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Institut de Biologie StructuraleGrenoble / France

An Unexpected P-Cluster like Intermediate En Route to the Nitrogenase FeMo-co

Nitrogenase is a key metalloprotein that catalyzes the reduction of nitrogen to ammonia at room temperature and ambient pressure. It thus plays a major role in the global nitrogen cycle. It uses two metal centers : the P-cluster, an atypical [Fe8S7] center, which allows electron transfer to the active site itself, which is an organometallic [MoFe7S9C-(R)-homocitrate] center. Its biosynthesis requires the action of a dozen accessory proteins that constitute the NIF (for NItrogen Fixation) assembly machinery. The NifB protein is the key enzyme in this mechanism because it is responsible for the fusion of two [Fe4S4] centers combined with a carbide ion insertion and the addition of a sulfide ion to produce a [Fe8S9C] precursor termed NifB-co. Recently, American colleagues published a crystal structure of the NifB protein with all its metal centers. Unfortunately, they did not model their crystallographic data well and missed the identity of cluster bound to the active site. By reusing these data, we were able to highlight the presence of a unique [Fe8S8] center resulting from the fusion of [Fe4S4] centers. This crystal structure allowed us to redefine the order of the reaction steps showing that the FeS center fusion must take place before the carbide ion insertion. The particular coordination of this intermediate highlights the role of the protein matrix in the organization of the NifB-co biosynthetic steps, thus revealing the mechanism of the enzyme.

An unexpected P-cluster like intermediate en route to the nitrogenase FeMo-co. Leon P. Jenner, Mickael V. Cherrier, Patricia Amara, Luis M. Rubio and Yvain Nicolet. Chemical Science DOI : 10.1039/D1SC00289A