C-type cytochromes

Seeliger S, R Cord-Ruwisch B Schink (1998) A periplasmic and extracellular c-type cytochrome of Geobacter sulfurreducens acts as a ferric iron reductase and as an electron carried to other acceptors or to partner bacteria. J Bacteriol 180 3686-3691. 

The reductase in Geobacter sulfurreducens is located in the outer membrane and a soluble Fe(III) reductase has been characterized from cells grown anaerobically with acetate as electron donor and Fe(III) citrate or fumarate as electron acceptor (Kaufmann and Lovley 2001). The enzyme contained Fe, acid-labile S, and FAD. An extracellular c-type cytochrome is distributed in the membranes, the periplasm, and the medium, and functions as a reductase for electron transfer to insoluble iron hydroxides, sulfur, or manganese dioxide (Seeliger et al. 1998). 

Kolb S, S Seeliger, N Springer, W Ludwig, B Schink (1998) The fermenting bacterium Malonomonas rubra is phylogenetically related to sulfur-reducing bacteria and contains a c-type cytochrome similar to those of sulfur and sulfate reducers. SystAppl Microbiol 21 340-345. 

Fang and Burris (63) isolated a c-type cytochrome from the cells of Hydrogenomonas eutropha. Type c cytochromes have been isolated from the cells of a number of other bacterial species (64) and a three dimensional model has been proposed for the cytochrome C551 of Pseudomonas aeru-... 

Teixeira, M., Campos, A.P., Aguilar, A.P., Costa, H.S., Santos, H., Turner, D.L., and Xavier, A.V. 1993. Pitfalls in assigning heme axial coordination by EPR. c-Type cytochromes with atypical Met-His ligation. FEBS Letters 317 233-236. 

Y. Sallez, P. Bianco, and E. Lojou, Electrochemical behavior of c-type cytochromes at clay-modified carbon electrodes a model for the interaction between proteins and soils. J. Electroanal. Chem. 493, 37-49 (2000). 

After reduction of the oxidised special pair by a c-type cytochrome, the energy of a second photon is used to transfer a second electron to QB ... 

By interesting coincidence, the old dogma that cytochromes are not present in anaerobes was demolished by discovery, at about the same time, of c-type cytochromes in Desulfovibrio and anoxygenic photosynthetic bacteria (Kamen and Vernon 1955). 

Pollock WBR, Voordouw G. 1994. Molecular biology of c-type cytochromes from Desulfovibrio vulgaris Hildenborough. Biochimie 76 554-60. 

In vitro studies established that electrons are transferred efficiently from both NiFe- and iron-only hydrogenases to cytochrome C3. This small 13 kDa c-type cytochrome has four hemes (reduction potential -200 to -300 mV)... 

Figure 8.2. The gene arrangement of the hmc operon in D. vulgaris Hildenborough and a putative hmc operon in A. fulgidus. The sequence homology of the two HmcA proteins, which are multiheme c-type cytochromes, is also shown.

Lojou E, Bianco P, Bruschi M. 1998a. Kinetic studies on the electron transfer between various c-type cytochromes and iron (III) using a voltametric approach. Electrochim Acta 43 2005-13. 

Fig. 1. (a) The heme group in a c-type cytochrome showing the covalent attachment, (b) The di heme group. 

High-resolution crystal structures are known for both the diheme enzymes to be discussed in this article. We describe these structures and relate them to the mechanisms of the two enzymes before trying to compare the common features of these two types of enzyme. For example, they are both able to interact, at least in vitro, with c-type cytochromes... 

The structure of P pantotrophus (fomerly Thiospharea pantotropha, (3)) cytochrome cdi, the first of this type to have its crystal structure solved (4), shows that the enz5une is a homodimer of 567 aminoacid residues and each subunit contains both a c-type cytochrome center and a di heme center (Fig. 3). The di heme (Fig. 1) is unique to this class of enzyme and on that basis alone might be expected to be the catalytic site. The c-type cytochrome centers, which are defined by the covalent attachment of the heme to the polypeptide are usually, but... 

Fe atoms. It had been anticipated that the c-type cytochrome center would have His/Met coordination, but His/His is observed. The former is the more usual coordination, especially at the high potential end E° > +200 mV) ofthe typical bacterial electron transfer chain to which the nitrite reductase is connected (Fig. 2) (7). The second curious feature is that the di heme iron is also six-coordinate thus, the enzyme does not offer a substrate-binding site at either heme. In addition to an expected axial histidine ligand there was an axial tyrosine (residue 25) ligand to the d heme (Fig. 4a). Each monomer is organized into two domains. 

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