Multiheme

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.

Pereira lAC, Romao CV, Xavier AV, et al. 1998. Electron transfer between hydrogenases and mono- and multiheme cytochromes in Desulfovibrio spp. J Biol Inorg Chem 3 494-8. 

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. 

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). 

The involvement of HA during bacterial conversion of nitrate to NH3 (known also as the nitrate ammonification phase of the nitrogen cycle) has been studied at the molecular level as part of an effort to delineate the mechanism of conversion of nitrite to NH3 by a group of multiheme cytochromes of bacterial origin. The overall reduction reaction is depicted in equation 3 for cytochrome c-nitrite reductase " ... 

Fe Desulfovibrio vulgaris, Hildenborough strain Periplasm, soluble H2 consumption/ production 46 + 10 [H] + 2[4Fe-4S] Multiheme cytochrome c... 

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. ... 

Multiheme c cytochromes and enzymes form distinct families from the monoheme c cytochromes. They are characterized by a low amino acid residues to heme ratio (resulting in significant exposure of heme to the solvent) and bis-histidinyl coordination, with few known exceptions. These two characteristics are responsible for the low heme redox potentials of this family. In some cases, the multiheme... 

Redox Metalloenzymes in Electrocatalytic Action Imaged at the Single-Molecule Level Multicopper and Multiheme Nitrite Reductases... 

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... 

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