Description

Catalase or hydroperoxidase II  (Kat or HPII): Catalase is haem-containing enzyme able to dismutate H₂O₂.

It is a key enzyme in protecting all types of organisms against reactive oxygen species. It is controlled as part of the sS regulon in bacteria. Besides hydrogen peroxide and aliphatic peroxides it can oxidise certain 2-electron acceptors. Catalase catalyse the degradation of H₂O₂ via the two-stage mechanism.

One hydrogen peroxide molecule oxidizes the heme (Por-Fe^III) and the second hydrogen peroxide molecule is utilized as a reductant of compound I (Por^+.-Fe^IV = O) to regenerate the initial enzyme state.
Enz (Por-Fe^III) + H₂O₂ → Cpd I (Por^+.-Fe^IV = O) + H₂O
Cpd I (Por^+.-Fe^IV= O)  + H₂O₂ → Enz (Por-Fe^III) + H₂O + O₂

Heme-containing catalases are subdivided based on their size: large (> 75 kDa) or small (< 60 kDa) subunits. Phylogenetic analyses have demonstrated the existence of two distinct clades or subgroupings of small subunit enzymes and one clade of large subunit enzymes among the monofunctional catalases (Nicholls et al., 2004; Zamocky et al., 2008).

Further two evolutionary convergent families of catalase can be found:

bifunctional haem-containing peroxidase/catalases (CP) and monofuctional non-haem manganese-containing catalases (MnCat).

The monofunctional (typical) catalase (Kat) is detected in all aerobic and facultative aerobic organisms. The number of kat-isoforms in genomes can vary from 1 to 3 copies.

Kat-Lipoxyxenase KatLox is fusion protein detected marginally in 2 Metazoa and 1 cyanobacteria. N-terminal encodes oxide synthase domain - similar to catalase and C-terminal encodes 8R-lipoxygenase domain belonging to the lipoxygenase family.

This protein converts arachidonic acid to an allene oxide with an 8R-hydroperoxide intermediate. Kat and Lox domains have also been detected in Plesiocystis pacifica (proteobacteria): the two domains are in tandem (2 frames) and in inverse orientation.