Extreme differences between hemoglobins I and II of the clam Lucina pectinalis in their reactions with nitrite.
The clam Lucina pectinalis supports its symbiotic bacteria by H₂S transport in the open and accessible heme pocket of Lucina Hb I and by O₂ transport in the narrow and crowded heme pocket of Lucina Hb II. Remarkably, air-equilibrated samples of Lucina Hb I were found to be more rapidly oxidized by nitrite than any previously studied Hb, while those of Lucina Hb II showed an unprecedented resistance to oxidation induced by nitrite. Nitrite-induced oxidation of Lucina Hb II was enabled only when O₂ was removed from its active site. Structural analysis revealed that O₂ "clams up" the active site by hydrogen bond formation to B10Tyr and other distal-side residues. Quaternary effects further restrict nitrite entry into the active site and stabilize the hydrogen-bonding network in oxygenated Lucina Hb II dimers. The dramatic differences in nitrite reactivities of the Lucina Hbs are not related to their O₂ affinities or anaerobic redox potentials, which were found to be similar, but are instead a result of differences in accessibility of nitrite to their active sites; i.e. these differences are due to a kinetic rather than thermodynamic effect. Comparative studies revealed heme accessibility to be a factor in human Hb oxidation by nitrite as well, as evidenced by variations of rates of nitrite-induced oxidation that do not correlate with R and T state differences and inhibition of oxidation rate in the presence of O₂. These results provide a dramatic illustration of how evolution of active sites with varied heme accessibility can moderate the rates of inner-sphere oxidative reactions of Hb and other heme proteins.
Bonaventura
Celia
C
Nicholas School of the Environment, Duke University Marine Laboratory, Beaufort, NC 28516, USA. bona@duke.edu
Henkens
Robert
R
De Jesus-Bonilla
Walleska
W
Lopez-Garriga
Juan
J
Jia
Yiping
Y
Alayash
Abdu I
AI
Siburt
Claire J Parker
CJ
Crumbliss
Alvin L
AL
D000818:Animals; D001665:Binding Sites; D049872:Bivalvia / Q000378:metabolism*; D006418:Heme; D006454:Hemoglobins / Q000737:chemistry* / Q000378:metabolism*; D006801:Humans; D006860:Hydrogen Bonding; D007700:Kinetics; D009573:Nitrites / Q000737:chemistry / Q000378:metabolism*; D010084:Oxidation-Reduction; D010100:Oxygen / Q000378:metabolism*
D006454:Hemoglobins; D009573:Nitrites; D006418:Heme; C026236:hemoglobin I, non-mammalian; D010100:Oxygen