Recombinant expression and redox properties of triheme c membrane-bound quinol peroxidase

The qpo gene of Aggregatibacter actinomycetemcomitans encodes a triheme c-containing membrane-bound enzyme, quinol peroxidase (QPO) that catalyzes peroxidation reaction in the respiratory chain and uses quinol as the physiological electron donor. The QPO of A. actinomycetemcomitans is the only characterized QPO, but homologues of the qpo gene are widely distributed among many gram-negative bacteria, including Haemophils ducreii, Bacteroides fragilis, and Escherichia coli. One-third of the amino acid sequence of QPO from the N-terminal end is unique, whereas two-thirds of the sequence from the C-terminal end exhibits high homology with the sequence of the diheme bacterial cytochrome c peroxidase. In order to obtain sufficient protein for biophysical studies, the present study aimed to overproduce recombinant QPO (rQPO) from A. actinomycetemcomitans in E. coli. Coexpression of qpo with E. coli cytochrome c maturation (ccm) genes resulted in the expression of an active QPO with a high yield. Using purified rQPO, we determined the midpoint reduction potentials of the three heme molecules.