Is histidine dissociation a critical component of the NO/H-NOX signaling mechanism? Insights from X-ray absorption spectroscopy

Abstract

The H-NOX (Heme-Nitric oxide/OXygen binding) family of diatomic gas sensing hemoproteins has attracted great interest. Soluble guanylate cyclase (sGC), the well-characterized eukaryotic nitric oxide (NO) sensor is an H-NOX family member. When NO binds sGC at the ferrous histidine-ligated protoporphyrin-IX, the proximal histidine ligand dissociates, resulting in a 5-coordinate (5c) complex; formation of this 5c complex is viewed as necessary for activation of sGC. Characterization of other H-NOX family members has revealed that while most also bind NO in a 5c complex, some bind NO in a 6-coordinate (6c) complex or as a 5c/6c mixture. To gain insight into the heme pocket structural differences between 5c and 6c Fe(II)–NO H-NOX complexes, we investigated the Extended X-ray Absorption Fine Structure (EXAFS) of the Fe(II)–unligated and Fe(II)–NO complexes of H-NOX domains from three species, Thermoanaerobacter tengcongensis, Shewanella woodyi, and Pseudoalteromonas atlantica. Although the Fe(II)–NO complex of TtH-NOX is formally 6c, we found the Fe-NHis bond is substantially lengthened. Furthermore, although NO binds to SwH-NOX and PaH-NOX as a 5c complex, consistent with histidine dissociation, the EXAFS data do not exclude a very weakly associated histidine. Regardless of coordination number, upon NO-binding, the Fe–Nporphyrin bond lengths in all three H-NOXs contract by ~0.07 Å. This study reveals that the overall heme structure of 5c and 6c Fe(II)–NO H-NOX complexes are substantially similar, suggesting that formal histidine dissociation may not be required to trigger NO/H-NOX signal transduction. The study has refined our understanding of the molecular mechanisms underlying NO/H-NOX signaling.

Bacterial Haemoprotein Sensors of NO

“…Many bacterial H-NOX domains have now been cloned, expressed, purified, and spectroscopically characterized (Arora & Boon, 2012; Boon et al, 2006; Dai, Farquhar, Arora, & Boon, 2012; Henares et al, 2012; Karow et al, 2004; Liu et al, 2012; Ma, Sayed, Beuve, & van den Akker, 2007; Mukhopadyay, Sudasinghe, Schaub, & Yukl, 2016; Price, Chao, & Marletta, 2007; Tsai, Berka, Martin, & Olson, 2012; Tsai et al, 2010; Wang et al, 2010; Wu, Liu, Berka, & Tsai, 2015). UV/vis spectroscopy, paired with resonance Raman (rR) spectroscopy, has demonstrated that ferrous H-NOX domains from facultative aerobes form high-spin 5-coordinate Fe(II)-unligated complexes (Boon et al, 2006; Karow et al, 2004).…”

“…Extended X-ray absorption fine structure (EXAFS) measurements of H-NOX from several species known to form 5- or 6-coordinate complexes with NO were determined (Dai et al, 2012). Shewanella woodyi and Pseudoalteromonas atlantica H-NOX proteins are known to form 5-coordinates complexes with NO (Arora & Boon, 2012; Liu et al, 2012), which is consistent with dissociation of the proximal histidine.…”

“…Yet, the EXAFS structure was consistent with a residual weak interaction with the proximal histidine. Cs-H-NOX, on the other hand, known to form a 6-coordinate complex with NO, also had appreciable Fe–His bond lengthening (Dai et al, 2012). In short, the Fe–His bond lengths of these three H-NOX structures were not significantly different, which is consistent with a hypothesis that Fe–His bond disruption is not the primary factor leading to signal transduction.…”

“…Several key structural features have been identified in NO-activation of H-NOX, including the dissociation of a proximal histidine ligand (Dai et al, 2012; Erbil et al, 2009; Herzik et al, 2014; Olea, Herzik, et al, 2010) and flattening of a haem cofactor (Dai & Boon, 2011; Erbil et al, 2009; Olea et al, 2008; Olea, Kuriyan et al, 2010; Sun et al, 2016) that is otherwise exceedingly distorted. Features specific to some H-NOX proteins have also been distinguished, including a hydrogen-bonding network that stabilizes O 2 binding in H-NOX from obligate anaerobes (Boon et al, 2005; Hespen et al, 2016) and a zinc-binding feature that may be connected to redox sensing in H-NOX domains from some gammaproteobacteria (Mukhopadyay et al, 2016).…”

Chemie und Biologie der Stimulatoren und Aktivatoren der löslichen Guanylatcyclase

“…Neuere Studien diskutieren jedoch, dass zur Auslçsung der Signaltransduktion unter Umständen keine vollständige Dissoziation notwendig ist. [135] Die Konformationsänderung in der H-NOX-Domäne setzt sich in die heterodimere katalytische Domäne fort, wodurch die Produktion von cGMP verstärkt wird. Als Bindeglied zwischen der NO-Bindung und der sGC-Aktivierung wurde eine Spannungsverminderung des Häm-Kofaktors bei der Dissoziation von His105 (oder Verlängerung der Fe-His-Bindung) in Kombination mit Bewegungen der N-Termini vorgeschlagen [136] und mit einer sich entspannenden Feder bildhaft verglichen.…”