THE AUTO-REDUCIBLE CYTOCHROMES-C OF THE METHYLOTROPHS METHYLOPHILUS-METHYLOTROPHUS AND PSEUDOMONAS AM1

The 2 types of soluble cytochrome c (cytochrome cH, cytochrome cI) found in methylotrophs (including M. methylotrophus and Pseudomonas AM1) are completely distinct proteins; one type is not a dimer or degradation product of the other. Free thiol groups are probably not involved in the usually rapid autoreduction of the cytochromes at high pH. The axial ligands to the heme Fe, histidine and methionine, are the same as in other low-spin cytochromes c. The methionine ligand is displaced at high pH by an alternative strong-field ligand. This displacement does not occur on reduction of cytochorme cL by methanol dehydrogenase, but this does not rule out the possibility that the autoreduction mechanism is involved in the interaction of the dehydrogenase and cytochrome c.