Structure, redox, pKa, spin. A golden tetrad for understanding metalloenzyme energetics and reaction pathways

After a review of the current status of density functional theory (DFT) for spin-polarized and spin-coupled systems, we focus on the resting states and intermediates of redox-active metalloenzymes and electron transfer proteins, showing how comparisons of DFT-calculated spectroscopic parameters with experi- ment and evaluation of related energies and geome- tries provide important information. The topics we examine include (1) models for the active-site structure of methane monooxygenase intermediate Q and ribo- nucleotide reductase intermediate X; (2) the coupling of electron transfer to proton transfer in manganese superoxide dismutase, with implications for reaction kinetics; (3) redox, pKa, and electronic structure issues in the Rieske iron-sulfur protein, including their con- nection to coupled electron/proton transfer, and an analysis of how partial electron delocalization strongly alters the electron paramagnetic resonance spectrum; (4) the connection between protein-induced structural distortion and the electronic structure of oxidized high- potential 4Fe4S proteins with implications for cluster reactivity; (5) an analysis of cluster assembly and central-atom insertion into the FeMo cofactor center of nitrogenase based on DFT structural and redox potential calculations.