- Still a puzzle: why is haem covalently attached in c-type cytochromes?
Paul D Barker and Stuart J Ferguson
Summaryc-Type cytochromes are a group of proteins with diverse structures and functions. Their common feature is covalent attachment of haem to one or more CXXCH motifs. There does not seem to be a single advantageous reason for this covalent attachment.
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- The Structure of Acidithiobacillus ferrooxidans c4-Cytochrome
Structure, Volume 11, Issue 5, 1 May 2003, Pages 547-555
Chantal Abergel, Wolfgang Nitschke, Guillaume Malarte, Mireille Bruschi, Jean-Michel Claverie and Marie-Thérèse Giudici-Orticoni
SummaryThe study of electron transfer between the copper protein rusticyanin (RCy) and the c4-cytochrome CYC41 of the acidophilic bacterium Acidithiobacillus ferrooxidans has evidenced a remarkable decrease of RCy's redox potential upon complex formation. The structure of the CYC41 obtained at 2.2 Å resolution highlighted a specific glutamate residue (E121) involved in zinc binding as potentially playing a central role in this effect, required for the electron transfer to occur. EPR and stopped-flow experiments confirmed the strong inhibitory effect of divalent cations on CYC41:RCy complex formation. A docking analysis of the CYC41 and RCy structure allows us to propose a detailed model for the complex-induced tuning of electron transfer in agreement with our experimental data, which could be representative of other copper proteins involved in electron transfer.
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- The Cyanide Degrading Nitrilase from Pseudomonas stutzeri AK61 Is a Two-Fold Symmetric, 14-Subunit S...
Structure, Volume 11, Issue 11, 1 November 2003, Pages 1413-1422
B.T. Sewell, M.N. Berman, P.R. Meyers, D. Jandhyala and M.J. Benedik
SummaryThe quaternary structure of the cyanide dihydratase from Pseudomonas stutzeri AK61 was determined by negative stain electron microscopy and three-dimensional reconstruction using the single particle technique. The structure is a spiral comprising 14 subunits with 2-fold symmetry. Interactions across the groove cause a decrease in the radius of the spiral at the ends and the resulting steric hindrance prevents the addition of further subunits. Similarity to two members of the nitrilase superfamily, the Nit domain of NitFhit and N-carbamyl-D-amino acid amidohydrolase, enabled the construction of a partial atomic model that could be unambiguously fitted to the stain envelope. The model suggests that interactions involving two significant insertions in the sequence relative to these structures leads to the left-handed spiral assembly.
- Background: Cytochromes c4 are dihaem cytochromes c found in a variety of bacteria. They are assumed to take part in the electron-transport systems associated with both aerobic and anaerobic respiration. The cytochrome c4 proteins are located in the periplasm, predominantly bound to the inner membrane, and are able to transfer electrons between membrane-bound reduction systems and terminal oxidases. Alignment of cytochrome c4 sequences from three bacteria, Pseudomonas aeruginosa, Pseudomonas stutzeri and Azotobacter vinelandii, suggests that these dihaem proteins are composed of two similar domains. Two distinctly different redox potentials have been measured for the Ps. stutzeri cytochrome c4, however.Results: The crystal structure of the dihaem cytochrome c4 from Ps. stutzeri has been determined to 2.2 Å resolution by isomorphous replacement. The model, consisting of two entire cytochrome c4 molecules and 138 water molecules in the asymmetric unit, was refined to an R value of 20.1% for all observations in the resolution range 8–2.2 Å. The molecule is organized in two cytochrome c-like domains that are related by a pseudo-twofold axis. The symmetry is virtually perfectly close to the twofold axis, which passes through a short hydrogen bond between the two haem propionic acid groups, connecting the redox centre of each domain. This haem–haem interaction is further stabilized by an extensive symmetrical hydrogen-bond network. The twofold symmetry is not present further away from the axis, however, and the cytochrome c4 molecule can be considered to be a dipole with charged residues unevenly distributed between the two domains. The haem environment in the two domains show pronounced differences, mainly on the methionine side of the haem group.Conclusions: The structure, in conjunction with sequence alignment, suggests that the cytochrome protein has evolved by duplication of a cytochrome c gene. The difference in charge distribution around each haem group in the two domains allows the haem group in the N-terminal domain to be associated with the lower redox potential of 241 mV and the C-terminal haem group with the higher potential of 328 mV. The molecular dipole characteristic of cytochrome c4 is important for its interaction with, and recognition of, its redox partners. In cytochrome c4, the hydrogen-bond network (between residues that are conserved in all known cytochrome c4 subspecies) seems to provide an efficient pathway for an intramolecular electron transfer that can ensure cooperativity between the two redox centres. The C-pyrrole corners of the haem edges are potential sites for external electron exchange.