Thursday, July 14, 2011

Fe-S clusters

Biological Fe-S clusters come in many sizes and flavours:
  • 2Fe-2S clusters ligated by four cysteines
  • 2Fe-2S clusters ligated by three cysteines and one aspartate
  • 2Fe-2S clusters ligated by cysteines and histidines (the so-called Rieske clusters)
  • 3Fe-4S clusters ligated by three cysteines
  • 4Fe-4S clusters ligated by four cysteines
  • 4Fe-4S clusters ligated by three cysteines and one aspartate
  • the hideously complex cluster present in hybrid cluster protein (also known as fuscoredoxin or "prismane protein")
  • the P-cluster in nitrogenase
  • etc., etc., etc.
    The large number of electrons in Fe and the complexity of the possible couplings between spin states make the theoretical analysis of the electronic structures in Fe-S clusters quite difficult.
    Takano et al. have recently published a paper on the differences between a Cys3Asp ligated 4Fe-4S cluster and the "regular" (all Cys) 4Fe-4S cluster. The authors nicely analyze the influence of the Asp (and other) ligands on the electronic structure of the 4Fe-4S cluster, observe a -0.10 V difference in redox potential (vs. normal 4Fe-4S) in high dielectric constants, and offer this observation as the reason for the low potential of this cluster.
    I do not accept this last conclusion for two reasons:
  • redox potentials of Cys-ligated 4Fe-4S clusters may differ by >0.4 V from each other, which shows that the influence of the charge distribution of the protein is much more important than the small difference observed by the authors
  • the 0.1 V difference found amounts to ca. 2.3 kcal/mol, which is well within the error range of the computational methods used.
  • No comments:

    Post a Comment