Understanding the NO-sensing mechanism at molecular level

Abstract

We present here how ultrafast time-resolved spectroscopy improves our understanding of a new class of proteins: Nitric Oxide sensors. Nitric oxide (NO) is a small, short-lived, and highly reactive gaseous molecule and it acts as a second messenger in several physiological systems. NO sensors are proteins which bind NO and are able to translate this binding into a signal for mammal cells as well as in bacteria. We have studied NO-sensors with the goal of understanding the activation and deactivation mechanism of the human NO-receptor, the enzyme guanylate cyclase (sGC), which is involved in communication between cells. Some bacterial sensors of NO (SONO) have structural homologies and common properties with sGC, but also have differences with sGC which make them valuable system to get structural and physiological information on sGC. To understand how NO-sensors interact with NO and control its reactivity, it is essential to probe dynamics and interactions when NO is present within protein core and what are the associated structural changes. For this purpose, we have used time-resolved absorption spectroscopy in the picoseconds (10-12s) time domain. NO can be photodissociated from heme by the pulse of femtosecond laser. Time-resolved transient absorption spectra on NO-sensors were recorded and NO-protein interacttion were recorded. In case of cytochrome c′, we identified the formation of 5-coordinate (5c)-NO and 5c-His hemes from 4c-heme and demonstrate that proximal histidine precludes NO rebinding at the proximal site. In bacteria, the adaptation of SONO to temperature changes was not achieved by a simple temperature-dependent NO binding equilibrium, but by a change of the proportion between 5c-NO and 6c-NO species. This amplifies the response to temperature changes since a fast NO rebinding is the only property of a 5c-NO leading to 4c-heme after dissociation. Our results of NO dynamics provide a model for the regulation at molecular level in NO-sensing function.