The kinetics of CO binding to the horse myoglobin fragment Mb-(32-139), the so-called "mini-Mb," were investigated by laser flash photolysis in 0.1 M phosphate buffer and in buffer with 75% (vol/vol) glycerol. The reaction displays complex time courses that can be approximated satisfactorily only with a sum of five exponentials. The features of the kinetic components and a comparison of the deoxy-minus-carbonyl difference spectra of mini-Mb and horse Mb obtained under equilibrium conditions, with the kinetic difference spectra resulting from the global analysis of the traces recorded between 400 and 450 nm, show that CO binding to mini-Mb is accompanied by large structural changes. In view of the fact that mini-Mb is an approximation of the Mb-(31-105) fragment encoded by the central exon of the Mb gene, this finding is particularly relevant. On the basis of our data and previous reports [De Sanctis, G., Falcioni, G., Giardina, B., Ascoli, F. & Brunori, M. (1988) J. Mol. Biol. 200, 725-733; De Sanctis, G., Falcioni, G., Grelloni, F., Desideri, A., Polizo, F., Giardina, B., Ascoli, F. & Brunori, M. (1992) J. Mol. Biol. 222, 637-643], we propose that the protein fragment encoded by the central exon of the Mb gene is the domain responsible for ligand-linked conformational transitions, while the two terminal fragments dampen the amplitude of the structural changes that accompany ligand binding, thus rendering the protein stable and kinetically more efficient in its physiological function.