TRIS - AN ALLOSTERIC EFFECTOR OF TARANTULA HEMOCYANIN

The effect of the chemical buffering component Tris (hydroxy-methyl-am ino-methane) and of chloride ions on the oxygen binding of tarantula h emocyanin was studied at constant pH. It revealed that Tris at micromo lar concentrations decreases the oxygen pressure at half-saturation (p (50)) by a factor of more than two, whereas chloride does not influenc e oxygen affinity. A thermodynamic analysis in terms of the nested mod el of allostery [(1987) Proc. Natl. Acad. Sci. 84, 1891-1895] indicate d that Tris acts a an allosteric activator of oxygen binding by influe ncing the interaction between the 12-meric half-molecules of the 24-me ric tarantula haemocyanin.