Bacterial nitric oxide reductase (NorBC) models employing click chemistry

• Published in: Journal of inorganic biochemistry Vol. 246; p. 112280
• Main Authors: Harland, Jill B., Samanta, Subhra, Lehnert, Nicolai
• Format: Journal Article
• Language: English
• Published: United States Elsevier Inc 01-09-2023
• Subjects: Bacteria - metabolism; Bacterial nitric oxide reductase; Click Chemistry; Dinitrosyl iron complexes; Heme - chemistry; Iron - chemistry; Model complexes; Nitric oxide; Nitric Oxide - metabolism; Oxidation-Reduction; Spectroscopy; Spectroscopy; Bacterial nitric oxide reductase; Click chemistry; Nitric oxide; Dinitrosyl iron complexes; Model complexes
• ISSN: 0162-0134; 1873-3344
• DOI: 10.1016/j.jinorgbio.2023.112280

Bacterial NO Reductase (NorBC or cNOR) is a membrane-bound enzyme found in denitrifying bacteria that catalyzes the two-electron reduction of NO to N2O and water. The mechanism by which NorBC operates is highly debated, due to the fact that this enzyme is difficult to work with, and no intermediates of the NO reduction reaction could have been identified so far. The unique active site of NorBC consists of a heme b3/non-heme FeB diiron center. Synthetic model complexes provide the opportunity to obtain insight into possible mechanistic alternatives for this enzyme. In this paper, we present three new synthetic model systems for NorBC, consisting of a tetraphenylporphyrin-derivative clicked to modified BMPA-based ligands (BMPA = bis(methylpyridyl)amine) that model the non-heme site in the enzyme. These complexes have been characterized by EPR, IR and UV–Vis spectroscopy. The reactivity with NO was then investigated, and it was found that the complex with the BMPA-carboxylate ligand as the non-heme component has a very low affinity for NO at the non-heme iron site. If the carboxylate functional group is replaced with a phenolate or pyridine group, reactivity is restored and formation of a diiron dinitrosyl complex was observed. Upon one-electron reduction of the nitrosylated complexes, following the semireduced pathway for NO reduction, formation of dinitrosyl iron complexes (DNICs) was observed in all three cases, but no N2O could be detected.
Three new diiron complexes assembled using click chemistry, modeling the active site of NorBC, were investigated for reactivity with NO. The BMPA-Pr clicked complex has surprisingly low reactivity with NO, forming a minimal amount of heme nitrosyl, while the TMPA and BMPAPhO clicked complexes form diiron dinitrosyl adducts. Upon addition of cobaltocene, DNIC formation is observed for all three compounds. [Display omitted]
•Bacterial Nitric Oxide Reductase (NorBC) is an important enzyme in the nitrogen cycle, responsible for the generation of N2O in bacterial denitrification.•Three new model complexes were assembled from suitable heme and non-heme ligands in a modular approach using click chemistry.•This synthetic approach allows for a variety of non-heme ligands and metal combinations to be investigated in a straightforward way.•The clicked complex with a propionate group in the non-heme ligand scaffold shows minimal reactivity with NO, as evidenced by UV–Vis, IR and EPR spectroscopy, while the analogous pyridine and phenolate derivatives form heme(NO)/non-heme(NO) diiron dinitrosyl complexes.•Addition of 1 equiv. of cobaltocene to the nitrosylated clicked complexes (testing for the semireduced NO reduction mechanism) results in the formation of Dinitrosyl Iron Complexes (DNICs), and not N2O.