Crowding effect on electron transfer in inducible nitric oxide synthase

The large family of mononuclear molybdenum and tungsten enzymes all possess the special ligand molybdopterin (MPT), which consists of a metal-binding dithiolene chelate covalently bound to a pyranopterin group. These molybdoenzymes catalyze a variety of oxygen atom transfer (OAT) and hydroxylation reactions on a broad range of substrates. MPT pyran cyclization/scission processes have been proposed to modulate the reactivity of the metal center during the course of catalysis. However, the role of MPT and its participation in the catalytic reaction is still under considerable debate and is a focus of this study. We have designed several small molecule models for the Mo-MPT cofactor that allow detailed investigation into how pyran cyclization modulates electronic communication between the dithiolene and pterin moieties, and how this cyclization alters the electronic environment of the molybdenum catalytic site. Using a combination of cyclic voltammetry (CV), vibrational spectroscopy (FT-IR and rR), electronic absorption spectroscopy, and x-ray absorption spectroscopy (XAS), distinct changes in the Mo ≡O stretching frequency, Mo(V/IV) reduction potential, and electronic structure across the pterin-dithiolene ligand are observed as a function of pyran ring closure. The results are significant for they reveal that a dihydropyranopterin is electronically coupled into the Mo-dithiolene group due to a coplanar conformation of the pterin and dithiolene units. This coplanarity creates extended π conjugation from the Mo-dithiolene unit through the pterin, providing a mechanism for the electron-deficient pterin to modulate the Mo environment. A spectroscopic signature identified for the pyranodihydropterin-dithiolene ligand on Mo is a strong dithioleneàpterin charge transfer transition. In the absence of a pyran group bridge between pterin and dithiolene, the pterin rotates out of plane, largely decoupling the system. The results support a hypothesis that pyran cyclization/scission processes in MPT may function as a molecular switch to electronically couple and decouple the pterin and dithiolene to adjust the redox properties in certain pyranopterin molybdenum enzymes.