The survival of Mycobacterium tuberculosis requires detoxification of host center dot NO. Oxygenated Mycobacterium tuberculosis truncated hemoglobin N catalyzes the rapid oxidation of nitric oxide to innocuous nitrate with a second-order rate constant (k'NOD approximate to 745 x 10(6) M(-1 center dot)s(-1)), which is similar to 15-fold faster than the reaction of horse heart myoglobin. We ask what aspects of structure and/or dynamics give rise to this enhanced reactivity. A first step is to expose what controls ligand/substrate binding to the heme. We present evidence that the main barrier to ligand binding to deoxy-truncated hemoglobin N (deoxy-trHbN) is the displacement of a distal cavity water molecule, which is mainly stabilized by residue Tyr(B10) but not coordinated to the heme iron. As observed in the Tyr( B10)/Gln(E11) apolar mutants, once this kinetic barrier is lowered, CO and O-2 binding is very rapid with rates approaching 1 - 2 x 10(9) M(-1 center dot)s(-1). These large values almost certainly represent the upper limit for ligand binding to a heme protein and also indicate that the iron atom in trHbN is highly reactive. Kinetic measurements on the photoproduct of the center dot NO derivative of met-trHbN, where both the center dot NO and water can be directly followed, revealed that water rebinding is quite fast (similar to 1.49 x 10(8) s(-1)) and is responsible for the low geminate yield in trHbN. Molecular dynamics simulations, performed with trHbN and its distal mutants, indicated that in the absence of a distal water molecule, ligand access to the heme iron is not hindered. They also showed that a water molecule is stabilized next to the heme iron through hydrogen-bonding with Tyr(B10) and Gln(E11).
|Titolo:||Ligand binding to truncated hemoglobin N from Mycobacterium tuberculosis is strongly modulated by the interplay between the distal heme pocket residues and internal water|
|Settore Scientifico Disciplinare:||Settore BIO/10 - Biochimica|
|Data di pubblicazione:||2008|
|Digital Object Identifier (DOI):||10.1074/jbc.M804215200|
|Appare nelle tipologie:||01 - Articolo su periodico|