Crystal structure of recombinant trypsin-solubilized fragment of cytochrome b(5) and the structural comparison with Val61His mutant

The crystal structure of the recombinant trypsin-solubilized fragment of th e microsomal cytochrome b(5) from bovine liver has been determined at 1.9 A ngstrom resolution and compared with the reported crystal structure of the lipase-solubilized fragment of the membrane protein cytochrome b(5), The tw o structures are similar to each other. However, some detailed structural d ifferences are observed: the conformation of the segment Asn16-Ser20 is qui te different, some helices around the heme and some segments between the he lices are shifted slightly, the heme is rotated about the normal of the mea n plane of heme, one of the propionates of the heme exhibits a different co nformation. The average coordination distances between the iron and the two nitrogen atoms of the imidazole ligands are the same in the two structures . Most of the structural differences can be attributed to the different int ermolecular interactions which result from the crystal packing, The wild-ty pe protein structure is also compared with its Val6His mutant, showing that the heme binding and the main chain conformations are basically identical with each other except for the local area of the mutation site. However, wh en Val61 is mutated to histidine, the large side chain of His61 is forced t o point away from the heme pocket toward the solvent region, disturbing the micro-environment of the heme pocket and influencing the stability and the redox potential of the protein. (C) 2000 Wiley-Liss, Inc.