Rates of reaction of native human globin with some hemes.

It was possible to account for the time course of the reactions of both carboxyheme and monocyanide heme with globin by use of suitable values for k3 and the ratio kr/kz, the two latter constants being too great to measure directly by the stopped flow method. It has been known for more than 30 years that mesoand deuterohemes can combine with globin to form compounds able to react reversibly with oxygen and other ligands (2). Some of the properties of these unnatural hemoglobins and myoglobins have been studied in detail (3-6) and are generally similar in kind to those of protohemoglobin, alt,hough showing considerable quantitative differences. In this paper the kinetic experiments with protoheme and globin are extended to meso-, deutero-, hemato-, and some other hemes, and the effects of chemical substitution in the heme on the rate and course of the reaction are examined. The results are consistent with the mechanism already proposed, and the considerable differences in rates can be correlated with the chemical modifications in the heme and with the structure of the protein as revealed by x-ray analysis. As a necessary preliminary, the absorption spectra and extinction coefficients of the heme derivatives were determined in different solvents, both to provide the data required for quantitative treatment of the kinetic results and to give information about t.he degree of polymerization in the solutions.