Bacterial cytochrome c and

Cytochrome Ch is similar in most respects to other typical bacterial cytochromes c and C2- However its X-ray structure (Read et al., 1999) shows a number of unusual features it bears a closer gross resemblance to mitochondrial cytochrome c than to the bacterial cytochrome C2 and the left hand side of the haem cleft is unique. In particular it is highly hydrophobic, the usual water is absent, and the iconservediTyr67 is replaced by tryptophan. A number of features of the structure demonstrate that the usual hydrogen bonding network involving water in the haem channel is not essential, and that other mechanisms for modulation of redox potentials may exist in this cytochrome. It should be emphasised that the unique character of this cytochrome does not appear to be related in any way to its special involvement in oxidising cytochrome Ci in the methylotroph electron transport chain. 

Two amicyanins from the methylotropic bacteria Pseudomonas AMI and T. versutus have been a recent focus of attention (42, 43). Their function is to mediate electron transfer between bacterial cytochrome c and methylamine dehydrogenase in a relatively short electron transport chain. 

Such an involvement of an amino acid side-chain ligand switch within each catalytic cycle was a novel proposal and as such needs to be scrutinized by a variety of experimental procedures as well as analysis in the context of information known for cytochrome cd nitrite reductase from another source (see later discussion). However, it is interesting to note that something similar has been proposed for the protocate-chuate 3,4-dioxygenase enzyme from Pseudomonas putida (15). On the other hand, bacterial cytochrome c peroxidase offers an example where ligand switching seemingly relates only to an activation phenomenon. 

It is nearly 50 years since a c-type cytochrome was shown to catalyze peroxidase activity in crude extracts of Pseudomonas fluorescens (40). The enzyme responsible was first purified some 20 years later by Ell-folk and Soninen from the closely related P. aeruginosa and shown to be a diheme cytochrome c peroxidase (CCP) (41). These bacterial diheme CCPs are quite distinct from the superfamily of plant and yeast peroxidases (42) and are widely distributed among the Gram-negative bacteria (41, 43 6). Diheme CCPs are located in the periplasm (Fig. 2), where they catalyze the two-electron reduction of H2O2 to H2O by soluble one-electron donors such as cytochromes c and cupredoxins. 

Blom was the first to demonstrate, in 1928, the formation of HA by an unknown mixture of bacteria which utilized nitrate as their sole nitrogen source to produce ammonia , an observation substantiated by Lindsey and Rhines who generalized this reaction to a diverse set of microorganisms capable of producing NH3 by reduction of both nitrites and nitrates. The mechanism of the 6-electron reduction of nitrite to ammonia (i.e. conversion of the [N + 02] species to by bacterial cytochrome c nitrite reductase... 

A different type of concerted reaction involves the bacterial cytochrome c peroxide, where two hemes are coupled together, so that hydrogen peroxide undergoes a two-electron reduction to water without the formation of radical species. In a number of dioxygenases, dioxygen is reduced to peroxide by concerted electron transfer from [2Fe-2S] and non-heme Fe11 centres. 

Cytochromes from bacterial, yeast, and mammalian sources have been investigated by Mossbauer spectroscopy (114—117). Horseheart cytochrome c and the c-type cytochrome from T. utilis show spectra characteristic of low-spin Fe(III) in the oxidized form of the protein and low-spin Fe(II) for the reduced form of the protein. Lang et al. (115) have analyzed the Mossbauer data in terms of a low-spin Hamiltonian in some detail. Cooke and Debrunner (116) present quadrupole data on dehydrated forms of oxidized and reduced cytochrome c the quadrupole splittings for hydrated and dehydrated forms of the reduced protein are quite similar in contrast to a difference of the oxidized form. No spin-state change is reported for either form of cytochrome c. 

Homologues of mitochondrial bc complex are found in photosynthetic bacteria and other prokaryotes. Some bacterial foC] complexes contain only the three redox-active subunits, cytochrome b, cytochrome C], and the Rieske protein. Another bc homologue, bgE complex in chloroplasts and cyanobacteria, has a shorter cytochrome bg, cytochrome E, Rieske protein and a subunit 4. The absence of the extra subunits in the bacterial complexes seems to indicate that the non-redox subunits present in the mitochondrial bc complex are generally not directly involved in electron transfer and proton translocation per se. 

Hosier, J. P., Ferguson-Miller, S., Calhoun, M. W., Thomas, J. W., Hill, J., Lemieux, L., Ma, J., Georgiou, C., Fetter, J., Shapleigh, J., Tecklenburg, M. M. J., Babcock, G. T., and Gennis, R. B., 1993, Insight into the active site structure and function of cytochrome oxidase by analysis of site-directed mutants of bacterial cytochrome aaj and cytochrome bo, J. Bioener. Biomemhr. 25 121nl36. 

Ostermeier, C., Iwata, S., Lubwig, B., and Michel, H., 1995, Fv fragment-mediated crystallization of the membrane protein bacterial cytochrome c oxidase, Nature Structural Biology, 2 842n846. 

The bacterial cytochromes c are known to contain five-coordinate heme c, with no distal residues present capable of hydrogen-bonding to endogenous ligands. 4159 Mogg and... 

Bacterial cytochromes c are either periplasmic or attached to the periplasmic side of the membrane, and this led to the suggestion by Wood that the covalent attachment of heme evolved as a way of preventing its loss from periplasmic proteins to... 

Photosynthetic reaction centers plug into the chemiosmotic scheme by using light-excited states to create both an oxidant and a reductant. For the purple bacterial reaction centers, these oxidants and reductants are the redox carriers already described, oxidized cytochrome c and reduced ubiquinone QH2. Thus, in combination with Complex III, light drives a relatively straightforward cyclic electron transfer that generates a transmembrane electric field and proton gradient. 

The most basic design requirements of the bacterial photosynthetic reaction centers then, are a light energy absorbing center and two chains of redox centers that connect this light-activatable center to cytochrome c and quinone on opposite sides of the membrane. The physical process by which electrons are transferred between members of the chains in reaction centers, and indeed in the vast majority of electron transfer proteins, was also revealed by Chance, together with Devault, in these same years [4]. 

We find also some interesting results from the data shown in Table 3.4. Comparing the amino acid sequence of cytochrome c from N. winogradskyi with those of cytochromes c from human, monkey, and tuna, the numbers of the different amino acids at the corresponding positions of the sequence are found to be 62, 61, and 59, respectively. The number of different amino acids in the sequence is 72 between yeast and the bacterial cytochrome c. From these data, the evolutionary distance between yeast and the bacterium is farther than that between the animals and the bacterium. The number of different amino acids in the sequence of cytochrome c is 44 between human and yeast cytochromes c. By simple analysis of the figures, we are led to a conclusion that the bacterium has appeared on Earth evolutionarily after human or that yeast has appeared after human. This is a mystery of molecular evolution that needs to be solved in future studies. 

A FIGURE 8-18 Molecular structure of the core of the cytochrome c oxidase complex in the inner mitochondrial membrane. Mitochondrial cytochrome c oxidases contain 13 different subunits, but the catalytic core of the enzyme consists of only three subunits I (green), II (blue), and III (yellow). The function of the remaining subunits (white) is not known. Bacterial cytochrome c oxidases contain only the three catalytic subunits. Hemes a and are shown as purple and orange space-filling models, respectively the three copper atoms are dark blue spheres. [Adapted from I Tsukihara et at, 1996, Science 272 1136.]... 

The three-dimensional folding of bacterial cytochromes ca and caso as described in refs. S0 31 and 36-38 is shown in Figs. 9 and 10, and the comparison of amino acid sequences with that of tuna c appears in Table VII, The sequence of csso was unknown prior to the X-ray analysis, but an alignment of c and ca sequences substantially identical to that of Table VII had been proposed by Dus et al. 33) when they reported the sequence of Cj from Rhodospirillum rubrum. Recently, the cz sequences from three rhodopseudomonads have been completed by Ambler and co-workers, and these have been included in Table VII for comparison. 

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