An optical signal correlated with the allosteric transition in Scapharca inaequivalvis HbI.

The transient absorbance change within the first 2 mus of photolysis of COHbI (from Scapharca inaequivalvis) reported by Chiancone et al. [Chiancone, E., Elber, R., Royer, W. E., Regan, R., and Gibson, Q. H. (1993) J. Biol. Chem. 268, 5711-5718] has been studied in several mutants. Evidence is presented that this change (rts) is associated with the allosteric transition between R and T states. Two different rts spectra relate to Hb2 and Hb2CO. No rts has been observed for mutants at position 97 (normally Phe). Correlation of ligand binding and rts shows that protein function changes at or near the rates of rts, typically, 2 x 10(6) s-1 (Hb2) and 5 x 10(5) s-1 (Hb2CO). Unique values of allosteric parameters for several mutants have been obtained by combining kinetic and equilibrium data. The effect of mutation on function thus may be assigned to allostery or to a change in intrinsic heme reactivity.