Molecular mechanisms of metal-mediated biological functions for NO, O2, and HNO

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NO has considerable biomedical significance in cardiovascular regulation, immune response, neurotransmission, and global N-cycle. O2 is vital for many fundamental biological functions such as bioenergy, metabolism, and redox signaling. HNO also plays significant roles in vascular relaxation, enzyme activity regulation, and neurological function regulation. Despite numerous progress in this area, many important questions have not been answered. Building on our long-term research on biological complexes of NO, O2, and HNO with metalloproteins and models especially the successful preliminary data in the current grant period, we will provide some novel results to address three significant questions. Our first objective is to determine one- electron NO-to-N2O conversion mechanisms via heme models activated by Lewis acids reported recently, which is different from the conventional two-electron process by bacterial nitric oxide reductases. To provide a complete understanding of the kinetic and thermodynamic factors of this new reaction, systematic computational studies will be done to reveal the full reaction pathways of the reported heme models and explore the pathways for other biologically available metal, ligand environments, and Lewis acids. Our second objective is to determine rewiring mechanisms of NO/O2-sensing functions of a heme enzyme. How enzymes differentiate between two important redox reagents NO and O2 despite their similarity in shape, size, and charge remain unknown. Our experimental collaborator has recently reversed the NO sensing heme protein DosS to be O2 sensing via a triple mutant. The proposed work will reveal specific contributions of each mutation and their combinations on geometric and electronic properties and protein environment effects. The identified correlations of structural and electronic features with sensitivity functions will help rational design to rewire redox sensing functions in future biomedical research. Our third objective is to determine HNO formation mechanisms of a clinical drug hydroxyurea via heme proteins. The reactions have been experimentally studied using horseradish peroxidase (HRP) and catalase (CAT) with different reactivities. However, HNO formation mechanistic details and the origin for such reactivity difference are yet to be elucidated. The proposed work will calculate the complete reaction pathways for HRP and CAT using active site models with varying size of nearby residues and full proteins, to reveal basic mechanisms and roles of active site residues and protein environments for their differential reactivities. Results from this systematic study may also help identify key structural features to assist drug design and understanding of related HNO-generation drugs.