Multiheme proteins

MCD spectroscopy in range 300 to 2000 nm at both ambient and liquid helium (4.2 K) temperatures can yield information about the spin, oxidation, and coordination states of each heme in a multiheme protein such as CCP (75). This technique, in combination with low-temperature X-band EPR spectroscopy, was used to great effect in characterizing the properties of the fully oxidized and MV forms of the P. aeruginosa CCP in solution. At 4.2 K, both hemes in the oxidized enzyme are low-spin ferric, with diagnostic features in the near infrared-MCD (NIR-MCD) spectrum consistent with one heme with His/Met axial coordination and the other with bis-histidine axial coordination this is entirely consistent with the crystal structure. In contrast, at room temperature only the low-potential (bis-histidine coordinated) heme in the C-terminal domain remains completely low-spin, whereas the high-potential (His/Met coordinated) heme exists as mixture of high- and low-spin forms 58). 

Although complex, the cytochromes c3 provide the opportunity to obtain information that will greatly extend our knowledge of biological electron transfer and the interaction of redox centers in multiheme proteins. Moreover, because of unique electrochemistry and electrical properties, the cytochromes c3 provide the opportunity to develop a system useful as a model for bioelectronic devices. Much research remains to be done to understand fully the redox properties of the cytochromes c3. However, the data discussed clearly define interesting and important issues, which include (1) the paths by which electrons move between hemes (2) how electrons enter and exit the cytochrome c3 molecule during physiological electron transfer (3) the nature of the factors that control the interaction potentials between hemes (4) the factors responsible for the observed behavior on metal surfaces and, importantly, (5) the specific molecular features responsible for the behavior of... 

Microbial Electrosynthesis, Fig. 2 (a) Electron trans- pathway via multiheme proteins in Shewanella oneidensis fer pathway via multiheme proteins and conductive pili in (for further details, see text)... 

The multiheme protein cytochrome C3, an electron transfer protein from the sulfate-reducing bacteria Desulfovibrio vulgaris (strain Miyazaki), was the first example of a heme protein exhibiting a reversible electrode reaction at a mercury electrode. Due to the sophisticated structure of proteins, there is great difficulty in most cases to achieve electron transfer of enzyme molecules via an electrode. 

Moreover, an electron transfer chain could be reconstituted in vitro that is able to oxidize aldehydes to carboxylic acids with concomitant reduction of protons and net production of dihydrogen (213, 243). The first enzyme in this chain is an aldehyde oxidoreductase (AOR), a homodimer (100 kDa) containing one Mo cofactor (MOD) and two [2Fe—2S] centers per subunit (199). The enzyme catalytic cycle can be regenerated by transferring electrons to flavodoxin, an FMN-con-taining protein of 16 kDa (and afterwards to a multiheme cytochrome and then to hydrogenase) ... 

The first test case was the ferrous high-spin state (Fe, S = 2) in the picket-fence porphyrin acetate complex [Fe(CH3COO)(TPpivP)] [13, 23], which is a model for the prosthetic group termed P460 of the multiheme enzyme hydroxyl-amine oxidoreductase from the bacterium Nitrosomonas europeae. Both the picket-fence porphyrin and the protein P460 exhibit an extraordinarily large quadrupole splitting, as observed by conventional Mossbauer studies [56]. 

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.

In the pursuit of mechanisms for electron transfer, various cytochromes and enzymes have been isolated and purified. A list of proteins implicated in the electron transfer processes with reduction of electron acceptor is given in Table 16.5. Although reductases have considerable specificity for reduction, it is apparent that the low-potential multiheme cytochromes interface with numerous different electron acceptors. 

Most known multiheme cytochromes and enzymes belong to the family of cytochromes c (see Iron Heme Proteins Electron Transport), which contain Fe-protoporphyrin IX covalently attached to the polypeptide chain by two thioether bonds, formed by addition of two cysteinyl residues to the vinyl side-chains of the porphyrin ring. The two cysteines form a characteristic amino acid sequence motif CXXCH, usually indicative of heme c ligation, and where the histidine is the axial fifth ligand to the iron. For some cytochromes (see Section 2), the number of residues between the two cysteines can be three or four. The heme redox potentials in cytochromes c cover a wide range and are tuned by several factors, usually dominated by the type of axial ligation and the extent of solvent exposure of the heme. ... 

Myoglobin, a single-heme system, and hemoglobin, a multiheme system, both contain an identical heme prosthetic group however, their O2 binding properties are significantly different, as is the second coordination shell of iron. Consequently, this leads these two important proteins to have different... 

Table 3. Multiheme cytochromes c in proteins from anaerobes ...

After they were first isolated from D. vulgaris in 1952 [48], tetraheme cytochromes c3 have been detected in large quantities in the periplasm of the Desulfovibrionacea family as well as in other sulfate reducers, where they can function as a coupling protein to hydrogenase [49]. These small, very soluble and stable proteins, which are low-spin in both oxidation states, are specially well suited for physico-chemical and structural studies, being the best characterized multiheme cytochromes. 

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