title: H-NOX from Clostridium botulinum, like H-NOX from Thermoanaerobacter tengcongensis, Binds Oxygen but with a Less Stable Oxyferrous Heme Intermediate.
abstract: Heme nitric oxide/oxygen binding protein isolated from the obligate anaerobe Clostridium botulinum (Cb H-NOX) was previously reported to bind NO with a femtomolar K(D) (Nioche, P. et al. Science 2004, 306, 1550-1553). On the other hand, no oxyferrous Cb H-NOX was observed despite full conservation of the key residues that stabilize the oxyferrous complex in the H-NOX from Thermoanaerobacter tengcongensis (Tt H-NOX) (the same study). In this study, we re-measured the kinetics/affinities of Cb H-NOX for CO, NO, and O2. K(D)(CO) for the simple one-step equilibrium binding was 1.6 × 10(-7) M. The K(D)(NO) of Cb H-NOX was 8.0 × 10(-11) M for the first six-coordinate NO complex, and the previous femtomolar K(D)(NO) was actually an apparent K(D) for its multiple-step NO binding. An oxyferrous Cb H-NOX was clearly observed with a K(D)(O2) of 5.3 × 10(-5) M, which is significantly higher than Tt H-NOX's K(D)(O2) = 4.4 × 10(-8) M. The gaseous ligand binding of Cb H-NOX provides another supportive example for the "sliding scale rule" hypothesis (Tsai, A.-L. et al. Antioxid. Redox Signal. 2012, 17, 1246-1263), and the presence of hydrogen bond donor Tyr139 in Cb H-NOX selectively enhanced its affinity for oxygen.
authors.affiliation: Division of Hematology, Department of Internal Medicine, The University of Texas-Medical School at Houston , 6431 Fannin Street, Houston, Texas 77030, United States.
authors.forename: Gang
authors.initials: G
authors.lastname: Wu
authors.affiliation: Division of Hematology, Department of Internal Medicine, The University of Texas-Medical School at Houston , 6431 Fannin Street, Houston, Texas 77030, United States.
authors.forename: Wen
authors.initials: W
authors.lastname: Liu
authors.affiliation: Division of Hematology, Department of Internal Medicine, The University of Texas-Medical School at Houston , 6431 Fannin Street, Houston, Texas 77030, United States.
authors.forename: Vladimir
authors.initials: V
authors.lastname: Berka
authors.affiliation: Division of Hematology, Department of Internal Medicine, The University of Texas-Medical School at Houston , 6431 Fannin Street, Houston, Texas 77030, United States.
authors.forename: Ah-Lim
authors.initials: AL
authors.lastname: Tsai
mesh_terms: D001426:Bacterial Proteins / Q000378:metabolism*; D001906:Botulism / Q000382:microbiology; D002248:Carbon Monoxide / Q000378:metabolism; D003014:Clostridium botulinum / Q000378:metabolism*; D006418:Heme / Q000378:metabolism*; D006420:Hemeproteins / Q000378:metabolism*; D006801:Humans; D007700:Kinetics; D009569:Nitric Oxide / Q000378:metabolism; D010100:Oxygen / Q000378:metabolism*; D011485:Protein Binding; D046968:Thermoanaerobacter / Q000378:metabolism*
chemical_list: D001426:Bacterial Proteins; D006420:Hemeproteins; D009569:Nitric Oxide; D006418:Heme; D002248:Carbon Monoxide; D010100:Oxygen