Abstract
The double-cubane cluster (DCC) refers to an [Fe8S9] iron-sulfur complex that is otherwise only known to exist in nitrogenases. Containing a bridging µ2-S ligand, the DCC in the DCC-containing protein (DCCP) is covalently linked to the protein scaffold via six coordinating cysteine residues. In this study, the nature of spin coupling and the effect of spin states on the cluster’s geometry are investigated computationally. Using density functional theory (DFT) and a broken symmetry (BS) approach to study the electronic ground state of the system, we computed the exchange interaction between the spin-coupled spins of the four FeFe dimers contained in the DCC. This treatment yields results that are in excellent agreement with both computed and experimentally determined exchange parameters for analogously coupled di-iron complexes. Hybrid quantum mechanical (QM)/molecular mechanical (MM) geometry optimizations show that cubane cluster A closest to charged amino acid side chains (Arg312, Glu140, Lys146) is less compact than cluster B, indicating that electrons of the same spin in a charged environment seek maximum separation. Overall, this study provides the community with a fundamental reference for subsequent studies of DCCP, as well as for investigations of other [Fe8S9]-containing enzymes.
Highlights
Enzymes catalyzing ATP-driven redox reactions usually consist of two components
Hybrid quantum mechanical (QM)/molecular mechanical (MM) geometry optimizations show that cubane cluster A closest to charged amino acid side chains (Arg312, Glu140, Lys146) is less compact than cluster B, indicating that electrons of the same spin in a charged environment seek maximum separation
This study provides the community with a fundamental reference for subsequent studies of DCCcontaining protein (DCCP), as well as for investigations of other [Fe8S9]-containing enzymes
Summary
The first component is an ATPase, which contains an FeS cluster that provides the electron that is to be transferred. The second component is another metalloenzyme, the electron acceptor, which is ready to accept a highly energetic electron since it contains one or more metal centers that cannot be reduced at physiological reduction potentials [1]. The two components of a system of this type were characterized and the electronaccepting component was found to be a novel double-cubane [Fe8S9]-cluster (DCC) [1] (Figure 1). This cluster catalyzes reductive reactions otherwise associated only with the complex iron-sulfur clusters of nitrogenases [2]. Double-cubane [Fe8S9]-clusters have been reported only once for a biological system, they have already been synthesized and studied [3,4,5,6]
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