Hemoglobin I from Lucina pectinata: A model for distal heme-ligand control

Structure and Ligand Selection of Hemoglobin II from Lucina pectinata

“…The combination of these factors facilitates the strength of the Fe-O 2 complex in the distal heme pocket. Moreover, the small distal cavity volume of HbII Lp and its hydrogen bonding network with the heme-O 2 moiety is consistent with the relationship between the Fe-O 2 stretching mode and its oxygen dissociation rate constant (k off ) (46). This is supported by data in Table 3 61 s Ϫ1 , respectively.…”

“…6) must have an important role to explain the observed experimental data. Similarly, this is also supported by studies of CO complexes showing three different conformers at 1912, 1956, and 1965 cm Ϫ1 for Hb Asc -CO, whereas the HbII Lp -CO complex show only the presence of the A 3 and A 0 conformers at 1924 and 1964 cm Ϫ1 in the infrared spectra (15,46). Therefore, the interplay between the small HbII Lp heme pocket structure, the hydrogen bonding network to the proximal and distal heme environments, the His(F8) trans-effect, and the orientation of the oxygen molecule in the oxy complex are responsible for the stability of the oxyHbII Lp complex.…”

“…This is supported by data in Table 3 61 s Ϫ1 , respectively. Similarly, HbI Lp Gln(E7) 3 Asp and HbI Lp Gln(E7) 3 Val mutants show an increase in the dissociation rate constant from 140 s Ϫ1 (for HbIr specie) to 375 and 500 s Ϫ1 , whereas the Fe-O normal mode frequency decreases to 563 cm Ϫ1 (46). The faster oxygen dissociation rates suggest that the interactions of the E7 residue in these two mutants are much weaker than in HbI Lp , thus corroborating that the absence of a hydrogen bond induces a lower Fe-O vibrational frequency.…”

Hydrogen Sulfide and Hemeproteins: Knowledge and Mysteries

“…Indeed, early spectroscopic studies of HbI with other well-known ligands (such as CO, cyanide, H 2 O, and O 2 ) suggested that glutamine is flexible and controls ligand access to the HbI heme pocket and that hydrogen bonding and other interactions with the Phecage contribute to ligand stability (15,51,60). Using sitedirected mutagenesis, and spectroscopic and theoretical approaches, we have shown that protein fluctuations are required to allow H 2 S access to the HbI distal heme site.…”

“…Once in the distal heme site, the flexibility of glutamine allows the ligand to bind rapidly to the ferric iron (24,43,(59)(60)(61)63). The gaseous ligand is stabilized in part by a hydrogen bonding interaction with glutamine (61,66).…”