title: Equilibrium and kinetic evidence for a transition between six- and five-coordinate ferrous heme in the nitric oxide derivative of Aplysia myoglobin.
abstract: The pH dependence in the range 3--7 of the optical absorption and electron paramagnetic resonance of the nitric oxide adduct of ferrous Aplysia myoglobin is reported. Optical spectra in the Soret region show a transition between two conformers with an apparent pK in the range 3.5--5 depending on the presence of carboxylic anions as third component. In the same pH range, the EPR spectrum undergoes a change from a 9-line to a 3-line hyperfine pattern in the g. region, similar to that reported for synthetic heme derivatives and for other hemoproteins. The structural interpretation of the pH-induced transition experienced by Aplysia myoglobin nitric oxide is that of a proton-linked cleavage of the proximal bond as suggested by several lines of evidence. Temperature-jump measurements allowed an estimation of the relaxation time for the process, which is of the order of 0.3 ms at 25 degrees C.
mesh_terms: D000818:Animals; D001048:Aplysia / Q000378:metabolism*; D004578:Electron Spin Resonance Spectroscopy; D006418:Heme / Q000378:metabolism*; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D009211:Myoglobin / Q000378:metabolism*; D009569:Nitric Oxide; D013053:Spectrophotometry; D013113:Spin Labels
chemical_list: D009211:Myoglobin; D013113:Spin Labels; D009569:Nitric Oxide; D006418:Heme