1H NMR Study of the Role of Individual Heme Propionates in Modulating Structural and Dynamic Properties of the Heme Pocket in Myoglobin

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The relative importance of the two heme propionate salt bridges to the protein matrix in determining the equilibrium orientational preference of the synthetic hemin, heptamethyl-monopropionate-porphine-iron(III), reconstituted into sperm whale and horse myoglobin metcyano complexes was determined using the nuclear Overhauser effect, NOE. The preferences for occupying the crystallographic 7-propionate position which makes a link to interior His FG3 in both proteins, versus that of the 6-propionate group interacting with the surface CD3 residue, were found to be ~ 1:2 for the sperm whale and 3:2 for the horse protein. Since the FG3 residue is conserved (His), and the CD3 residue differs in the two proteins (Arg for spermwhale, Lys for horse), we conclude that the 6-propionate-Lys CD3 link in horse Mb is less stable by ~0.64 kcal/mol than the 6-propionate-Arg CD3 salt bridge in sperm whale Mb. Since this salt bridge, in part, holds closed a likely distal ligation channel to the heme pocket, the lower stability of this link in the horse relative to the sperm whale protein provides a structural basis for interpreting earlier highly differential labile hydrogen dynamics for the two proteins (Lecomte, J. T. J.; La Mar, G. N. Biochemistry 1985, 24, 7388-7395). Reconstitution of horse Mb with protohemin IX derivatives having individual propionates selectivityreplaced by methyls confirms that vinyl contacts are much more important than propionate contacts in determining the orientational preference of the heme in the pocket. Each of the monopropionate hemins revealed that the orientational isomer with a propionate in the crystallographic 6-position is in dynamic equilibrium (-103 s_1) with another isomeric form which isdeduced to be the heme orientation with the propionate in the crystallographic 8-position. The interconversion involves simply 90° rotational “hopping” of the heme about the iron-His bond, and this process is concluded to be significantly faster in the horse thansperm whale protein, again suggesting that the 6-propionate salt bridge is weaker in the former Mb. © 1990, American Chemical Society. All rights reserved.

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Journal of the American Chemical Society

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