Catalase-peroxidase haem <p>Haem-containing catalase-peroxidases are bifunctional antioxidant enzymes that exhibit both catalase (<db_xref db="EC" dbkey="1.11.1.6"/>) and peroxidase (<db_xref db="EC" dbkey="1.11.1.7"/>) activity, and which are present predominantly in bacterial species [<cite idref="PUB00053578"/>]. Several evolutionary lineages are present also in archaeal, fungal, and protistan species. These enzymes provide protection against oxidative stress by dismutating hydrogen peroxide to oxygen and water [<cite idref="PUB00053579"/>]. Phylogenetically they are closely related to ascorbate peroxidases and cytochrome c peroxidases [<cite idref="PUB00000617"/>] and can be divided in two distinct clades [<cite idref="PUB00053580"/>]. They do not share sequence similarity with mono-functional, haem-containing catalases (<db_xref db="INTERPRO" dbkey="IPR002226"/>) that are ubiquitous in aerobic organisms, nor with non-haem manganese-containing catalases found in bacteria (<db_xref db="INTERPRO" dbkey="IPR007760"/>). Catalases perform a unique two-step reaction cycle that cleaves two hydrogen peroxide molecules heterolytically to alternately oxidise and reduce the haem iron thus releasing water and molecular oxygen [<cite idref="PUB00053579"/>]. Contrary, peroxidases use hydrogen peroxide only to oxidise the haem iron, but use different electron donors such as NADH or ascorbate to then reduce the haem.</p><p>The structure of the catalase-peroxidase from the archaeon, <taxon tax_id="2238">Haloarcula marismortui</taxon> (Halobacterium marismortui), reveals a dimer of two identical subunits [<cite idref="PUB00015058"/>], although some catalase-peroxidases can exist also as homotetramers. The general topology, as well as the arrangement of the catalytic residues and haem in the active site, are similar to other class I peroxidases. However, the location of the haem group deeply buried inside the domain is typical of a catalase. The primary structure of the subunit can be divided into two similar halves, which very probably arose from a gene duplication event [<cite idref="PUB00000617"/>, <cite idref="PUB00053582"/>]. A similar structure was obtained also for a catalase-peroxidase from the proteobacterium Burkholderia pseudomallei [<cite idref="PUB00027476"/>]. </p>