title: Tyrosine residues as redox cofactors in human hemoglobin: implications for engineering nontoxic blood substitutes.
abstract: Respiratory proteins such as myoglobin and hemoglobin can, under oxidative conditions, form ferryl heme iron and protein-based free radicals. Ferryl myoglobin can safely be returned to the ferric oxidation state by electron donation from exogenous reductants via a mechanism that involves two distinct pathways. In addition to direct transfer between the electron donor and ferryl heme edge, there is a second pathway that involves "through-protein" electron transfer via a tyrosine residue (tyrosine 103, sperm whale myoglobin). Here we show that the heterogeneous subunits of human hemoglobin, the alpha and beta chains, display significantly different kinetics for ferryl reduction by exogenous reductants. By using selected hemoglobin mutants, we show that the alpha chain possesses two electron transfer pathways, similar to myoglobin. Furthermore, tyrosine 42 is shown to be a critical component of the high affinity, through-protein electron transfer pathway. We also show that the beta chain of hemoglobin, lacking the homologous tyrosine, does not possess this through-protein electron transfer pathway. However, such a pathway can be engineered into the protein by mutation of a specific phenylalanine residue to a tyrosine. High affinity through-protein electron transfer pathways, whether native or engineered, enhance the kinetics of ferryl removal by reductants, particularly at low reductant concentrations. Ferryl iron has been suggested to be a major cause of the oxidative toxicity of hemoglobin-based blood substitutes. Engineering hemoglobin with enhanced rates of ferryl removal, as we show here, is therefore likely to result in molecules better suited for in vivo oxygen delivery.
authors: Department of Biological Sciences, University of Essex, Wivenhoe Park, Colchester, Essex CO4 3SQ, UK. reedb@essex.ac.uk
authors: Brandon J
authors: BJ
authors: Reeder
authors: Marie
authors: M
authors: Grey
authors: Radu-Lucian
authors: RL
authors: Silaghi-Dumitrescu
authors: Dimitri A
authors: DA
authors: Svistunenko
authors: Leif
authors: L
authors: Bülow
authors: Chris E
authors: CE
authors: Cooper
authors: Michael T
authors: MT
authors: Wilson
mesh_terms: D000818:Animals; D001802:Blood Substitutes / Q000737:chemistry*; D004579:Electron Transport; D005609:Free Radicals / Q000737:chemistry*; D006454:Hemoglobins / Q000737:chemistry* / Q000235:genetics; D006736:Horses; D006801:Humans; D007501:Iron / Q000737:chemistry*; D007700:Kinetics; D009211:Myoglobin / Q000737:chemistry / Q000235:genetics; D010084:Oxidation-Reduction; D010100:Oxygen / Q000737:chemistry*; D015202:Protein Engineering; D021122:Protein Subunits / Q000737:chemistry / Q000235:genetics; D050661:Sperm Whale; D014443:Tyrosine / Q000737:chemistry* / Q000235:genetics
chemical_list: D001802:Blood Substitutes; D005609:Free Radicals; D006454:Hemoglobins; D009211:Myoglobin; D021122:Protein Subunits; C066317:ferryl iron; D014443:Tyrosine; D007501:Iron; D010100:Oxygen