NMR Study of Heme Pocket Polarity/Hydrophobicity of Myoglobin Using Polypropionate-Substituted Hemins

The products of the reconstitution of sperm whale apomyoglobins with a series of synthetic hemins possessing a wide variety of substitution patterns for two and three propionates on the methyl-containing heme periphery were studied by NMR in the metaquo and metcyano derivatives to investigate the prospects for, and structural and thermodynamic consequences of, the location of propionates in the hydrophobic interior of the pocket. The orientations for the propionates for the various hemins were determined by the nuclear Overhauser effect in the metcyano complexes. In all but one case, a single, unique orientation was found for each hemin which resulted in the occupation by a propionate, in at least one case, in each of the possible eight heme pocket sites without disruption of the heme pocket structure. While propionate orientations into the protein interior were readily attainable, indicating a surprising polar pocket environment, these propionates exhibited extraordinarily elevated pks \u3e 8 indicative of the highly hydrophobic contacts in the interior of the heme pocket. For propionates situated in the sites of the usual protohemin vinyl groups, we observe pH-modulated structural transitions involving deprotonation of the propionate that results in changes in the orientations of the hemins. The detection of saturation transfer between these alternate orientations identifies the interconversion mechanism as rotational “hopping” of the hemin about the intact iron-histidine bond. The thermodynamics of the carboxylate side chains and dynamics of isomer interconversion can serve as sensitive probes of heme pocket structure among both natural and synthetic myoglobin variants. © 1990, American Chemical Society. All rights reserved