Crystal structural analysis of human serum albumin complexed with hemin and fatty acid

BACKGROUND: Human serum albumin (HSA) is an abundant plasma protein that binds a wide variety of hydrophobic ligands including fatty acids, bilirubin, thyroxine and hemin. Although HSA-heme complexes do not bind oxygen reversibly, it may be possible to develop modified HSA proteins or heme groups that will confer this ability on the complex. RESULTS: We present here the crystal structure of a ternary HSA-hemin-myristate complex, formed at a 1:1:4 molar ratio, that contains a single hemin group bound to subdomain IB and myristate bound at six sites. The complex displays a conformation that is intermediate between defatted HSA and HSA-fatty acid complexes; this is likely to be due to low myristate occupancy in the fatty acid binding sites that drive the conformational change. The hemin group is bound within a narrow D-shaped hydrophobic cavity which usually accommodates fatty acid; the hemin propionate groups are coordinated by a triad of basic residues at the pocket entrance. The iron atom in the centre of the hemin is coordinated by Tyr161. CONCLUSION: The structure of the HSA-hemin-myristate complex (PDB ID 1o9x) reveals the key polar and hydrophobic interactions that determine the hemin-binding specificity of HSA. The details of the hemin-binding environment of HSA provide a structural foundation for efforts to modify the protein and/or the heme molecule in order to engineer complexes that have favourable oxygen-binding properties

Evaluation of Safety and Efficacy of Hemoglobin-Vesicles and Albumin-Hemes

Since the discovery of a red-colored saline solution of a heme derivative that reversibly binds and releases O2 (1983), significant efforts have been made to realize an O2 infusion as a red cell substitute based on the sciences of both molecular assembling phenomena and macromolecular metal complexes. We have specified that hemoglobin (Hb)-vesicles (HbV) and recombinant human serum albumin -hemes (rHSAhemes) would be the best systems that meet the clinical requirements. The HbV encapsulates ultrapure cone. Hb solution, that is free of any infectious elements, with a phospholipid bimolecular membrane (diameter, 250nmφ), and its solution properties can be adjusted comparable with blood. Surface modification of HbV with a water-soluble polymer ensures stable dispersion state and storage over a year at 20°C. In vivo tests have clarified the efficacy for extreme hemodilution and resuscitation from hemorrhagic shock, and safety in terms of biodistribution, metabolism in RES, clinical chemistry, blood coagulation, etc.. The HbV does not induce vasoconstriction thus maintains blood flow and tissue oxygenation. The rHSAheme is a totally synthetic O2 carrier that incorporates 8 heme derivatives (axial base substituted hemes) as O2 binding sites in the hydrophobic pockets of rHSA, which is now manufactured in Japan as a plasma-expander. Hb binds endothelium-derived relaxation factor, NO, and induces vasoconstriction. The rHSA-heme binds NO as Hb does, however, it does not induce vasoconstriction due to its low pI (4.8) and the resulting low permeability across the vascular wall (1/100 of Hb). A 5%-albumin solution possesses a physiologic oncotic pressure. Therefore, to increase the 02-transporting capacity, albumin dimer is effective. Albumin dimer can incorporate totally 16 hemes with a regulated oncotic pressure. The rHSA-heme is effective not only as a red cell substitute but also for O2 therapeutics (e.g., oxygenation for tumor). Significant efforts have been made to produce HbV and rHSA-hemes with a facility of GMP standard, and to start preclinical and finally clinical trials