Characterization of the periplasmic cytochromes c of Paracoccus denitrificans: identification of the electron acceptor for methanol dehydrogenase, and description of a novel cytochrome c heterodimer

Abstract

Summary: This paper describes periplasmic c-type cytochromes from two strains of Paracoccus denitrificans NCIB 8944 grown in heterotrophic or methylotrophic conditions. It is shown that the functions of two monomeric, monohaem cytochromes induced during growth on methanol have been wrongly designated in previous work. The CO-reactive cytochrome c ₅₅₃ (30 kDa) is not the electron acceptor for methanol dehydrogenase; this is shown to be the role of the cytochrome c ₅₅₂ (22 kDa). The monomeric 45 kDa cytochrome induced in conditions of oxygen insufficiency is a dihaem c-type cytochrome and does not contain haem b as previously assumed. In addition to these cytochromes, the Oxford strain of NCIB 8944 contains two cytochrome c complexes. One of these (150 kDa), produced in relatively small amounts, consists of a non-haem protein plus four haemoproteins (28, 33, 41 and 47 kDa). The second complex is a novel dimeric multi-haem cytochrome c (46 kDa) which constitutes about 25% of the periplasmic c-type cytochrome. It reacts with CO and has no methionine ligands. One subunit (16 kDa) has two low-spin haems; the larger subunit (30 kDa) has three haems which have low-spin characteristics in the oxidized state and are high-spin in the reduced state. The subunits were readily separated at pH 12 and could be subsequently reconstituted into a complex indistinguishable from the original. The 30 kDa subunit was denatured on prolonged exposure to high pH, which also converted it to a low-spin cytochrome. No function could be designated for these novel c-type cytochrome complexes.