FIG. 1.

SenX3 binds heme and is responsive to O₂, NO, and CO. (A) SenX3 reacts reversibly with O₂. UV-visible absorption spectra of native SenX3 (3 μM in 50 mM sodium phosphate, 200 mM NaCl, and pH 8.0) indicate that a hexa-coordinated heme bound to the protein. Treatment of native SenX3 with a 100-fold molar excess of DTH followed by re-exposure to air leads to reversible spectral changes, as observed with classical hypoxia/O₂ sensors. (B, C) NO and CO are ligands of SenX3. DTH-treated SenX3 was exposed to a 50-fold molar excess of NO donor ProliNONOate in an anaerobic glove box, and absorption spectra were recorded (B). DTH-treated SenX3 was exposed to a 100-fold molar excess of CO donor CORM-2, and the absorption spectra were recorded (C). Insets display enlarged images of the 500–600 nm regions. Numbers in parentheses indicate the absorption maxima in nanometers. CO, carbon monoxide; CORM-2, carbon monoxide-releasing molecule-2; DTH, sodium dithionite; NO, nitric oxide; O₂, oxygen. To see this illustration in color, the reader is referred to the web version of this article at www.liebertpub.com/ars