New Mechanistic Insights from Structural Studies of the Oxygen-Sensing Domain of Bradyrhizobium japonicum FixL^,

Abstract

The FixL heme domain serves as the dioxygen switch in the FixL/FixJ two-component system of Rhizobia. Recent structural studies of the Bradyrhizobium japonicum FixL heme domain (BjFixLH) have suggested an allosteric mechanism that is distinct from the classical hemoglobin model. To gain further insight into the FixL sensing mechanism, structures of BjFixLH bound to dioxygen, imidazole, and nitric oxide have been determined. These structures, particularly the structure of BjFixLH bound to its physiological ligand, dioxygen, have helped to address a number of important issues relevant to the BjFixLH sensing mechanism. On the basis of the oxy-BjFixLH structure, a conserved arginine is found to stabilize the dioxygen ligand in a mode reminiscent of the distal histidine in classical myoglobins and hemoglobins. The structure of BjFixLH bound to imidazole elucidates the structural requirements for accommodating sterically bulky ligands. Finally, the structure of BjFixLH bound to nitric oxide provides evidence for a structural intermediate in the heme-driven conformational change.

Identification of Residues Crucially Involved in the Binding of the Heme Moiety of Soluble Guanylate Cyclase

“…It might be that this interaction is part of intramolecular signal transmission cascades involved in translating changes of the porphyrin geometry and heme status into structural changes of the protein as observed for other heme proteins (38). This proposed signal transmission triad consisting of histidine 105, tyrosine 135, and arginine 139 may also explain why sGC is activated by PPIX or CO to a lesser extent than by NO, namely due to differences in porphyrin geometry.…”

“…The sGC prosthetic heme is embedded in a hydrophobic binding pocket between the histidine 105, tyrosine 135, and the arginine 139. Comparable interactions were described for various other heme binding proteins such as cytochrome P-450, cytochrome b, or FixL (37)(38)(39).…”

Two ground state isoforms and a chromophore D -ring photoflip triggering extensive intramolecular changes in a canonical phytochrome

“…Although we can rule out a C5-isomerization in Cph1, global changes in the pocket apparently lead to a partner swap between the B-ring propionate (C8 3 ) from Arg-254 in Pr to Arg-222 in Pfr (see above). Interestingly, the FixL "two-component" redox sensor might also signal via an analogous tetrapyrrole propionate-Arg partner swap (47). Like Arg-472, although Arg-254 is conserved, R254A has little effect photochemically or on Pfr stability.…”

Reactive oxygen species and cellular oxygen sensing

“…In parallel with the FNR system in E. Coli, the two-component Arc A/B system induces transcription of adaptive genes in response to the redox state of electron carriers during hypoxic stress [2,3]. During O 2 deprivation in Rhizobia and Bradyrhizobia, O 2 is released from a heme moiety bound to the histidine kinase FixL which subsequently undergoes autophosphorylation and activates the transcription factor FixJ [4,5].…”

“…A high concentration of mitochondrial SOD ensures no significant O 2 .− accumulation beyond its initial site of production and allows for the generation of H 2 O 2 which has second messenger effects in the cytosol [31,[38][39][40]. The endoplasmic reticulum (ER) is another cellular source of ROS where resident cytochrome P-450 and b 5 family members oxidize unsaturated fatty acids and xenobiotics to generate O 2 .− and H 2 O 2 [41][42][43]. In addition, plasma membrane-associated oxidases such as NADPH oxidases generate ROS by oxidizing intracellular NADPH to reduce O 2 into O 2 .− in order to achieve localized microbicidal function in phagosomes [44][45][46][47].…”