Effects of the bHLH domain on axial coordination of heme in the PAS-A domain of neuronal PAS domain protein 2 (NPAS2): conversion from His119/Cys170 coordination to His119/His171 coordination.

Neuronal PAS domain protein 2 (NPAS2), which is a CO-dependent transcription factor, consists of a basic helix-loop-helix domain (bHLH), and two heme-containing PAS domains (PAS-A and PAS-B). In our previous study on the isolated PAS-A domain, we concluded that His119 and Cys170 are the axial ligands of the ferric heme, while Cys170 is replaced by His171 upon reduction of heme (Uchida et al., J. Biol. Chem. 270, (2005) 21358-21368.). Recently, we characterized the PAS-A domain combined with the N-terminal bHLH domain, and found that some spectroscopic features were different from those of the isolated PAS-A domain (Mukaiyama et al., FEBS J. 273, (2006) 2528-2539.). Therefore, we reinvestigated the coordination structure of heme in the bHLH-PAS-A domain and prepared four histidine and one cysteine mutants. Resonance Raman spectrum of the Cys170Ala mutant is the same as that of wild type with a dominant 6-coordinate heme in the ferric form. In contrast, His119Ala and His171Ala mutants significantly increase amounts of the 5-coordinate species, indicating that His119 and His171, not Cys170, are axial ligands of the ferric heme in the bHLH-PAS-A domain. We had confirmed that the coordination structure of the isolated PAS-A domain is in equilibrium between Cys-Fe-His and His-Fe-His coordinated species but newly found that interaction of the PAS-A domain with the bHLH domain shifts the equilibrium toward the latter structure. Such flexibility in the heme coordination structure seems to be in favor of signal transduction in NPAS2. Copyright Â