Changes in Side Chain Reactivity Accompanying the Binding of Heme to Sperm Whale Apomyoglobin.

Considerable attention has been devoted recently to the study of conformational changes occurring during enzyme-substrate interaction (1, 2). Presumably the nature of the interaction between apoproteins and their prosthetic groups is essentially similar to that between enzymes and substrates, with the exception that no catalytic step occurs subsequent to the initial binding. Studies of the physicochemical changes accompanying the binding of heme to sperm whale apomyoglobin would seem to be of particular interest in t.hat the three-dimensional conformation of sperm whale metmyoglobin is now known (3), and conformational changes occurring on heme-globin interaction may therefore be more amenable to interpretation. Kinetic studies of heme-globin interaction suggest that the initial product of the reaction must rearrange to form the native heme-protein (4). Conceivably this rearrangement involves a change in protein conformation. Optical rotatory dispersion studies of metmyoglobin give values for the a-helix content of approximately 75% (5, 6), whereas similar studies of globin suggest a helix content of 50%.’ In view of the differing Cotton effects in the two proteins, however, it is difficult to assess the significance of this disparity. In a preliminary communication (7), this laboratory cited discrepancies between the H+ ion titration curves of globin and metmyoglobin which could possibly be interpreted as indications of subtle conformational differences. The primary aim of the present study is to examine in greater detail the significance of the relative reactivities of globin and myoglobin derivatives to H+ ion. Secondarily the observed differences in reactivity of imidazoles to H+ ion in the two proteins will be related to changes in reactivity to bromoacetic acid, a reagent for which the reaction with metmyoglobin imidazoles has been recently documented (8).