Relative Conformations of Sperm Whale Metmyoglobin and Apomyoglobin in Solution

Abstract

Recent studies of the side chain reactivities of sperm whale metmyoglobin (l-3) and its derivative apoprotein, apomyoglobin (4), have demonstrated significant differences between these two proteins.These differences have been interpreted to indicate a reversible change in metmyoglobin conformation upon removal of the hsme.However, reactivity differences between metmyoglobin and apomyoglobin may only reflect the effect of the heme on the ease with which equivalent.secondary structures are transformed to more open conformations.It seemed important, therefore, to investigate the relative conformations of metmyoglobin and apomyoglobin directly to determine whether the observed reactivity differences were indeed accompanied by an observable difference in struct.ure.The a-helix content of metMb1 has been determined in the crystalline state (5).In solution, est,imates of helix content have been made from optical rotatory dispersion st,udies by using both the Moffitt equation in the region 240 to 340 rnp (6-8) and measurements of the conformation-dependent 225 nip Cotton effect (7-9).The Moffitt equation has also been applied to estimate the a-helix content of globin (1O).2However, no criteria for the intactness of the native globin structure were given and no direct comparisons with t.he parent myoglobin were made.The apparently good agreement among the different studies of metMb suggested that the relative magnitude of the 225 rnp Cotton effect (11) in metMb and in globin would provide a convenient index of the relative helix contents of these two proteins.In metMb, however, the possibility that this band is affected by an optically active heme transition must be considered.In the present st.udy it will be demonstrated that the amplitude of this Cotton effect remains constant when ligands which produce marked alterations elsewhere in the spectrum are added to the heme.To a large extent also, the possible influence of distant heme transitions may be detect,ed by carrying out measurements * Some of the material is taken from the thesis of K. D. Hard-