Abstract
We present a new analysis of the EPR data and Mossbauer parameters of the redox state P-OX of the P-cluster of nitrogenase. In our model, each half of the cluster is formally equivalent to a ''classic'' [Fe4S4](+) unit containing one ferric and three ferrous ions. However, due to the presence of an additional serine ligand at one iron site, the spin properties of the two halves are distinct; one has a typical spin S-1 = 1/2 and the other, close to the serine, a spin S-2 = 7/2 (We considered also the case S-2 = 5/2) A parallel coupling of these two subspins (resulting in the state \S-1, S-2, S-t] with S-t equal to 4) is found to be more likely than the antiparallel \1/2, 7/2, 3]. This conclusion is based on two lines of evidence. First, the ferrous ions (identified from: their large isomer shifts) present both positive and negative hyperfine parameters, which points to a ferromagnetic coupling of the two cubane subspins S-1 and S-2. Second, we propose the use of a simple empirical quantity a(test) = Sigma A(iz) where A(iz) are the measured hyperfine parameters for P-OX. Comparison of the values of a(test) (which is markedly St-dependent) for P-clusters from Clostridium pasteurianum (Cp) and from Azotobacter vinelandii (Av) with theoretical estimates for the possible parallel-coupled states \1/2, 5/2, 3] and \1/2, 7/2, 4] clearly favors the latter. Our spin coupling model predicts a 5:3 (5 negative and 3 positive)pattern for the hyperfine parameters rather than 4:4 as originally measured in Av or 6:2 as in Cp (and also in Av after experimental reanalysis). This model for the P-OX state is combined with other information to provide a consistent picture of oxidation states and spin coupling patterns in all four observed P-cluster redox states.