Pietri Meléndez, Ruth

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    Hemoglobin I from Lucina pectinata: A model for hydrogen sulfide reactivity within hemeproteins
    (2009) Pietri Meléndez, Ruth; López Garriga, Juan; College of Arts and Sciences - Sciences; Vega Olivencia, Carmen A.; Cadilla, Carmen L.; Cardona Martínez, Nelson; Department of Chemistry; Banerjee, Jayanta
    The hemoglobin I (HbI) from the clam Lucina pectinata is an intriguing hemeprotein that binds and transports H2S to chemoautotrophic bacteria to maintain a symbiotic relationship and to protect the mollusk from H2S toxicity. Mutations at E7, B10 and E11 positions were introduced in the HbI heme pocket to define the reactivity of H2S with hemeproteins. The structural properties of the mutants and the recombinant HbI were first evaluated using CO as a sensor of the heme environment. The effects of these mutations were then studied in the ferric H2O and H2S derivatives. The results obtained with the HbI-CO mutants show that GlnE7Asn and GlnE7Val generate open distal structures, relative to rHbI, with slight or none ligand stabilization, while GlnE7His and PheB10Tyr exert strong stabilization mechanisms, creating in turn closed distal configurations. The results also suggest that distal structural rearrangements occur in the PheB10Leu, PheB10Val and E11 mutations, inducing movement of GlnE7. The closed distal configuration suggested in the ferrous GhE7His mutant was also observed in the H2O derivative. Moreover, the displacement of the GlnE7 residue, proposed in the ferrous PheB10Leu, PheB10Val and E11 mutants also induces closed distal conformers in the H2O complexes. These conformers are associated with H-bonding interactions between the E7 residues and the bound water. Similar H-bonding interactions are invoked for these HbI-H2S derivatives and the rHbI, altering in turn H2S reactivity within these proteins. This is evident in the resonance Raman spectra of these HbI-H2S complexes, which show reduction of heme iron as judged by the appearance of the ν4 oxidation state marker at 1356 cm-1, indicative of deoxy-FeII species. This reduction process depends strongly on distal mutations showing faster reduction for those HbI mutants exhibiting strongest H-bonding interactions. Furthermore, the X-ray absorption spectra of rHbI and several HbI mutants suggest that an “FeII-H2S” like intermediate is formed prior to the formation of the deoxy-FeII species. Overall, the results presented here show that: a. H2S association is regulated by external barriers; b. H2S release is controlled by two competing reactions involving simple sulfide dissociation and heme reduction followed by H2S liberation; c. at high H2S concentrations, reduction of the ferric center dominates; d. reduction of the heme is also enhanced in those HbI mutants having polar distal environments.