title: Kinetic studies on mammalian cytochrome c modified with 2-hydroxy-5-hydroxy-5-nitrobenzyl bromide.
abstract: The reduction of 2-hydroxy-5-nitrobenzyl tryptophyl cytochrome c by the chromous ion was studied by stopped-flow techniques. At pH6.5 the reduction of 2-hydroxy-5-nitrobenzyl tryptophyl cytochrome c is complex, showing the presence of three distinct phases. Two chromium concentration-dependent phases are observed (1.1 X 10(5) M-1-S-1, phase 1; 1.25 X 10(4)M-1-S-1, phase 2) and one slow first-order process (0.25S-1, phase 3). A comparison of the static and kinetic difference spectra, along with the data from the reduction of the reoxidized reduced protein, suggests that the slow chromium concentration-independent phase is due to a slow conformational event after fast reduction of the NO2 group. The rates of the chromium concentration-dependent phases show a marked variation with pH above 7.5. The activation energies for the three processes were also measured at 33.2, 38.6 and 69.7 kJ-mol-1 for phases 1, 2 and 3 respectively. The reaction of reduced 2-hydroxy-5-nitrobenzyl tryptophyl cytochrome c with CO was foollowed by means of both stopped-flow and flash photolysis. The combination with CO at pH 6.8 as measured in stopped-flow experiments showed two phases, one CO-dependent phase (phase 2, 2.4 X 10(2)M-1-S-1) and one CO-independent phase (phase 1, 0.015S-1). Investigation of the pH-dependence of the phases showed both the rates and amounts of each phase to be pH-invariant. CO recombination, after photolytic removal, was found to be biphasic; a CO-dependent phase (phase 2, 2.4 X 10(2)M-1-S-1) and a CO-independent phase (phase 1, 1.0s-1) were observed. A tentative model which can accommodate these observations is proposed.
mesh_terms: D015077:2-Hydroxy-5-nitrobenzyl Bromide / Q000494:pharmacology; D001665:Binding Sites; D002151:Calorimetry; D002248:Carbon Monoxide / Q000494:pharmacology; D002857:Chromium / Q000494:pharmacology; D003574:Cytochrome c Group / Q000378:metabolism; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D009596:Nitrophenols; D010084:Oxidation-Reduction; D011485:Protein Binding; D013053:Spectrophotometry; D013816:Thermodynamics
chemical_list: D003574:Cytochrome c Group; D009596:Nitrophenols; D002857:Chromium; D015077:2-Hydroxy-5-nitrobenzyl Bromide; D002248:Carbon Monoxide