This is a family of distinct cytochrome c peroxidases (CCPs) that contain two haem groups. Similar to other cytochrome c peroxidases, they reduce hydrogen peroxide to water using c-type haem as an oxidisable substrate. However, since they possess two, instead of one, haem prosthetic groups, bacterial CCPs reduce hydrogen peroxide without the need to generate semi-stable free radicals. The two haem groups have significantly different redox potentials. The high potential (+320 mV) haem feeds electrons from electron shuttle proteins to the low potential (-330 mV) haem, where peroxide is reduced (indeed, the low potential site is known as the peroxidatic site) [1]. The CCP protein itself is structured into two domains, each containing one c-type haem group, with a calcium-binding site at the domain interface. This family also includes MauG proteins, whose similarity to di-haem CCP was previously recognised [2].

1: Crystal structure of the di-haem cytochrome c peroxidase from Pseudomonas aeruginosa. Fulop V, Ridout CJ, Greenwood C, Hajdu J; Structure 1995;3:1225-1233. PMID:8591033

2: Organization of methylamine utilization genes (mau) in 'Methylobacillus flagellatum' KT and analysis of mau mutants. Gak ER, Tsygankov YD, Chistoserdov AY; Microbiology 1997;143:1827-1835. PMID:9202457