The model for carbon monoxide or dioxygen recombination with heme proteins developed by the group at the University of Illinois is reexamined. We propose that the carbon monoxide or dioxygen molecule enters the protein at essentially a diffusion-limited rate determined by the solvent viscosity and that the protein offers no important barriers to this entry. The viscosity dependence of the entry rate k(ED), its magnitude (1 x 10(10) M(-1)s(-1), and the rate of quenching of triplet states of protoprophyrin IX in apomyoglobin by dioxygen are used as supporting evidence. Comparison is made to the model of a fluctuating protein developed by G. Weber.
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