Since the discovery of soluble guanylate cyclase (sGC) as the mammalian receptor for nitric oxide (NO), numerous studies have been performed in order to understand how sGC transduces the NO signal. However, the structural basis of sGC activation is still not completely elucidated. Spectroscopic and kinetic studies showed that the key step in the activation mechanism was the NO-induced breaking of the iron proximal histidine bond in the so-called 6c-NO to 5c-NO transition. The main breakthrough in the understanding of sGC activation mechanism came, however, from the elucidation of crystal structures for two different prokaryotic heme NO oxygen (HNOX) domains, which are homologues to the sGC heme domain. In this work we present computer simulation results of Thermoanaerobacter tencogensis HNOX that complement these structural studies, yielding molecular explanations to several poorly understood properties of these proteins. Specifically, our results explain the differential ligand binding patterns of the HNOX domains according to the nature of proximal and distal residues. We also show that the natural dynamics of these proteins is intimately related with the proposed conformational dependent activation process, which involves mainly the alphaFbeta1 loop and the alphaA-alphaC distal subdomain. The results from the sGC models also support this view and suggest a key role for the alphaFbeta1 loop in the iron proximal histidine bond breaking process and, therefore, in the sGC activation mechanism.
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