Tyrosine B10 and heme–ligand interactions of Lucina pectinata hemoglobin II: control of heme reactivity

Structure and Ligand Selection of Hemoglobin II from Lucina pectinata

“…At basic pH values, the barrier for the reaction increases as the tyrosine adopts a closed conformation and the heme (Fe III ) hydroxyl complex replaces the met-aquo species, which suggested the existence of an open and closed conformation due to the interactions between Tyr(B10) and the heme iron. The presence of these conformers were confirmed by resonance Raman spectroscopy showing that, in a neutral environment, met-aquo HbII Lp was present as a mixture of coordination and spin states with values for the v 2 mode at 1558 cm Ϫ1 (6C HS) and 1580 cm Ϫ1 (6C LS) and for the v 3 mode at 1479 cm Ϫ1 (6C HS), 1492 cm Ϫ1 (5C), and 1503 cm Ϫ1 (6C LS) (9). The infrared spectra of the complex HbII Lp CO also showed the presence of the A 3 (closed) and A 0 (open) conformers at 1924 and 1964 cm Ϫ1 , respectively (9).…”

“…The presence of these conformers were confirmed by resonance Raman spectroscopy showing that, in a neutral environment, met-aquo HbII Lp was present as a mixture of coordination and spin states with values for the v 2 mode at 1558 cm Ϫ1 (6C HS) and 1580 cm Ϫ1 (6C LS) and for the v 3 mode at 1479 cm Ϫ1 (6C HS), 1492 cm Ϫ1 (5C), and 1503 cm Ϫ1 (6C LS) (9). The infrared spectra of the complex HbII Lp CO also showed the presence of the A 3 (closed) and A 0 (open) conformers at 1924 and 1964 cm Ϫ1 , respectively (9). We proposed that in the open conformation Tyr(B10) swings away from the heme iron, whereas in the closed conformation Tyr(B10) is closer to and may interact with the ligand.…”

“…HbI Lp only shares 32% of its amino acid sequence with HbII Lp , but both have a conserved glutamine in the E7 position, and a phenylalanine and a tyrosine in the B10 position for HbI Lp and HbII Lp , respectively. The Tyr(B10) and Gln(E7) distal heme pocket residues of HbII Lp are also found in other hemoglobins (8,9). Hemoglobins from Ascaris suum (Hb Asc ) (Tyr(B10) and Gln(E7)) (10) and Mycobacterium tuberculosis hemoglobin N (Tyr(B10) and Leu(E7)) (11,12) show high oxygen affinity and a very slow release of the bound oxygen.…”

“…We proposed that in the open conformation Tyr(B10) swings away from the heme iron, whereas in the closed conformation Tyr(B10) is closer to and may interact with the ligand. Similarly, the reactions between hydrogen peroxide and both HbII Lp and the HbI Lp Phe(B10) 3 Tyr mutant showed that Tyr(B10) tailors, in two very distinct ways, the reactivity of compounds I and II ferryl species (9). First, increasing the reaction pH from 4.9 to 7.5, and then to 11.2, the second order rate constant for HbII Lp decreases from 141.60 to 77.78 M Ϫ1 s Ϫ1 , and to 2.96 M Ϫ1 s Ϫ1 , respectively.…”

“…The low oxygen off rates were attributed to differences in the hydrogen bonding network of both oxy-protein complexes. Additional studies using UV-visible spectroscopy and pH titration of HbII Lp and a HbI Lp Phe(B10) 3 Tyr mutant have revealed bands at 486, 541, 577, and 603 nm for both proteins at neutral conditions (3,9). At basic pH values, the barrier for the reaction increases as the tyrosine adopts a closed conformation and the heme (Fe III ) hydroxyl complex replaces the met-aquo species, which suggested the existence of an open and closed conformation due to the interactions between Tyr(B10) and the heme iron.…”