Cytochrome c '' from the obligate methylotroph Methylophilus methylotrophus, an unexpected homolog of sphaeroides heme protein from the phototroph Rhodobacter sphaeroides

The complete primary structure of an unusual soluble cytochrome c isolated from the obligate methylotrophic bacterium Methylophilus methylotrophus has been determined to contain 124 amino acids and to have an average molecula r mass of 14 293.0 Da. The sequence has two unusual features: firstly, the location of the heme-binding cysteines is far downstream from the N-terminu s, namely at positions 49 and 52; secondly, an extra pair of cysteine resid ues is present near the C-terminus. In both respects, cytochrome c" is simi lar to the oxygen-binding heme protein SHP from the purple phototrophic bac terium Rhodobacter sphaeroides. In contrast to SHP, cytochrome c" changes f rom low-spin to high-spin upon reduction, due to dissociation of a sixth he me ligand histidine which is identified as His-95 by analogy to the class I cytochromes c. The distance of His-95 from the heme (41 residues) and the presence of certain consensus residues suggests that cytochrome C" is the s econd example of a variant class I cytochrome c. (C) 1999 Elsevier Science B.V. All rights reserved.