Multiheme c-type cytochromes

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.

Multiheme cytochromes c play an important role in bacterial iron respiration. In this process, soluble Fe chelates or insoluble Fe oxides serve as terminal electron acceptors of the anaerobic respiratory chain. In Shewanella and Geobacter spp., the two most studied Fe -reducing organisms, a great increase in cellular multiheme c-type cytochromes is observed upon growth by iron respiration. In the case of the genus Shewanella, electron transport to Fe involves CymA in the cytoplasmic membrane, and several... 

Figure 2 Cross-eyed stereo views of the hemes and their axial ligands found in electron-transfer cytochromes. (a) Bis-His ligation is generally found in cyts b and multiheme cyts c (PDB codelWDB). (b) His/ Met ligation is common in class I cyts c (PDB code lYCQ. (c) His/amine ligation has been observed only in cyt / (PDB code ICTM). For c-type cytochromes, the conserved Cys-Xxx-Yyy-Cys-His sequence and its covalent linkages to the heme are shown, (d) Bis-Met ligation is found in bacterioferritin (PDB code IBCF) and has been engineered into some cyt mutants. Note the variation in amplitudes and compositions of...

The enzyme of Desulfomicrobium sp. str. Ben-RB is either a c-type cytochrome or is associated in the membrane with such a cytochrome. This enzyme is therefore different from that of C. arsenatis (see Sec. II.D). If it is a c-type cytochrome, it may be similar to a number of multiheme c-type metal reductase cytochromes found in other sulfate-, sulfur-, and iron (Ill)-reducing bacteria. These metal reductase activities are specific to the polyheme cytochrome c from class III as defined by Ambler (21). Such acfivity was first demonstrated for the soluble cytochrome C3 of Desulfovibrio vulgaris, shown to reduce both chromium... 

As opposed to the multiheme cytochrome c-type metal reductases described above, which are soluble, usually periplasmic proteins, the c-type cytochrome involved in arsenate reduction by Desulfomicrobium sp. str. Ben-RB is in the membrane. Whether the enzyme that catalyzes the reduction of arsenate to arse-nite is in fact a c-type cytochrome or associated with one remains to be determined. 

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. ... 

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