STRUCTURE OF A MONOCLINIC CRYSTAL FORM OF CYTOCHROME B1 (BACTERIOFERRITIN) FROM ESCHERICHIA-COLI

Crystals of E. coli cytochrome bl, alias bacterioferritin, were grown from a low ionic strength solution. The resulting monoclinic P2(1) str ucture was solved by molecular replacement and refined using noncrysta llographic symmetries applied to the fundamental unit, consisting of t wo protein subunits and a single haem. From the Patterson self-rotatio n results it was shown that the asymmetric unit of the monoclinic crys tal consists of 12 such dimers and corresponds to a complete, nearly s pherical, molecule of bacterioferritin (M-r = 450 kDa) of 432 point-gr oup symmetry. It is thus the most symmetrical cytochrome. As previousl y determined for the tetragonal form, the haem is located in a special position on a local twofold axis of the dimer. A bimetal centre is al so observed within the four-helix bundle of each monomer; a metal-bind ing site is located on the fourfold axis.