A new scaffold for binding haem in the cytochrome domain of the extracellular flavocytochrome cellobiose dehydrogenase

Background: The fungal oxidoreductase cellobiose dehydrogenase (CDH) degrad es both lignin and cellulose, and is the only known extracellular flavocyto chrome. This haemoflavoenzyme has a multidomain organisation with a b-type cytochrome domain linked to a large flavodehydrogenase domain. The two doma ins can be separated proteolytically to yield a functional cytochrome and a flavodehydrogenase. Here, we report the crystal structure of the cytochrom e domain of CDH. Results: The crystal structure of the b-type cytochrome domain of CDH from the wood-degrading fungus Phanerochaete chrysosporium has been determined a t 1.9 Angstrom resolution using multiple isomorphous replacement ncluding a nomalous scattering information. Three models of the cytochrome have been r efined: the in vitro prepared cytochrome in its redox-inactive state (pH 7. 5) and redox-active state (pH 4.6), as well as the naturally occurring cyto chrome fragment. Conclusions: The 190-residue long cytochrome domain of CDH folds as a beta sandwich with the topology of the antibody Fab V-H domain. The haem iron is ligated by Met65 and His 163, which confirms previous results from spectro scopic studies. This is only the second example of a b-type cytochrome with this ligation, the first being cytochrome b(562). The haem-propionate grou ps are surface exposed and, therefore, might play a role in the association between the cytochrome and flavoprotein domain, and in interdomain electro n transfer. There are no large differences in overall structure of the cyto chrome at redoxactive pH as compared with the inactive form, which excludes the possibility that pH-dependent redox inactivation results from partial denaturation. From the electron-density map of the naturally occurring cyto chrome, we conclude that it corresponds to the proteolytically prepared cyt ochrome domain.