br Structural studies of the cyclase

Structural studies of the cyclase catalytic domains
Structural determinants for catalytic activity The structure of the active αβGCcat structure has remained elusive. However, several studies have suggested how specific structural elements are involved in αβGCcat transitioning from the open inactive conformation to the closed active state (Fig. 5).
Protein-protein interactions regulating GC-1 activity
Conclusions Much progress has been made toward characterizing NO-stimulated GC-1, but we still lack a mechanistic understanding of how αβGCcat transitions between inactive and active forms in the full-length enzyme. Structural studies of GC catalytic domains from different organisms and the C1/C2 domain from AC have aided in identifying catalytic residues, potential allosteric sites for small molecules, and regions in the catalytic domain possibly involved in protein-protein interactions either with other GC-1 domains or other protein partners. Determination of the activated αβGCcat conformation will expand on small molecules that can target αβGCcat. These small molecules will aid, not only in structural studies aimed at characterizing the catalytic domain, but in creating a novel class of pharmaceuticals to target GC-1 independent of the NO-sensor domain. Further studies are required to determine what role βHNOX plays in regulating αβGCcat and to identify additional protein-protein interaction partners that assist GC-1. While we still lack a high-resolution structure of GC-1, this should not impede progress in the field, but rather drive researchers to utilize alternative methods toward understanding and targeting dysfunctional GC-1 in a variety of diseases.
Declarations of interest
Funding Research reported in this publication was supported by the National Eye Institute of the National Institutes of Health under Award Number R21EY026663 (EDG) and by the National Institute of Health training grant T32 GM066706 (KCC). The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Institutes of Health.
Introduction Soluble guanylyl cyclase (sGC), a heterodimer consisting of one α and one β subunit, is the major physiological receptor for nitric oxide (NO). In humans, there are two major isoforms catalysing the conversion of GTP to the second messenger cGMP: α1/β1 and α2/β1. Both heterodimeric Sal 003 coexist in a wide range of tissues including blood vessels and the lung. However, α1/β1 is the predominating isoform. Only in brain tissue both isoforms are present in similar amounts [1]. sGC is an important target for therapeutic intervention: NO-liberating drugs have been used for decades to treat angina pectoris [2], [3]. Today, NO/cGMP signalling is widely recognised to be important in different disease pathomechanisms including pulmonary hypertension [4], [5], [6], chronic heart failure [7], [8] and fibrotic diseases [9], [10], [11]. In addition to NO-liberating drugs and phosphodiesterase (PDE) 5 inhibitors, the discovery of sGC agonists provides a new opportunity to modulate the cGMP pathway which can overcome the limitations of NO-donors and PDE inhibitors [12], [13]. There are two types of sGC agonists, sGC stimulators and sGC activators. sGC stimulators can stimulate sGC NO-independently but also act synergistically with endogenous NO and potentiate NO-mediated cGMP signalling when NO is bound to sGC. sGC stimulators require the NO-binding heme group for sGC activation (for excellent review, see [14]). Riociguat is the first marketed sGC stimulator and is approved as therapeutic for chronic thromboembolic pulmonary hypertension (CTEPH) and pulmonary arterial hypertension (PAH) [5]. Phase II studies of riociguat in patients with scleroderma-associated digital ulcers (ClinicalTrails.gov Identifier NCT02915835) and systemic sclerosis (ClinicalTrails.gov Identifier NCT02283762) are ongoing. Furthermore, the sGC stimulator vericiguat showed beneficial effects in patients with chronic heart failure with reduced ejection fraction (HFreF) [7], [15]. A phase III trial of vericiguat in HFrEF patients is ongoing [7], [16]. In contrast to sGC stimulators, sGC activators act as heme-mimetics and do not require the NO-binding heme group [17]. A common assumption is that sGC activators selectively target oxidised or heme-free sGC [18], [19]. As these enzyme states predominantly occur under disease situations accompanied by oxidative stress, this binding mode has quite some appeal for the clinical use of activator drugs [12]. Cinaciguat, an amino dicarboxylic acid, was the first characterised drug of this new class of sGC activators [17]. However, cinaciguat failed to pass phaseII clinical trials because of hypotension [20], [21]. Although a number of other sGC activators have been characterised, no candidate of this drug class is available for patients to date.