Nitrosylation of the Diiron Core Mediated by the N Domain of YtfE

The YtfE protein catalyzes the reduction of NO to N₂O, protecting iron-sulfur clusters from nitrosylation. The structure of YtfE has a two-domain architecture, with a diiron-containing C-terminal domain linked to an N-terminal domain, in which the function of the latter is enigmatic. Here, by using electron spin resonance (ESR) spectroscopy, we show that YtfE exists in two conformational states, one of which has not been reported. Under high osmotic stress, YtfE adopts a homogeneous conformation (C state) similar to the known crystal structure. In a regular buffer, the N-terminal domain switches between the C state and a previously unidentified conformation (C' state), the latter of which has more space at the domain interface to allow the trafficking of NO molecules and thus is proposed to be a functionally active state. The conformational switch between the C and C' states is pivotal for facilitating NO access to the diiron core.