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Equilibrium and kinetic measurements of carbon monoxide binding to hemoglobin kansas in the presence of inositol hexaphosphate
Authors:CL Castillo  S Ogawa  JM Salhany
Institution:1. Bell Laboratories, Inc., Murray Hill, New Jersey 07974 U.S.A.;2. Departments of Biochemistry and Biomedicinal Chemistry and the Cardiovascular Center, University of Nebraska Medical Center and Veterans Administration Hospital, Omaha, Nebraska 68105 U.S.A.
Abstract:Previous proton nuclear magnetic resonance (nmr) studies have indicated that inositol hexaphosphate (IHP) can stabilize hemoglobin (Hb) Kansas in a deoxy-like quaternary structure even when fully liganded with carbon monoxide (CO) (S. Ogawa, A. Mayer, and R. G. Shulman, 1972, Biochem. Biophys. Res. Commun., 49, 1485–1491). In the present report we have investigated both CO binding at equilibrium and the CO binding and release kinetics to determine if Hb Kansas + IHP is devoid of cooperativity, as would be suggested by the nmr studies just quoted. The equilibrium measurements show that Hb Kansas + IHP has a very low affinity for CO (P12 = 1.2 mm Hg and Keq = 5.4 × 105M?1) and almost no cooperativity (n = 1.1) at pH 7, 25 °C. The CO “on” and “off” kinetics also show no evidence for cooperativity. In addition, the equilibrium constant estimated from the kinetic rate constants (Keq = 5.2 × 105M?1 with kon = 1.03 × 105M?1 · S? and koff = 0.198 S?1) is in excellent agreement with the equilibrium constant determined directly. Thus, both kinetic and equilibrium measurements allow us to conclude that CO binding to Hb Kansas + IHP occurs without significant cooperativity.
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