<i>Escherichia coli</i> Triheme Enzyme YhjA: Structure and Reactivity

It has been recently realized that some Gram-negative organisms such as <i>Escherichia coli</i> produce a multiheme cytochrome <i>c</i> to serve as a quinol peroxidase that couples electrons from the quinol pool directly to H₂O₂. The <i>E. coli</i> version of this enzyme, termed YhjA, has been predicted to be a member of the bacterial cytochrome <i>c</i> peroxidase (bCCP) superfamily, where a novel N-terminal single-heme binding domain is fused to the canonical bCCP diheme domain found widely in Gram-negative bacteria. Here, we present an X-ray crystal structure of YhjA, revealing the triheme architecture that nature has employed to couple the quinol pool to the reduction of H₂O₂. We also show kinetic, spectroscopic, and electrochemical data that detail the differences between the three hemes that are observed in the structure, where two of the heme irons are both six-coordinate, ligated by Met and His residues, and the third peroxidatic heme is found to be five-coordinate. Electrocatalytic voltammetry of YhjA illustrates how the high-potential hemes serve as relays to the peroxidatic active site. Together, these data suggest a model of the catalytic chemistry of YhjA, illustrating how this member of the bCCP family may react with substrates and engage in multielectron redox reactions.