Journal:JBSD:9

=== Carbon monoxide binding to the heme group at the dimeric interface modulates structure and copper accessibility in the Cu,Zn superoxide dismutase from Haemophilus ducreyi: in silico and in vitro evidences ===

<scene name='Journal:JBSD:9/Cv/5'>Superoxide dismutases</scene> (SODs) are metalloenzymes playing a vital role in the defense mechanism against the oxidative stress; they catalyze the dismutation of superoxide, the one-electron reduction product of oxygen, to hydrogen peroxide and molecular oxygen, thus protecting living organism from oxidative lethality (<scene name='Journal:JBSD:9/Cv/7'>conserved SODs regions</scene> among prokaryotes and eukaryotic organisms are highlighted in different colors: <font color='crimson'><b>SS subloop (residues Glu73-Gly93) in crimson</b></font>; (<span style="color:salmon;background-color:black;font-weight:bold;">Zn subloop (residues Gly94-Ala119) in salmon</span>; (<span style="color:violet;background-color:black;font-weight:bold;">Greek key loop (residues Pro135-Gly145) in violet color</span>; (<span style="color:pink;background-color:black;font-weight:bold;">7,8 loop (residues Ala152-Pro169) in pink color)</span>. Haemophilus ducrey, the causative agent of the sexually transmitted human genital ulcerative disease known as chancroid, expresses one of the most interesting examples of bacterial Cu,Zn SOD (#HdSOD) with the unique feature of binding a heme molecule at the interface between the two subunits asymmetrically bound by residues <scene name='Journal:JBSD:9/Cv/11'>His64 and His124 of subunits A and B</scene>, respectively (<span style="color:lime;background-color:black;font-weight:bold;">His64 and His124 are colored green</span> and <span style="color:yellow;background-color:black;font-weight:bold;">heme molecule is in yellow)</span>. The heme molecule proved to be able to bind small gaseous ligands, such as nitric oxide or carbon monoxide, as a sixth ligand thus displacing the distal histidine. In this study the structural and dynamic response of HdSOD to the <scene name='Journal:JBSD:9/Cv/12'>binding of CO to heme</scene> (<font color='magenta'><b>the carbon atom of CO colored in magenta</b></font> and <font color='red'><b>the oxigen one is in red</b></font>) was studied by means of a combinations of Molecular Dynamics Simulation and X-Ray absorbtion Spettroscopy and hypothesis formulated were further confirmed by in vitro experiments. All together the collected results evidenced that binding of the CO molecule produces a strong reduction in the asymmetric fluctuations of the two subunits and long range effects of the heme group on the <scene name='Journal:JBSD:9/Cv/13'>Cu active site structure</scene> (<span style="color:cyan;background-color:black;font-weight:bold;">active site residues: His70, His72, His95, His104, His113, Asp 116 and His 151 are in cyan</span>, <font color='gray'><b>Zn is in gray</b></font> and <span style="color:darkgoldenrod;background-color:black;font-weight:bold;">Cu is in darkgoldenrod</span>) which becomes more easily accessible to the solvent thus causing an increase in copper dismutasic activity. Based on this picture, we suggest a role of HdSOD as a heme-based-sensor protein in which conformational changes, triggered by the heme group, could help Haemophilus ducreyi to adapt at fluctuating levels of gaseous molecules, such as carbon monoxide or nitric oxide: in the presence of these gasses, the HdSOD would be able to increase its superoxide dismutation activity to subtract superoxide substrate and to prevent the formation of the dangerous peroxynitrite, the molecule generated by the reaction of nitric oxide with superoxide, which is even more harmful to the cell than superoxide. Further investigation to validate this attractive hypothesis is thus desirable.