title: Is Nostoc H-NOX a NO sensor or redox switch?
abstract: Nostoc sp. (Ns) H-NOX is a heme protein found in symbiotic cyanobacteria, which has approximately 35% sequence identity and high structural homology to the beta subunit of soluble guanylyl cyclase (sGC), suggesting a NO sensing function. However, UV-vis, EPR, NIR MCD, and ligand binding experiments with ferrous and ferric Ns H-NOX indicate significant functional differences between Ns H-NOX and sGC. (1) After NO binding to sGC, the proximal histidine dissociates from the heme iron, causing a conformational change that triggers activation of sGC. In contrast, formation of pentacoordinate (5c) NO heme occurs to only a limited extent in Ns H-NOX, even at >1 mM NO. (2) Unlike sGC, two different hexacoordinate (6c) NO complexes are formed in Ns H-NOX with initial and final absorbance peaks at 418 and 414 nm, and the conversion rate is linearly dependent on [NO], indicating that a second NO binds transiently to catalyze formation of the 414 nm species. (3) sGC is insensitive to oxygen, and ferric sGC prepared by ferricyanide oxidation has a 5c high-spin heme complex. In contrast, Ns H-NOX autoxidizes in 24 h if exposed to air and forms a 6c ferric heme complex, indicating a major conformational change after oxidation and coordination by a second histidine side chain. Such a large conformational transition suggests that Ns H-NOX could function as either a redox or a NO sensor in the cyanobacterium.
authors.affiliation: Division of Hematology, Internal Medicine, University of Texas Houston Medical School, Houston, Texas 77030, USA. Ah-lim.tsai@uth.tmc.edu
authors.forename: Ah-Lim
authors.initials: AL
authors.lastname: Tsai
authors.forename: Vladimir
authors.initials: V
authors.lastname: Berka
authors.forename: Faye
authors.initials: F
authors.lastname: Martin
authors.forename: Xiaolei
authors.initials: X
authors.lastname: Ma
authors.forename: Focco
authors.initials: F
authors.lastname: van den Akker
authors.forename: Marian
authors.initials: M
authors.lastname: Fabian
authors.forename: John S
authors.initials: JS
authors.lastname: Olson
mesh_terms: D000458:Cyanobacteria; D006162:Guanylate Cyclase / Q000378:metabolism / Q000502:physiology; D009247:NADH, NADPH Oxidoreductases / Q000378:metabolism / Q000502:physiology*; D009569:Nitric Oxide / Q000378:metabolism*; D046937:Nostoc / Q000201:enzymology* / Q000378:metabolism; D010084:Oxidation-Reduction; D011485:Protein Binding; D011487:Protein Conformation; D018160:Receptors, Cytoplasmic and Nuclear / Q000378:metabolism / Q000502:physiology; D017386:Sequence Homology, Amino Acid; D000071756:Soluble Guanylyl Cyclase; D013057:Spectrum Analysis; D040681:Structural Homology, Protein
chemical_list: D018160:Receptors, Cytoplasmic and Nuclear; D009569:Nitric Oxide; D009247:NADH, NADPH Oxidoreductases; D006162:Guanylate Cyclase; D000071756:Soluble Guanylyl Cyclase