acid exchanges, but some modifications lead to decreased protohaem binding. AH isolated mutants (most pronounced the VillA-species) show decreased catalatic and markedly increased peroxidatic activity, both with aliphatic and aromatic substrates. These effects can in part be explained by steric effects, but also reveal destabilisation of compound I.

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