Functional importance of Glutamate-445 and Glutamate-99 in proton-coupled electron transfer during oxygen reduction by cytochrome bd from Escherichia coli

Abstract

<p class="first" id="P1">The recent X-ray structure of the cytochrome <i>bd</i> respiratory oxygen reductase showed that two of the three heme components, heme <i>d</i> and heme <i>b ₅₉₅ </i>, have glutamic acid as an axial ligand. No other native heme proteins are known to have glutamic acid axial ligands. In this work, site-directed mutagenesis is used to probe the roles of these glutamic acids, E445 and E99 in the <i>E. coli</i> enzyme. It is concluded that neither glutamate is a strong ligand to the heme Fe and they are not the major determinates of heme binding to the protein. Although very important, neither glutamate is absolutely essential for catalytic function. The close interactions between the three hemes in cyt <i>bd</i> result in highly cooperative properties. For example, mutation of E445, which is near heme <i>d</i>, has its greatest effects on the properties of heme <i>b ₅₉₅ </i> and heme <i>b ₅₅₈ </i>. It is concluded that 1) O ₂ binds to the hydrophilic side of heme <i>d</i> and displaces E445; 2) E445 forms a salt bridge with R448 within the O ₂ binding pocket, and both residues play a role to stabilize oxygenated states of heme <i>d</i> during catalysis; 3) E445 and E99 are each protonated accompanying electron transfer to heme <i>d</i> and heme <i>b ₅₉₅ </i>, respectively; 4) All protons used to generate water within the heme <i>d</i> active site come from the cytoplasm and are delivered through a channel that must include internal water molecules to assist proton transfer: [cytoplasm]→E107→E99 (heme <i>b ₅₉₅ </i>)→E445 (heme <i>d</i>)→oxygenated heme <i>d</i>. </p><p id="P2"> <div class="figure-container so-text-align-c"> <img alt="" class="figure" src="/document_file/3b3e6995-4c95-43bf-94f9-6e6870636351/PubMedCentral/image/nihms967665u1.jpg"/> </div> </p>