Introduction Soluble guanylyl cyclase sGC is a
Soluble guanylyl cyclase (sGC) is a key protein in the regulation of multiple physiological functions in mammalian physiology [1,2]. Upon stimulation by nitric oxide (NO) , sGC generates cyclic guanosine monophosphate (cGMP) from guanosine-5′ triphosphate and activates protein kinase G-dependent signaling needed for various signaling cascades in several cell types . It is well known that dysregulation of sGC signaling leads to cardiovascular, neuronal, and gastrointestinal system failure and disease acceleration [2,4].
Structural information on sGC is partial, and only broad structural characteristics have been identified. Studies on the sGC interactome are also incomplete. The physiologically relevant mammalian sGC heterodimer is comprised of the α1 and β1 subunits and is expressed in most cell types and tissues; however, two other subunits of sGC, α2 and β2, have also been identified . Other mixed heterodimers have been formed – particularly α2β1, which is expressed in the brain, placenta, spleen, and uterus .
Each sGC α and β subunit forming the heterodimer constitutes a four-domain architecture: the H-NOX domain, a Per-ARNT-Sim domain, a coiled-coil signaling helix, and the catalytic cyclase domain. These domains are fused into a single polypeptide chain measuring 600 and 700 Nocodazole to form a subunit (Fig. 1). A heme moiety in the ferrous state is bound to the β subunit H-NOX domain, which is responsible for transmitting the NO signal and is coordinated to the β-H105 residue. The α subunit counterpart to the H-NOX cannot coordinate a heme moiety and is often referred to as a pseudo H-NOX . H-NOX crystal structures have been elucidated for various bacterial homologs [, , ], and mammalian H-NOX molecular models are based on the cyanobacterial structures [6,10]. Hydrogen/deuterium exchange mass spectrometry has yielded information critical to understanding crucial conformational changes following binding of NO .
A short linker connects the H-NOX to the PAS domain. PAS domains can exist as a component of multi-domain signaling proteins  and function in binding co-factors or chromophores, acting as sensors in signal transduction. However, no ligand binding has been shown for the sGC PAS domain. Following the PAS domain is the coiled-coil (CC) signaling domain, as seen in sGC domain architecture (Fig. 1). The CC domain contains long helices, one from each subunit. PAS-CC modules are found in many proteins and form structural units for effective intramolecular signaling [6,13] and activation of catalysis, as is the case for sGC . Crystal structures discovered for the rat sGC CC domain are thought to come together as parallel helices in the α1β1 heterodimer [14,15].
Both α and β subunits contain a cyclase homology domain (CHD) and belong to a family of class III cyclases . Because the sGC active site is at the heterodimer interface, two active sites are possible, but as the subunits join, a catalytic pocket and a pseudosymmetric pocket are formed [, , , ]. While the pseudosymmetric pocket has lost catalytic capabilities, it still retains ligand-binding affinities. In the case of adenylate cyclase, forskolin is shown to bind to the pseudosymmetric pocket and enhance cAMP production [20,21].
Various physiological studies have shown that the β1 subunit of the heterodimer is necessary, but not sufficient, for proper sGC signaling [2,22]; both the α1 and α2 subunits can heterodimerize with the β1 subunit to create a functional protein . A conserved histidine on the β1 subunit was found to coordinate the heme moiety  that preferentially binds NO and carbon monoxide (CO) over oxygen (O2) . Structural changes induced by NO binding activate the catalytic domain of the sGC protein, leading to the downstream effects of vasorelaxation. sGC structural characteristics have been reviewed by Derbyshire and Marletta .
Cysteine regulation of sGC