Different kinetic phases of the heme transfer reactions of apoHtsAM79A with ferric holoShpM66A and holoShpM153A. HoloShp mutant (0.9 μM) was mixed with 10 μM apoHtsA mutant in 20 mM Tris-HCl in a stopped-flow spectrophotometer. The arrows indicate the orientation of the spectral shift with time. A, spectral shift in the reaction of holoShpM66A with apoHtsAM79A. B, single-phase time course of ΔA418 and ΔA600 for the reaction in panel A. The black traces and gray curves represent the observed data and single exponential fitting curves, respectively. C, spectral shift in the reaction of holoShpM153A with apoHtsAM79A. D, kinetically biphasic time course of ΔA408 and ΔA601 for the reaction in panel C. The black traces and gray curves represent the observed data and double exponential fitting curves, respectively.