Effect of Heme Structure on O₂-Binding Properties of Human Serum Albumin−Heme Hybrids:  Intramolecular Histidine Coordination Provides a Stable O₂−Adduct Complex

Abstract

5,10,15,20-Tetrakis[(alpha,alpha,alpha,alpha-o-pivaloylamino)phenyl]porphinatoiron(II) and 5,10,15,20-tetrakis([alpha,alpha,alpha,alpha-o-(1-methylcyclohexanoylamino)]phenyl)porphinatoiron(II) complexes bearing a covalently bound 8-(2-methyl-1-imidazolyl)octanoyloxymethyl or 4-(methyl-L-histidinamido)butanoyloxymethyl side-chain [FeRP(B) series: R = piv or cyc, B = Im or His] have been synthesized. The histidine-bound derivatives [FepivP(His), FecycP(His)] formed five N-coordinated high-spin iron(II) complexes in organic solvents under an N(2) atmosphere and showed large O(2)-binding affinities in comparison to those of the 2-methylimidazole-bound analogues [FepivP(Im), FecycP(Im)] due to the low O(2)-dissociation rate constants. On the contrary, the difference in the fence groups around the O(2)-coordination site (pivaloyl or 1-methylhexanoyl) did not significantly influence to the O(2)-binding parameters. These four porphinatoiron(II)s were efficiently incorporated into recombinant human serum albumin (rHSA), thus providing the synthetic hemoprotein, the albumin-heme hybrid [rHSA-FeRP(B)]. An rHSA host absorbs a maximum of eight FeRP(B) molecules in each case. The obtained rHSA-FeRP(B) can reversibly bind and release O(2) under physiological conditions (in aqueous media, pH 7.3, 37 degrees C) like hemoglobin and myoglobin. As in organic solutions, the difference in the fence groups did not affect their O(2)-binding parameters, but the axial histidine coordination significantly increased the O(2)-binding affinity, which is again ascribed to the low O(2)-dissociation rates. The most remarkable effect of the heme structure appeared in the half-life (tau(1/2)) of the O(2)-adduct complex. The dioxygenated rHSA-FecycP(His) showed an unusually long lifetime (tau(1/2): 25 h at 37 degrees C) which is ca. 13-fold longer than that of rHSA-FepivP(Im).