Nitric oxide coupling to generate N2O promoted by a single-heme system as examined by density functional theory

Discovery of a Nitric Oxide-Responsive Protein in Arabidopsis thaliana

In plants, much like in animals, nitric oxide (NO) has been established as an important gaseous signaling molecule. However, contrary to animal systems, NO-sensitive or NO-responsive proteins that bind NO in the form of a sensor or participating in redox reactions have remained elusive. Here, we applied a search term constructed based on conserved and functionally annotated amino acids at the centers of Heme Nitric Oxide/Oxygen (H-NOX) domains in annotated and experimentally-tested gas-binding proteins from lower and higher eukaryotes, in order to identify candidate NO-binding proteins in Arabidopsis thaliana. The selection of candidate NO-binding proteins identified from the motif search was supported by structural modeling. This approach identified AtLRB3 (At4g01160), a member of the Light Response Bric-a-Brac/Tramtrack/Broad Complex (BTB) family, as a candidate NO-binding protein. AtLRB3 was heterologously expressed and purified, and then tested for NO-response. Spectroscopic data confirmed that AtLRB3 contains a histidine-ligated heme cofactor and importantly, the addition of NO to AtLRB3 yielded absorption characteristics reminiscent of canonical H-NOX proteins. Furthermore, substitution of the heme iron-coordinating histidine at the H-NOX center with a leucine strongly impaired the NO-response. Our finding therefore established AtLRB3 as a NO-interacting protein and future characterizations will focus on resolving the nature of this response.

Construction of valence bond structures for {FeNO}7 nitrosyl heme and non-heme complexes

For {FeNO}⁷ nitrosyl heme and non-heme complexes, with 17 valence shell electrons, Fe^IINO, Fe^IINO* and Fe^IIINO^- valence bond structures of the increased-valence type are generated primarily from Lewis valence bond structures by delocalizing non-bonding electrons into diatomic bonding molecular orbitals. Valence bond formulations for the reactions 2MbNO + 2H⁺ → 2metMb + N₂O + H₂O and cis MbNO + NO^- + 2H⁺ → metMb + N₂O + H₂O are also presented.