Pseudomonas denitrificans cytochromes

Amicyanins function as electron carriers in the respiratory chains of some me-thylotrophic bacteria, e.g., Thiobacillus versutus [92]. They transfer single electrons from methylamine dehydrogenase to a cytochrome c [78] which then transfers the electron to cytochrome c oxidase. Amicyanin from Pseudomonas denitrificans has a molecular mass of 11.6 kD and contains 106 amino acid residues. Amicyanin contains one /J-sheet more than the eight of plastocyanin and pseudoazurin, the result of several additional amino acids at its N-terminus [78] (Fig. 15). Like pseudoazurin, amicyanin is found exclusively in bacteria. 

The best understood system of dissimilatory nitrate metabolism is that of Pseudomonas denitrificans, with the proposed transport scheme shown in Fig. 39 (406 408). The nitrogen intermediates shown between nitrate and Nz are those which have been isolated and identified with a particular reductase enzyme. There remains the possibility of additional intermediates in the form of transient oxidation states of nitrogen. The single electron chain of Oz respiration here has been replicated into parallel pathways, each with the general form donor 6 —> c — reductase acceptor. On the NzO reductase path, the particulate cytochrome Cb52... 

A second proteinaceous fraction, nitric oxide reductase, containing a bound c-type cytochrome converts nitric oxide to nitrous oxide (Matsubara and Iwasaki, 1971 Payne et al., 1971 Cox and Payne, 1973). The enzyme is soluble in Pseudomonas perfectomarinus (Cox and Payne, 1973), but is particulate in Alcaligenes faecalis and Pseudomonas denitrificans (Matsubara and Iwasaki, 1971 Miyata et al., 1969). Epr studies indicate no metal involvement but the formation of a different heme nitric oxide complex during release of nitrous oxide (Payne et al., 1971). The physiological electron donors are not known. FADHa, reduced phenazine methosulphate, and reduced viologen dyes have been found to be effective (Thauer et al., 1977). 

Little is known of the catalytic properties of cytochrome b investigators must rely on the spectrum characterization of cytochrome b rather than on its enzymic activity to evaluate the purity of the preparation. Little is known of the heme group of mammalian cytochrome b. It is significant that cytochrome b extracted from Micrococcus denitrificans and Pseudomonas denitrificans contains protoheme, the same heme as is in cytochrome c. The nature of the binding between the prosthetic group and the protein is not known. The firm thioester bond in cytochrome c probably is not present in cytochrome b. 

Cytochrome c peroxidase may also be isolated from Pseudomonas spp. and differs from the yeast enzyme described above. Pseudomonas cytochrome c peroxidase contains two covalently bound heme c moieties in a single polypeptide chain, of molecular weight 50 000. The enzyme from Ps. denitrificans has a molecular weight of 63 000 with two heme c groups.1370... 

This is a remarkable reaction because the transition metal chemistry of N2O is sparse, especially with copper. Most N2O reductases are soluble, periplasmic homodimers however, there are examples of membrane-associated enzymes. " The best characterized N2O reductases are from Paracoccus denitrificans, Pseudomonas nautica, and Pseudomonas stutzeri, and most of the information presented here is derived from experiments on these enzymes. Where comparable data are available, N2O reductases from various organisms appear to be fairly similar, with the exception of the enzyme from Wolinella succinogenes, as noted above. The crystal stractmes of N2O reductase from P. nautica and more recently from P. denitrificans show two distinct copper clusters per subunit a bis-thiolate bridged dinuclear electron-transfer site (Cua), which is analogous to the Cua site in cytochrome c oxidase see Cyanide Complexes of the Transition Metals), and a novel four-copper cluster ligated by seven histidines, the catalytic copper site (Cuz), where N2O is thought to bind and be reduced. Cuz was proposed to be a copper-histidine cluster on the basis of the presence of nine strictly conserved histidine residues, and this was supported by a H NMR study that identified two non-CuA associated resonances that were assigned as copper-histidine N-H protons. ... 

Iwata S, Ostermeier C, Ludwig B, Michel H (1995) Structure at 2.8A resolution of cytochrome c oxidase from Paracoccus denitrificans. Nature 376 660-669 Jannasch HW, Nelson DC, Wirsen CO (1989) Massive natural occurrence of unusually large bacteria (Beggiatoa sp.) at a hydrothermal deep-sea vent site. Nature 342 834—836 Jetten MSM, de Bruijn P, Kuenen JG (1997) Hydroxylamine metabolism in Pseudomonas PB16 involvement of a novel hydroxylamine oxidoreductase. Antonie van Leeuwenhoek. 71 69-74... 

Nitrite reduction to nitric oxide is catalyzed by dissimilatory nitrite reductase. The enzymes purified from Alcaligenes faecalis (Iwasaki and Matsub-ara, 1971), Pseudomonas aeruginosa (Walker and Nicholas, 1961), andMj-crococcus denitrificans (Newton, 1969) have been shown to contain c d-type cytochrome. The nitrite reductase from Achromobacter cycloclastes does not... 

There are two main types of NiRs involved in the reduction of nitrites, namely, the heme-containing cytochrome cdj NiR which was obtained and first purified from Thiosphaera pantotropha (261). The second kind of NiR is the copper-containing NiR which was first isolated from Alcaligenes xylosoxidans NCIB 11015, a bacterial isolated from a soil in Japan. Other Cu NiR have been isolated from, Achromohacter cycloclastes, Alcaligenes faecalis S-6, Bacillus halodenitrificans, Haloferax denitrificans, Nitrosomonas europaea, Pseudomonas aureofaciens, Rhodobacter sphaeroides, and Hyphomicrobium sp. (262 and references thereinj. In mammalian systems, nitrites are reduced by deoxyHb (263) and by ferrous myoglobin (264,265) to nitric oxide. In synthetic iron porphyrins. Ford and coworkers have demonstrated how nitrites inhibit the reductive nitrosylation process by forming ferric-nitrites species (266). 

Heme di (107), which was isolated by Timkovich et al (70) and Chang et al (71) occurs as one of two cofactors in the reductase cytochrome cd. Cytochrome cd participates in the reduction of nitrite to nitrous oxide (N2O) in chemoautotrophic bacteria, such as Pseudomonas aeruginosa, Paracoccus denitrificans, and Thiobacillus denitrificans (13). From recent investigations it seems very likely that cytochrome cdi mediates the nitrite reduction to nitric oxide (NO) and that a second enzyme produces N2O from NO (13). A structure was... 

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