Tyrosine Residues as Redox Cofactors in Human Hemoglobin: IMPLICATIONS FOR ENGINEERING NONTOXIC BLOOD SUBSTITUTES*S⃞

• Published in: Journal of Biological Chemistry
• Main Authors: Reeder, Brandon J., Grey, Marie, Silaghi-Dumitrescu, Radu-Lucian, Svistunenko, Dimitri A., Bülow, Leif, Cooper, Chris E., Wilson, Michael T.
• Format: Text
• Language: English
• Published: American Society for Biochemistry and Molecular Biology 2008
• Subjects: Enzyme Catalysis and Regulation; Sperm whale
• Online Access: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2662160; http://www.ncbi.nlm.nih.gov/pubmed/18728007; https://doi.org/10.1074/jbc.M804709200

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 α and β chains, display significantly different kinetics for ferryl reduction by exogenous reductants. By using selected hemoglobin mutants, we show that the α 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 β 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.