Two soluble, novel cytochromes <i>c </i>have been discovered in the purple phototrophic bacterium <i>Rhodobacter sphaeroides. </i> They have been designated Diheme cytochrome c (DHC) and <i>Sphaeroides </i> heme protein (SHP). DHC is a 16 kDa, low-spin diheme protein and SHP is a 12 kDa high-spin, monoheme protein, which is capable of binding oxygen. The crystal structure of SHP indicates that the sixth heme ligand is asparagines, which is the first known report of an amide as the sixth heme ligand in a wild-type protein. Upon reduction of the SHP heme, this asparagines swings away, rendering ferrous SHP as penta-coordinate and able to bind small ligands, such as CO and NO, as well as O<sub>2</sub>. CN<sup>-</sup> can bind to ferric SHP. The complete crystal structure of DHC has been solved and as well as indicating two bis-His ligated hemes, a lack of β-sheet character and a structurally unique second heme domain, it also shows pronounced acidic patches surrounding both heme regions, which complement the basic patch surrounding the SHP heme region. This provides evidence of an electrostatic attraction between the proteins. DHC and SHP are encoded on the same operon, which also encodes a membrane-bound cytochrome <i>b</i>. Thus DHC could be the natural electron donor for SHP, which could function as the terminal electron acceptor in a novel respiratory pathway. The midpoint reduction potentials of the hemes in DHC are -340 ± 4 and -210 ± 4 mV. The midpoint reduction potential of the SHP heme is -105 ± 2 mV. This indicates that electron transfer from DHC to SHP is thermodynamically favourable. DHC and SHP also bind very tightly to each other (<i>K<sub>d</sub></i> = 200 nM) at low salt 10 mM KC1). At higher salt concentrations, binding becomes weaker, again indicating an electrostatic interaction. Phenotypic studies on SHP have not yet uncovered its function but have ruled out its involvement in respiration on various inorganic substrates and in response to oxidative and nitrosative stress.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:651446 |
| Date | January 2005 |
| Creators | Gibson, Helen R. |
| Publisher | University of Edinburgh |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://hdl.handle.net/1842/14904 |
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