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Author(s): Marscha Hirschi [1]; Mark A. Herzik Jr [2]; Jinhong Wie [3]; Yang Suo [1]; William F. Borschel [1]; Dejian Ren [3]; Gabriel C. Lander (corresponding author) [2]; Seok-Yong Lee [...]

Globins are heme-containing proteins that are best known for their roles in oxygen (O2) transport and storage. However, more diverse roles of globins in biology are being revealed, including gas and redox sensing. In the nematode Caenorhabditis elegans, 33 globin or globin-like genes were recently identified, some of which are known to be expressed in the sensory neurons of the worm and linked to O2 sensing behavior. Here, we describe GLB-6, a novel globin-like protein expressed in the neurons of C. elegans. Recombinantly expressed full-length GLB-6 contains a heme site with spectral features that are similar to those of other bishistidyl ligated globins, such as neuroglobin and cytoglobin. In contrast to these globins, however, ligands such as CO, NO, and CN do not bind to the heme in GLB-6, demonstrating that the endogenous histidine ligands are likely very tightly coordinated. Additionally, GLB-6 exhibits rapid two-State autoxidation kinetics in the presence of physiological O2 levels as well as a low redox potential (-193 ± 2 my). A high-resolution (1.40 A) crystal structure of the ferric form of the heme domain of GLB-6 confirms both the putative globin fold and bis-histidyl ligation and also demonstrates key structural features that can be correlated with the unusual ligand binding and redox properties exhibited by the full-length protein. Taken together, the biochemical properties of GLB-6 suggest that this neural protein would most likely serve as a physiological sensor for O2 in C. elegans via redox signaling and/or electron transfer. [ABSTRACT FROM AUTHOR]

Nitric oxide (NO) signaling in mammals controls important processes such as smooth muscle relaxation and neurotransmission by the activation of soluble guanylate cyclase (sGC). NO binding to the heme domain of sGC leads to dissociation of the iron-histidine (Fe-His) bond, which is required for enzyme activity. The heme domain of sGC belongs to a larger class of proteins called H-NOX (Heme-Nitric oxide/OXygen) binding domains. Previous crystallographic studies on H-NOX domains demonstrate a correlation between heme bending and protein conformation. It was unclear, however, whether these structural changes were important for signal transduction. Subsequent NMR solution structures of H-NOX proteins show a conformational change upon disconnection of the heme and proximal helix, similar to those observed in the crystallographic studies. The atomic details of these conformational changes, however, are lacking in the NMR structures especially at the heme pocket. Here, a high-resolution crystal structure of an H-NOX mutant mimicking a broken Fe-His bond is reported. This mutant exhibits specific changes in heme conformation and major N-terminal displacements relative to the wild-type H-NOX protein. Fe-His ligation is ubiquitous in all H-NOX domains, and therefore, the heme and protein conformational changes observed in this study are likely to occur throughout the H-NOX family when NO binding leads to rupture of the Fe-His bond. [ABSTRACT FROM AUTHOR]

An abstract of the article "Tunnels modulate ligand flux in a heme nitric oxide/oxygen binding (H-NOX) domain," by Michael B. Winter, Mark A. Herzik Jr., John Kuriyan, and Michael A. Marletta is presented.