SUBSTRATE SPECIFICITY AT ALKANE BINDING SITES OF HEMOGLOBIN AND MYOGLOBIN

Binding of the ligands (L) iodobutane, pentane, neopentane, butane, and xenon, to ferrihemoglobin and ferrimyoglobin, and of pentane to oxy- and deoxyhemoand myoglobin (H is one subunit), at pH 7, 0°, is accurately described by two equilibria, HL = H + L, HL2 = HL + L, with dissociation constants K1 and K2, as found earlier for β-lactoglobulin. Apomyoglobin requires three. The constants vary (e.g., for pentane and β-lactoglobulin, apomyoglobin, oxyhemoglobin, oxymyoglobin, K1 = 0.12, 0.28, 0.34, 0.8, all × 10-3 M). K2 may approach 4K1 (e.g., xenon and butane) or may be very large (e.g., ferrihemoglobin pentane, and iodobutane; ferrimyoglobin and iodobutane). The ratios of K1 for different ligands and a given protein roughly approximate the ratios calculated from partition between water and dodecyl sulfate micelles (pentane: neopentane: iodobutane: butane: xenon = 1:1.7:2:3.8:120). An exception is neopentane, which binds moderately well to ferrihemoglobin, but is almost excluded from β-lactoglobulin and ferrimyoglobin. The xenon constants fall in the expected range. In both ferrihemoglobin and ferrimyoglobin, pentane and xenon compete for the same two interacting sites. It is concluded that each protein has a single, localized, but not strictly discrete, hydrophobic binding region (e.g., a circle has localized but continuous binding sites for two semicircles), which probably correspond to the xenon loci of ferrihemoglobin and ferrimyoglobin described by Schoenborn. © 1969, American Chemical Society. All rights reserved.