Effects of heme modification on oxygen affinity and cooperativity of human adult hemoglobin

Abstract

We substituted strongly electron-withdrawing trifluoromethyl ( CF3) group(s) as heme side chain(s) of human adult hemoglobin (Hb) to achieve large alterations of the heme electronic structure, in order to elucidate the relationship between the oxygen ( O2) binding properties of Hb and the electronic properties of heme peripheral side chains. The obtained results were compared with those of similar studies performed on myoglobin (Mb), e.g. (Nishimura R, Matsumoto D, Shibata T, Yanagisawa S, Ogura T, Tai H, Matsuo T, Hirota S, Neya S, Suzuki A, and Yamamoto Y. Inorg. Chem. 2014; 53: 9156–9165). These two proteins shared the common feature of a decrease in O2affinity upon the CF3substitution(s). Using the P50value, which is the partial pressure of O2required for 50% oxygenation of a protein, and the equilibrium constant ( p Ka) of the "acid-alkaline transition" in the met form of a protein as measures of the O2affinity and the electron density of heme Fe atom of the protein, respectively, a linear p Ka- log (1/P50) relationship was demonstrated for the Hb and Mb systems. The native Hb, however, deviated from the p Ka- log (1/P50) relationship, while the native Mb followed it. These results highlighted the significance of the vinyl side chains of the heme cofactor in the functional control of Hb through tertiary and quaternary structural changes upon the oxygenation of the protein.