Effect of heme structure on O(2)-binding properties of human serum albumin-heme hybrids: intramolecular histidine coordination provides a stable O(2)-adduct complex.

5,10,15,20-Tetrakis[(α,α,α,α-o-pivaloylamino)phenyl]porphinatoiron(II) and 5,10,15,20-tetrakis-{[α,α,α,α-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 02-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 °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 (τ 1 / 2 ) of the O 2 -adduct complex. The dioxygenated rHSA-FecycP(His) showed an unusually long lifetime (τ 1 / 2 : 25 h at 37 °C) which is ca. 13-fold longer than that of rHSA-FepivP(Im).